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The Linux 2.4 SCSI subsystem HOWTO

Douglas Gilbert

<dgilbert at interlog dot com>

Copyright (c) 2001, 2002, 2003, 2004 Douglas Gilbert

2003-08-24
Revision History                                                             
Revision 2.1           2004-08-24            Revised by: dpg                 
scsihosts change -> run mkinitrd, lk 2.4.21,22                               
Revision 2.0           2003-05-04            Revised by: dpg                 
lk2.4.20, linuxdoc->tldp, sATA and SAS, last sector on raw devs, blockdev    
Revision 1.9           2002-11-20            Revised by: dpg                 
convert to xml, lk2.4.19, spelling                                           
Revision 1.8           2002-05-05            Revised by: dpg                 
scsihosts comma delimiter, grub+lilo                                         
Revision 1.7           2002-04-27            Revised by: dpg                 
mkinitrd, scsi_debug, 2.4.18, more ATAPI                                     
Revision 1.6           2002-01-26            Revised by: dpg                 
ATAPI cdrom selection                                                        
Revision 1.5           2001-12-21            Revised by: dpg                 
16 byte SCSI commands, SCSI_IOCTL_GET_PCI                                    
Revision 1.4           2001-08-26            Revised by: dpg                 
spelling, dd_rescue, mkinitrd example, lk 2.4 changes, 1394.                 
Revision 1.3           2001-08-26            Revised by: dpg                 
ATAPI CDROM section, alter title, U320, iSCSI.                               
Revision 1.2           2001-03-25            Revised by: dpg                 
Information about scu, dt, "Alt" sequences, more notes.                      
Revision 1.1           2001-01-22            Revised by: dpg                 
Add osst description, _EXTRA_DEVS limitations.                               


  This document describes the SCSI subsystem as the Linux kernel enters the
2.4 production series. An external view of the SCSI subsystem is the main
theme. Material is included to help the system administration of the Linux
SCSI subsystem. There are also brief descriptions of ioctl()s and interfaces
that may be relevant to those writing applications that use this subsystem.

  Permission is granted to copy, distribute and/or modify this document under
the terms of the GNU Free Documentation License, Version 1.1 or any later
version published by the Free Software Foundation; with no Invariant
Sections, with no Front-Cover Texts, and with no Back-Cover Texts.

  For an online copy of the license see   www.fsf.org/copyleft/fdl.html.

-----------------------------------------------------------------------------
Table of Contents
1. Introduction
2. Architectural Overview
3. Names and Addresses
    3.1. SCSI Addressing
    3.2. Device Names
    3.3. Device Names in devfs
    3.4. Device Names in scsidev
   
   
4. Kernel Configuration
5. Boot Parameters
6. Modules and their Parameters
7. Proc pseudo file system
8. Mid Level, Unifying layer
    8.1. boot parameters
    8.2. module parameters
    8.3. proc interface
   
   
9. Upper level drivers
    9.1. Disk driver (sd)
    9.2. CDROM driver (sr or scd)
    9.3. Tape driver (st)
    9.4. Generic driver (sg)
   
   
10. Lower Level drivers
    10.1. Pseudo drivers
   
   
11. Raw devices
12. Devfs pseudo file system
A. Common bus types (SCSI and other)
B. Changes between lk 2.2 and (during) 2.4
    B.1. Mid level changes
    B.2. sd changes
    B.3. sr changes
    B.4. st changes
    B.5. sg changes
    B.6. Changes during the lk 2.4 series
   
   
C. Troubleshooting
D. Performance, Test and Debugging tools
E. Compile options and System calls including ioctls
    E.1. Mid level
    E.2. sd driver
    E.3. sr driver
    E.4. st driver
    E.5. sg driver
   
   
F. References, Credits and Corrections

-----------------------------------------------------------------------------
Chapter 1. Introduction

 This document describes the SCSI subsystem as the Linux kernel enters the
2.4 production series.

 An external view of the SCSI subsystem is the main theme. Material is
included to help the system administration of the Linux SCSI subsystem. There
are also brief descriptions of ioctl()s and interfaces that may be relevant
to those writing applications that use this subsystem. However internal data
structures and design issues are not addressed [see reference W2]. To
unclutter the presentation, compile options and system calls (including ioctl
()s) have been placed in Appendix E. Although not strictly part of the SCSI
subsystem, there is also a description of raw devices in Chapter 11.

 For those who have no interest in the SCSI subsystem and just want to get
their ATAPI cd writer going, see Section 9.2.4. It may also be useful to
browse Chapter 2.

 This document follows on from one written five years ago by Drew Eckhardt
called the SCSI-HOWTO [see reference W7]. That document described the SCSI
subsystem in Linux kernel 1.2 and 1.3 series. It is still available from the
Linux Documentation Project [LDP, see reference W8] in its "unmaintained"
section. Both documents have roughly similar structures although Drew's
document has a lot of information on the adapter drivers.

 This document can be found in electronic form at  www.tldp.org/HOWTO/
SCSI-2.4-HOWTO. The home site and perhaps the most up to date version of this
document can be found at  www.torque.net/scsi/SCSI-2.4-HOWTO (this is the
multi-page html version). At that location this document is rendered in txt,
pdf, ps, a single (long) page of html as well as multi-page html. For
example, a pdf version is at  www.torque.net/scsi/SCSI-2.4-HOWTO.pdf).

 This document was last altered on 24th August 2004.
-----------------------------------------------------------------------------

Chapter 2. Architectural Overview

 The SCSI subsystem has a 3 level architecture with the "upper" level being
closest to the user/kernel interface while the "lower" level is closest to
the hardware. The upper level drivers are commonly known by a terse two
letter abbreviation (e.g. "sd" for SCSI disk driver). The names of the
corresponding module drivers which, for historical reasons, sometimes differ
from the built in driver names are shown in braces in the following diagram.

[scsi-arch]

  The 3 level driver architecture of the SCSI subsystem.

 The upper level supports the user-kernel interface. In the case of sd and sr
this is a block device interface while for st and sg this is a character
device interface. Any operation using the SCSI subsystem (e.g. reading a
sector from a disk) involves one driver at each of the 3 levels (e.g. sd,
SCSI mid level and aic7xxx drivers).

 As can be seen from the diagram, the SCSI mid level is common to all
operations. The SCSI mid level defines internal interfaces and provides
common services to the upper and lower level drivers. Ioctls provided by the
mid level are available to the file descriptors belonging to any of the 4
upper level drivers.

 The most common operation on a block device is to "mount" a file system. For
a sd device typically a partition is mounted (e.g. mount -t ext2 /dev/sda6 /
home). For a sr device usually the whole device is mounted (e.g.  mount -t
iso9660 /dev/sr0 /mnt/cdrom). The dd command can be used to read or write
from block devices. In this case the block size argument ("bs") needs to be
set to the block size of the device (e.g. 512 bytes for most disks) or an
integral multiple of that device block size (e.g. 8192 bytes). A recent
addition to the block subsystem allows a device (or partition) to be mounted
more than once, at different mount points.

 Sd is a member of the generic disk family, as is the hd device from the IDE
subsystem. Apart from mounting sd devices, the fdisk command is available to
view or modify a disk's partition table. Although the hdparm command is
primarily intended for ATA disks (also known as IDE or EIDE disks), some
options work on SCSI disks.

 Sr is a member of the CD-ROM subsystem. Apart from mounting file systems
(e.g. iso9660), audio CDs can also be read. The latter action does not
involve mounting a file system but typically by invoking some ioctls. General
purpose Linux commands such as dd cannot be used on audio CDs.

 St is a char device for reading and writing tapes. Typically the mt command
is used to perform data transfers and other control functions.

 Sg is a SCSI command pass through device that uses a char device interface.
General purpose Linux commands should not be used on sg devices. Applications
such as SANE (for scanners), cdrecord and cdrdao (for cd writers) and 
cdparanoia (for reading audio CDs digitally) use sg.
-----------------------------------------------------------------------------

Chapter 3. Names and Addresses

 This section covers the various naming schemes that exist in Linux and the
SCSI worlds and how they interact.
-----------------------------------------------------------------------------

3.1. SCSI Addressing

 Linux has a four level hierarchical addressing scheme for SCSI devices:

��*�SCSI adapter number [host]
   
��*�channel number [bus]
   
��*�id number [target]
   
��*�lun [lun]
   

 "Lun" is the common SCSI abbreviation of Logical Unit Number. The terms in
brackets are the name conventions used by device pseudo file system (devfs).
"Bus" is used in preference to "channel" in the description below.

 The SCSI adapter number is typically an arbitrary numbering of the adapter
cards on the internal IO buses (e.g. PCI, PCMCIA, ISA etc) of the computer.
Such adapters are sometimes termed as HBAs (host bus adapters). SCSI adapter
numbers are issued by the kernel in ascending order starting with 0.

 Each HBA may control one or more SCSI buses. The various types of SCSI buses
are listed in Appendix A.

 Each SCSI bus can have multiple SCSI devices connected to it. In SCSI
parlance the HBA is called the "initiator" and takes up one SCSI id number
(typically 7). The initiator [1] talks to targets which are commonly known as
SCSI devices (e.g. disks). On SCSI parallel buses the number of ids is
related to the width. 8 bit buses (sometimes called "narrow") can have 8 SCSI
ids of which 1 is taken by the HBA leaving 7 for SCSI devices. Wide SCSI
buses are 16 bits wide and can have a maximum of 15 SCSI devices (targets)
attached. The SCSI 3 draft standard allows a large number of ids to be
present on a SCSI bus.

 Each SCSI device can contain multiple Logical Unit Numbers (LUNs). These are
typically used by sophisticated tape and cdrom units that support multiple
media.

 So Linux's flavour of SCSI addressing is a four level hierarchy:
<scsi(_adapter_number), channel, id, lun>                                    
Using the naming conventions of devfs this becomes:
<host, bus, target, lun>                                                     
-----------------------------------------------------------------------------

3.2. Device Names

 A device name can be thought of as a gateway to a kernel driver that
controls a device rather than the device itself. Hence there can be multiple
device names some of which may offer slightly different characteristics, all
mapping to the same actual device.

 The device names of the various SCSI devices are found within the /dev
directory. Traditionally in Linux, SCSI devices have been identified by their
major and minor device number rather than their SCSI bus addresses (e.g. SCSI
target id and LUN). The device pseudo file system (devfs) moves away from the
major and minor device number scheme and for the SCSI subsystem uses device
names based on the SCSI bus addresses [discussed later in Section 3.3 and see
reference: W5]. Alternatively, there is a utility called scsidev which
addresses this issue within the scope of the Linux SCSI subsystem and thus
does not have the same system wide impact as devfs. Scsidev is discussed
later in Section 3.4 and ref: W6.

 Eight block major numbers are reserved for SCSI disks: 8, 65, 66, 67, 68,
69, 70 and 71. Each major can accommodate 256 minor numbers which, in the
case of SCSI disks, are subdivided as follows:
[b,8,0]    /dev/sda                                                          
[b,8,1]    /dev/sda1                                                         
....                                                                         
[b,8,15]   /dev/sda15                                                        
[b,8,16]   /dev/sdb                                                          
[b,8,17]   /dev/sdb1                                                         
....                                                                         
[b,8,255]  /dev/sdp15                                                        

 The disk device names without a trailing digit refer to the whole disk (e.g.
/dev/sda) while those with a trailing digit refer to one of the 15 allowable
partitions [2] within that disk.

 The remaining 7 SCSI disk block major numbers follow a similar pattern:
[b,65,0]   /dev/sdq                                                          
[b,65,1]   /dev/sdq1                                                         
....                                                                         
[b,65,159] /dev/sdz15                                                        
[b,65,160] /dev/sdaa                                                         
[b,65,161] /dev/sdaa1                                                        
....                                                                         
[b,65,255] /dev/sdaf15                                                       
[b,66,0]   /dev/sdag                                                         
[b,66,1]   /dev/sdag1                                                        
....                                                                         
[b,66,255] /dev/sdav15                                                       
....                                                                         
[b,71,255] /dev/sddx15                                                       

 So there are 128 possible disks (i.e. /dev/sda to /dev/sddx) each having up
to 15 partitions. By way of contrast, the IDE subsystem allows 20 disks (10
controllers each with 1 master and 1 slave) which can have up to 63
partitions each.

 SCSI CD-ROM devices are allocated the block major number of 11.
Traditionally sr has been the device name but scd probably is more
recognizable and is favoured by several recent distributions. 256 SCSI CD-ROM
devices are allowed:
[b,11,0]   /dev/scd0          [or /dev/sr0]                                  
[b,11,255] /dev/scd255        [or /dev/sr255]                                

 SCSI tape devices are allocated the char major number of 9. Up to 32 tape
devices are supported each of which can be accessed in one of four modes (0,
1, 2 and 3), with or without rewind. The devices are allocated as follows:
[c,9,0]    /dev/st0     [tape 0, mode 0, rewind]                             
[c,9,1]    /dev/st1     [tape 1, mode 0, rewind]                             
....                                                                         
[c,9,31]   /dev/st31    [tape 31, mode 0, rewind]                            
[c,9,32]   /dev/st0l    [tape 0, mode 1, rewind]                             
....                                                                         
[c,9,63]   /dev/st31l   [tape 31, mode 1, rewind]                            
[c,9,64]   /dev/st0m    [tape 0, mode 2, rewind]                             
....                                                                         
[c,9,96]   /dev/st0a    [tape 0, mode 3, rewind]                             
....                                                                         
[c,9,127]  /dev/st31a   [tape 31, mode 3, rewind]                            
[c,9,128]  /dev/nst0    [tape 0, mode 0, no rewind]                          
....                                                                         
[c,9,160]  /dev/nst0l   [tape 0, mode 1, no rewind]                          
....                                                                         
[c,9,192]  /dev/nst0m   [tape 0, mode 2, no rewind]                          
....                                                                         
[c,9,224]  /dev/nst0a   [tape 0, mode 3, no rewind]                          
....                                                                         
[c,9,255]  /dev/nst31a  [tape 31, mode 3, no rewind]                         

 The SCSI generic (sg) devices are allocated the char major number of 21.
There are 256 possible SCSI generic (sg) devices:
[c,21,0]   /dev/sg0                                                          
[c,21,1]   /dev/sg1                                                          
....                                                                         
[c,21,255] /dev/sg255                                                        

 Note that the SCSI generic device name's use of a trailing letter (e.g. /dev
/sgc) is deprecated.

 Each SCSI disk (but not each partition), each SCSI CD-ROM and each SCSI tape
is mapped to an sg device. SCSI devices that don't fit into these three
categories (e.g. scanners) also appear as sg devices.

 Pseudo devices [see Section 10.1] can cause devices that are usually not
considered as SCSI to appear as SCSI device names. For example an ATAPI
CD-ROM may be picked up by the ide-scsi pseudo driver and mapped to /dev/scd0
.

 The linux/Documentation/devices.txt file supplied within the kernel source
is the definitive reference for Linux device names and their corresponding
major and minor number allocations.
-----------------------------------------------------------------------------

3.3. Device Names in devfs

 The device pseudo file system can be mounted as /dev in which case it
replaces the traditional Linux device subdirectory. Alternatively it can be
mounted elsewhere (e.g. /devfs) and supplement the existing device structure.

 Without devfs, devices names are typically maintained in the dev directory
of the root partition. Hence the device names (and their associated
permissions) have file system persistence. The existence of a device name
does not necessarily imply such a device (or even its driver) is present. To
save users having to create device name entries (with the mknod command) most
Linux distributions come with thousands of device names defined in the /dev
directory. When applications try to open() the device name then an errno
value of ENODEV indicates there is no corresponding device (or driver)
currently available.

 Devfs takes a different approach in which the existence of the device name
is directly related to the presence of the corresponding device (and its
driver).

 Assuming devfs is mounted on /dev then SCSI devices have primary device
names that might look like this:
/dev/scsi/host0/bus0/target1/lun0/disc    [whole disk]                       
/dev/scsi/host0/bus0/target1/lun0/part6   [partition 6]                      
/dev/scsi/host0/bus0/target1/lun0/generic [sg device for disk]               
                                                                             
/dev/scsi/host1/bus0/target2/lun0/cd      [CD reader or writer]              
/dev/scsi/host1/bus0/target2/lun0/generic [sg device for cd]                 
                                                                             
/dev/scsi/host2/bus0/target0/lun0/mt      [tape mode 0 rewind]               
/dev/scsi/host2/bus0/target0/lun0/mtan    [tape mode 3 no rewind]            
/dev/scsi/host2/bus0/target0/lun0/generic [sg device for tape]               
The sg device on the third line corresponds to the "whole disk" on the first
line since they have the same SCSI address (i.e. host0/bus0/target1/lun0). If
the sg driver is a module and it has not yet been loaded (or it has been
unloaded) then the "generic" device names in the above list will not be
present.

 [Notice the spelling of "disc" as the devfs author favours English spelling
over the American variant.] It can be seen that devfs's naming scheme closely
matches the SCSI addressing discussed in Section 3.1. It is worth noting that
the IDE subsystem uses a similar devfs device naming scheme with the word
"scsi" replaced with "ide". Devfs is discussed further in Chapter 12.
-----------------------------------------------------------------------------

3.4. Device Names in scsidev

 A utility program called scsidev adds device names to the /dev/scsi
directory that reflect the SCSI address of each device. The first 2 letters
of the name are the upper level SCSI driver name (i.e. either sd, sr, st or
sg). The number following the "h" is the host number while the number
following the "-" is meant for host identification purposes. For PCI adapters
this seems to be always 0 while for ISA adapters it is their IO address.
[Perhaps this field could be made more informative or dropped.] The numbers
following the "c", "i" and "l" are channel (bus), target id and lun values
respectively. Raw disks are shown without a trailing partition number while
partitions contained within them are shown with the partition number
following a "p".

 The scsidev would typically be run as part of the boot up sequence. It may
also be useful to run it after the SCSI configuration has changed (e.g.
adding or removing lower level driver modules, or the use of the add/
remove-single-device command). After scsidev has been run on my system which
contains 2 disks, a cd reader and writer plus a scanner, then the following
names were added in the /dev/scsi directory:
$ ls -l /dev/scsi/   # abridged                                              
total 0                                                                      
brw-------  8,   0 Sep  2 11:56 sdh0-0c0i0l0                                 
brw-------  8,   1 Sep  2 11:56 sdh0-0c0i0l0p1                               
...                                                                          
brw-------  8,   8 Sep  2 11:56 sdh0-0c0i0l0p8                               
brw-------  8,  16 Sep  2 11:56 sdh0-0c0i1l0                                 
brw-------  8,  17 Sep  2 11:56 sdh0-0c0i1l0p1                               
...                                                                          
brw-------  8,  24 Sep  2 11:56 sdh0-0c0i1l0p8                               
crw------- 21,   0 Sep  2 11:56 sgh0-0c0i0l0                                 
crw------- 21,   1 Sep  2 11:56 sgh0-0c0i1l0                                 
crw------- 21,   2 Sep  2 11:56 sgh1-0c0i2l0                                 
crw------- 21,   3 Sep  2 11:56 sgh1-0c0i5l0                                 
crw------- 21,   4 Sep  2 11:56 sgh1-0c0i6l0                                 
br-------- 11,   0 Sep  2 11:56 srh1-0c0i2l0                                 
br-------- 11,   1 Sep  2 11:56 srh1-0c0i6l0                                 
The mapping between the SCSI generic device names (sg) and their
corresponding names when controlled by other upper level drivers (i.e. sd, sr
or st) can be seen by looking for name matches when the second letter is
ignored. Hence "sdh0-0c0i0l0" and "sgh0-0c0i0l0" refer to the same device. By
process of elimination the "sgh1-0c0i5l0" filename is the scanner since that
class of devices can only be accessed via the sg interface.

 The scsidev package also includes the ability to introduce names like /dev/
scsi/scanner by manipulating the  /etc/scsi.alias configuration file. The
package also includes the useful rescan-scsi-bus.sh utility. For further
information about scsidev see W6. On my system, both devfs and scsidev
co-exist happily.
-----------------------------------------------------------------------------

Chapter 4. Kernel Configuration

 The Linux kernel configuration is usually found in the kernel source in the
file: /usr/src/linux/.config . It is not recommended to edit this file
directly but to use one of these configuration options:

��*�make config - starts a character based questions and answer session
   
��*�make menuconfig - starts a terminal-oriented configuration tool (using
    ncurses)
   
��*�make xconfig - starts a X based configuration tool
   

 The descriptions of these selections that is displayed by the associated
help button can be found in the flat ASCII file: /usr/src/linux/Documentation
/Configure.help

 Ultimately these configuration tools edit the .config file. An option will
either indicate some driver is built into the kernel ("=y") or will be built
as a module ("=m") or is not selected. The unselected state can either be
indicated by a line starting with "#" (e.g. "# CONFIG_SCSI is not set") or by
the absence of the relevant line from the .config file.

 The 3 states of the main selection option for the SCSI subsystem (which
actually selects the SCSI mid level driver) follow. Only one of these should
appear in an actual .config file:
CONFIG_SCSI=y                                                                
CONFIG_SCSI=m                                                                
# CONFIG_SCSI is not set                                                     

 Some other common SCSI configuration options are:
CONFIG_BLK_DEV_SD        [disk (sd) driver]                                  
CONFIG_SD_EXTRA_DEVS     [extra slots for disks added later]                 
CONFIG_BLK_DEV_SR        [SCSI cdrom (sr) driver]                            
CONFIG_BLK_DEV_SR_VENDOR [allow vendor specific cdrom commands]              
CONFIG_SR_EXTRA_DEVS     [extra slots for cdroms added later]                
CONFIG_CHR_DEV_ST        [tape (st) driver]                                  
CONFIG_CHR_DEV_OSST      [OnSteam tape (osst) driver]                        
CONFIG_CHR_DEV_SG        [SCSI generic (sg) driver]                          
CONFIG_DEBUG_QUEUES      [for debugging multiple queues]                     
CONFIG_SCSI_MULTI_LUN    [allow probes above lun 0]                          
CONFIG_SCSI_CONSTANTS    [symbolic decode of SCSI errors]                    
CONFIG_SCSI_LOGGING      [allow logging to be runtime selected]              
                                                                             
CONFIG_SCSI_<ll_driver>  [numerous lower level adapter drivers]              
CONFIG_SCSI_DEBUG        [lower level driver for debugging]                  
                                                                             
CONFIG_SCSI_PPA          [older parallel port zip drives]                    
CONFIG_SCSI_IMM          [newer parallel port zip drives]                    
                                                                             
CONFIG_BLK_DEV_IDESCSI   [ide-scsi pseudo adapter]                           
CONFIG_I2O_SCSI          [scsi command set over i2o bus]                     
CONFIG_SCSI_PCMCIA       [for SCSI HBAs on PCMCIA bus]                       
CONFIG_USB_STORAGE       [usb "mass storage" type]                           
                                                                             
CONFIG_MAGIC_SYSRQ       [Alt+SysRq+S for emergency sync]                    
                         [Alt+SyrRq+U for emergency remount ro]              

 If the root file system is on a SCSI disk then it makes sense to build into
the kernel the SCSI mid level, the sd driver and the host adapter driver that
the disk is connected to. In most cases it is usually safe to build the sr,
st and sg drivers as modules so that they are loaded as required. If a device
like a scanner is on a separate adapter then its driver may well be built as
a module. In this case, that adapter driver will need to be loaded before the
scanner will be recognized.

 Linux distributions have many of the SCSI subsystem drivers built as modules
since building all of them in would lead to a very large kernel that would
exceed the capabilities of the boot loader. This leads to a "chicken and the
egg" problem in which the SCSI drivers are needed to load the root file
system and vice versa. The 2 phase load used by the initrd device addresses
this problem (see Chapter 6 for more details).
-----------------------------------------------------------------------------

Chapter 5. Boot Parameters

 On a PC the motherboard's BIOS together with the SCSI BIOS provided by most
SCSI host adapters takes care of the problem of loading the boot loader's
image from a SCSI disk into memory and executing it. This may require some
settings to be changed in the motherboard's BIOS. When more than one SCSI
adapter is involved, the SCSI BIOS settings may need to change to indicate
which one contains the disk with the boot image. The boot image make also
come from an ATA (IDE) disk, a bootable CD-ROM or a floppy.

 Both lilo and grub are commonly used boot loaders with Linux. Their
configuration files are in /etc/lilo.conf and /etc/grub.conf [3]
respectively. One difference is that after changing lilo's configuration the 
lilo command must be executed for the changes to take effect (and there is no
equivalent requirement for grub). See their "man" pages for usage
information. An excellent paper on lilo and the Linux bootup sequence can be
found  ftp://icaftp.epfl.ch/pub/people/almesber/booting/bootinglinux-0.ps.gz.
For further information on grub see  www.gnu.org/software/grub.

 Some boot parameters related to the SCSI subsystem:
 single         [enter single user mode]                                     
 <n>            [enter run level <n> {0..6}]                                 
 root=/dev/sda6 [*]                                                          
 root=/dev/scsi/host0/bus0/target0/lun0/part6 [*]                            
 root=/dev/sd/c0b0t0u0p6   [*]                                               
 devfs=mount    [overrides CONFIG_DEVFS_MOUNT=n]                             
 devfs=nomount  [overrides CONFIG_DEVFS_MOUNT=y]                             
 init=<command> [executes <command> rather than init]                        
 quiet          [reduce output to console during boot]                       
 debug          [increase output to console during boot]                     
 nmi_watchdog=0 [turn off NMI watchdog on a SMP machine]                     
 max_scsi_luns=1  [limits SCSI bus scans to lun==0]                          
 scsi_allow_ghost_devices=<n>                                                
* When devfs is in use the initial read-only mount of the root partition can
be done via the old /dev/sd<a><n> notation or the new devfs notation (and two
of these are shown). The joint "root=/dev/sda6 single" may be useful when
disk or adapter changes have broken the kernel boot load.

 The "root=" argument may also be a hex number. For example, if the root
partition is on /dev/sda3 then "root=803" is appropriate. The last two digits
are the minor device number discussed in an earlier section.

 The default argument to the "init" parameter is /sbin/init (see man (8)
init). If files such as /etc/fstab have incorrect entries, it may be useful
to drop directly into a shell with "init=/bin/bash". However if shared
libraries files or their paths are inappropriate this may also fail. That
leaves "init=/sbin/sash" which is a statically linked shell with many useful
commands (for repairing a system) built in (see man (8) sash).

 When Linux fails to boot after reporting a message like:
  VFS: Cannot open root device 08:02                                         
then the kernel expected to find a root partition on device /dev/sda2 and did
not. The numbers in the error message are major and minor device numbers (in
hex) [see Section 3.2 for the mapping to device names]. In such situations
the "root" boot option can be useful (also the rdev command can be used to
modify where the boot image looks for the root partition).

 Lilo's configuration file /etc/lilo.conf can take the "root=" option in two
ways. The normal way is a line like: 'root=/dev/sda2'. In this case /dev/sda2
is converted into major and minor numbers based on the state of the system 
when the lilo command is executed. This can be a nuisance, especially if
hardware is going to be re-arranged. The other way is a line of the form:
'append="root=/dev/sda2"' In this case the /dev/sda2 is passed through to the
kernel the next time it is started. This is the same as giving the "root=/dev
/sda2" string at the kernel boot time prompt. It is interpreted by the kernel
at startup (once the HBAs and their attached devices have been recognized)
and thus is more flexible.
-----------------------------------------------------------------------------

Chapter 6. Modules and their Parameters

 There are many SCSI related modules. The mid and upper level modules are
listed below:

��*�scsi_mod.o
   
��*�sd_mod.o
   
��*�sr_mod.o
   
��*�st.o [osst.o]
   
��*�sg.o
   

 Notice that the first 3 have "_mod" appended to their normal driver names.
Lower level drivers tend to use the name (or an abbreviation) of the HBA's
manufacturer (e.g. advansys) plus optionally the chip number of the major
controller chip (e.g. sym53c8xx for symbios controllers based on the NCR
53c8?? family of chips).

 All SCSI modules depend on the mid level. This means if the SCSI mid level
is not built into the kernel and if scsi_mod.o has not already been loaded
then a command like modprobe st will cause the scsi_mod.o module to be
loaded. There could well be other dependencies, for example modprobe sr_mod 
will also cause the cdrom module to be loaded if it hasn't been already. Also
if the SCSI mid level is a module, then all other SCSI subsystem drivers must
be modules (this is enforced by the kernel build configuration tools).

 Modules can be loaded with the modprobe <module_name> command which will try
to load any modules that the nominated <module_name> depends on. Also <
module_name> does not need the trailing ".o" extension which is assumed if
not given. The insmod <module_name> command will also try and load <
module_name> but without first loading modules it depends on. Rules for how
modules can cause other modules to be loaded (with appropriate parameters
appended) are usually placed in the file /etc/modules.conf. [Note that in
earlier Linux kernels this file was often called /etc/conf.modules.] For
further information about the format of this file try man modules.conf.

 Any module can have its allowable command line parameters queried with this
command: modinfo -p <module_name>.

 When upper level drivers are initialized and if there are no hosts active
then the mid level will attempt to load a module called "scsi_hostadapter".
An "alias" can then be used to associate "scsi_hostadapter" with the actual
name of the lower level (adapter) driver. For example, a line like "alias
scsi_hostadapter aic7xxx" in the /etc/modules.conf file would cause the
aic7xxx module to be loaded (if there were no lower level drivers already
active). [4]

 There is a special relationship between the module parameter
"scsi_hostadapter" and the initrd file system. For more information see man
initrd and man mkinitrd. [5]
-----------------------------------------------------------------------------

Chapter 7. Proc pseudo file system

 The proc pseudo file system provides some useful information about the SCSI
subsystem. The kernel configuration option that selects "proc_fs" is
CONFIG_PROC_FS and in almost all cases it should be selected. SCSI specific
information is found under the directory /proc/scsi. Probably the most
commonly accessed entry is cat /proc/scsi/scsi which lists the attached SCSI
devices. See Section 8.3 for more details.

 The lower level drivers are allocated proc_fs entries of the form:
    /proc/scsi/<driver_name>/<scsi_adapter_number>                           
where the <driver_name> is something like "aic7xxx" or "BusLogic". The <
scsi_adapter_number> (also known as the host number) is the same number that
was discussed in Section 3.1. Note that one driver may control one or more
hosts. What is stored in this file is lower level driver dependent (and in
the case of some adapter drivers it is possible to set parameters via this
file). When reporting problems to newsgroups or maintainers it is useful to
include the output of this file (e.g. cat /proc/scsi/aic7xxx/0 ).

 The cdrom driver provides information about attached cdrom devices in the /
proc/sys/dev/cdrom directory. This will include both SCSI devices (i.e. those
controlled by the sr driver) and IDE devices (i.e. those controlled by the
ide-cd driver). See Section 9.2.3.

 The sg driver provides information about its state and attached hosts and
devices in the /proc/scsi/sg directory. See Section 9.4.3.

 More general information on the proc pseudo file system can be found in the
kernel source file: /usr/src/linux/Documentation/filesystems/proc.txt.
-----------------------------------------------------------------------------

Chapter 8. Mid Level, Unifying layer

 The SCSI mid level is common to all usage of the SCSI subsystem. Probably
its most important role is to define internal interfaces and services that
are used by all other SCSI drivers. These internal mechanisms are not
discussed in this document [see ref: W2].

 The primary kernel configuration parameter "CONFIG_SCSI" determines whether
the mid level is built in (when "=y") or a module (when "=m"). If
"CONFIG_SCSI=m" then all other SCSI subsystem drivers must also be modules.

 When the mid level is built as a module then it probably never needs to be
loaded explicitly because using 'modprobe' to load any other SCSI subsystem
module will cause the mid level to be loaded first (if it is not already).

 Some upper and lower level drivers do not (fully) load if there are no
devices for that driver to control. Sometimes the report is loud as in this
case for the imm driver which controls zip drives connected to a parallel
port:
$ modprobe imm                                                               
   imm.o: init_module: No such device                                        
lsmod will not show the "imm" module as loaded. In other cases the result is
more subtle. For example, if the sg driver is loaded in a system with no
(real or pseudo) scsi devices then the /proc/scsi/sg directory will not
appear. [It will be created when the first scsi device is recognized.]
-----------------------------------------------------------------------------

8.1. boot parameters

 SCSI drivers that are built into the kernel are checked in a pre-determined
order to see if HBAs that they can control are present. The user has no
control over this order which in most cases is arbitrary but in the case of
some older ISA adapters is required to stop misidentification [6] .
scsi_logging=<n>                                                             
    where <n> is 0 to turn logging off                                       
    where <n> is non-zero to turn logging on                                 
                                                                             
max_scsi_luns=<n>                                                            
    where <n> is a number between 1 and 8 (< lk 2.4.7),                      
        >= lk 2.4.7 the upper limit can be much larger                       
                                                                             
scsi_allow_ghost_devices=<n>                                                 
    where (<n> - 1) is the maximum lu number to ghost if the                 
        the corresponding device is offline. When <n>==0                     
        (default) then don't ghost any devices (in lk 2.4.26                 
        and later)                                                           
                                                                             
scsihosts=host0:hosts1::host3                                                

 The recently introduced devfs defines a "scsihosts" boot time parameter to
give the user some control over this. See the devfs documentation [ref: W5]
for a description. The host names given in the list to the "scsihosts" boot
option are the names of lower level drivers (e.g. "scsihosts=advansys:imm::
ide-scsi"). [7] [8] Devfs does not need to be present for "scsihosts" to be
used. The "scsihosts" parameter, if given, is echoed during in the boot up
messages. For example:
scsi: host order: advansys:imm::ide-scsi                                     
Also if multiple HBA are present in a system then they are scanned in a fixed
order (see footnote). The "scsihosts" parameter only effects how these HBAs
are indexed (i.e. which SCSI adapter numbers are associated with them by the
kernel). In the above example, if the "imm" driver is not found during boot
up, then the scsi adapter number "1" is not allocated. If the "imm" driver is
later loaded as a module, then it will adopt scsi adapter number "1". If a
driver that is not named in "scsihosts" is found, then it will get the next
available scsi adapter number (e.g. a built in aic7xxx driver would get scsi
adapter number "2" in the above example).

 A full list of kernel parameters with some explanations can be found in the
file /usr/src/linux/Documentation/kernel-parameters.txt .
-----------------------------------------------------------------------------

8.2. module parameters

 If SCSI disks are present in the system then it usually is better to build
the mid level driver into the kernel. However if the SCSI subsystem is only
being used periodically (e.g. to burn CD-Rs on an ATAPI CD writer) then
building the mid level as a module is fine. The module load time options are
the same as the driver's built in options:
scsi_logging_level=<n>                                                       
    where <n> is the logging level mask (0 for logging off)                  
max_scsi_luns=<n>                                                            
scsihosts=host0::host2                                                       
scsi_allow_ghost_devices=<n>                                                 
-----------------------------------------------------------------------------

8.3. proc interface

 To display the SCSI devices currently attached (and recognized) by the SCSI
subsystem use cat /proc/scsi/scsi.

 The output looks like this:
    Attached devices:                                                        
    Host: scsi0 Channel: 00 Id: 02 Lun: 00                                   
      Vendor: PIONEER  Model: DVD-ROM DVD-303  Rev: 1.10                     
      Type:   CD-ROM              ANSI SCSI revision: 02                     
    Host: scsi1 Channel: 00 Id: 00 Lun: 00                                   
      Vendor: IBM      Model: DNES-309170W     Rev: SA30                     
      Type:   Direct-Access       ANSI SCSI revision: 03                     

 After the "Attached devices:" line there are 3 lines for each recognized
device. The first of these lines is SCSI address information discussed in 
Section 3.1. The following 2 lines of data are obtained from a INQUIRY
command that was performed on the device when it was attached. See Section
9.4 for the relationship between the ordering of these devices compared with
the sg driver's ordering (which most of the time is the same).

 Existing devices can be removed using   echo "scsi remove-single-device <h>
<b> <t> <l>" > /proc/scsi/scsi where the variables are host, bus (channel),
target (scsi id) and lun. The success (or otherwise) of this command can be
determined by sending a subsequent cat /proc/scsi/scsi command. The removal
will fail if the device is busy (e.g. if a file system on the device is
mounted).

 New devices can be added using   echo "scsi add-single-device <h> <b> <t> <l
>" > /proc/scsi/scsi where the variables are host, bus (channel), target
(scsi id) and lun. The success (or otherwise) of this command can be
determined by sending a subsequent cat /proc/scsi/scsi command. [9]

 The SCSI subsystem does not support hot-plugging of SCSI devices (there may
also be electrical issues on the associated SCSI parallel bus). It is
recommended that those who use add+remove-single-device make sure that other
devices on that SCSI bus are inactive if re-plugging is going to take place.

 To output a list of internal SCSI command blocks use   echo "scsi dump <n>"
> /proc/scsi/scsi where the numeric value of <n> doesn't matter. This is
probably only of interest to people chasing down bugs within the SCSI
subsystem.

 To start (or stop) logging information being sent to the console/log use  
echo "scsi log <token> <n>" > /proc/scsi/scsi where <token> is one of: {all,
none, error, timeout, scan, mlqueue, mlcomplete, llqueue, llcomplete,
hlqueue, hlcomplete, ioctl} and <n> is a number between 0 and 7. The tokens
"all" and "none" don't take an <n> argument. Prefix meanings:
    hl    upper level drivers [exception: sg uses "timeout"]                 
    ml    mid level                                                          
    ll    lower level drivers                                                
           [adapter drivers often have there own flags]                      
The value "0" turns off logging while "7" maximizes the volume of output.
Logging information will only be output if CONFIG_SCSI_LOGGING was selected
in the kernel build.

Warning Warning: "scsi log all" (and several other variants) can cause a     
        logging infinite loop if the log file (typically /var/log/messages ) 
        lies on a SCSI disk. Either turn off the kernel logging daemon or    
        direct its output to a non SCSI device.                              
-----------------------------------------------------------------------------

Chapter 9. Upper level drivers

 The upper level drivers maintain the kernel side of the OS interface for the
logical class of devices they represent (e.g. disks). They are also
responsible for managing certain kernel and SCSI subsystem resources such as
kernel memory and SCSI command structures. Applications in the user space
access these drivers by opening a special file (block or char) typically
found in the /dev directory tree.
-----------------------------------------------------------------------------

9.1. Disk driver (sd)

 Two types of SCSI devices are accessible via the sd driver:

��*�"direct access" devices which are usually magnetic disks. [SCSI
    peripheral device code is 0]
   
��*�"Optical memory devices" which are often called MOD disks. [SCSI
    peripheral device code is 7]
   

The sd driver is a block device which means that it is closely associated
with the block subsystem. It also supports the concept of partitions. [man sd
dates from 1992.]

 The sd driver is capable of recognizing 128 disks when it is loaded at
kernel boot time or later as a module. However, once it is loaded, it will
only recognize a fixed number of additional disks. The number of additional
disks that can be accommodated is set by the kernel configuration parameter
CONFIG_SD_EXTRA_DEVS whose default value is 40.
-----------------------------------------------------------------------------

9.1.1. sd boot parameters

 None.
-----------------------------------------------------------------------------

9.1.2. sd module parameters

 The sd driver takes no parameters when loaded as a module. Note that its
module name is sd_mod.o.
-----------------------------------------------------------------------------

9.2. CDROM driver (sr or scd)

 CDROM and DVD drives (and WORM devices) are accessible via the sr upper
level device driver. While "sr" is the device driver name, "sr_mod" is its
module name. The device file name is either /dev/sr<n> or /dev/scd<n>.

 Following is a diagram illustrating the CDROM subsystem of which sr is a
part:

[cdrom]

  The architecture of the CD-ROM subsystem.

 This diagram glosses over some of the differences between the protocol
stacks. CDROM device names are not maintained by the uniform CDROM layer but
rather by each individual protocol stack. In the case of the SCSI subsystem,
device names are maintained by the sr driver while the IDE subsystem
maintains device names with its central "ide" driver (i.e. not by the ide-cd
driver). USB and IEEE1394 cd devices names are maintained by their respective
stacks. This may partially explain why the /dev/cdrom is often a symbolic
link to the appropriate subsystem's device name.

 Two types of SCSI devices are accessible via the sr driver:

��*�CD-ROM devices (including DVD players) [SCSI peripheral device code is 5]
   
��*�"Write-once read-multiple" devices which are known as WORMs. [SCSI
    peripheral device code is 4]
   

 The sr driver is capable of recognizing 256 CDROM/DVD drives when it is
loaded at kernel boot time or later as a module. However, once it is loaded,
it will only recognize a fixed number of additional drives. The number of
additional drives that can be accommodated is set by the kernel configuration
parameter CONFIG_SR_EXTRA_DEVS whose default value is 2.

 People often use the dd command to read a data CDROM containing an iso9660
file system. If a count argument is not given then the dd command will read
the number of 2048 byte sectors indicated by the SCSI Read Capacity command.
Unfortunately this can include unwritten (or "run out") sectors at the end of
the image that can cause I/O errors. Use the isosize command (see its man
page) to find the true length of the iso9660 image and use that in the "count
=" argument given to the dd command.
-----------------------------------------------------------------------------

9.2.1. sr boot parameters

 None.
-----------------------------------------------------------------------------

9.2.2. sr module parameters

 Doing a test to find out if a cdrom drive supports XA mode (mode 2) triggers
firmware bugs on some drives. Consequently the check for XA mode support is
turned off by default. The following module parameter is provided:
    xa_test=<0|1>                                                            
to override the default. [Currently there seems to be no way to turn on XA
mode testing when the sr driver is built into the kernel.]
-----------------------------------------------------------------------------

9.2.3. sr proc interface

 All the following files are readable by all and produce ASCII output when
read:
 /proc/sys/dev/cdrom/autoclose                                               
 /proc/sys/dev/cdrom/autoeject                                               
 /proc/sys/dev/cdrom/check_media                                             
 /proc/sys/dev/cdrom/debug                                                   
 /proc/sys/dev/cdrom/info                                                    
 /proc/sys/dev/cdrom/lock                                                    
They reflect the current state of the CDROM subsystem. This location is part
of the procfs's window through to the sysctl configuration mechanism (see man
sysctl). All but info are writable by the superuser. There is a column for
each CDROM and DVD player in the system in info (not just SCSI devices).

 As an example, the auto eject feature can be turned on by the superuser with
the command echo "1" > /proc/sys/dev/cdrom/autoeject. This will cause cdroms
to be ejected from the drive when unmounted.
-----------------------------------------------------------------------------

9.2.4. ATAPI cdroms

 Many Linux users have no SCSI devices (or adapters) in their systems. They
become a little perplexed as to why cd writer software (e.g. cdrecord and 
cdrdao) and cd music reading programs (e.g. cdparanoia) use the Linux SCSI
subsystem. The answer is that these programs need lower level access to these
devices. ATAPI (ATA Packet Interface) is essentially a SCSI command set sent
over an ATA [10] transport. [The discussion in this section is also
applicable to ATAPI tape drives and ATAPI floppy drives.]

 Currently both cdrecord and cdparanoia interface to the SCSI generic driver
(sg) and, in the case of ATAPI cd devices, use the ide-scsi pseudo device
driver to access the hardware. This may change in the future as in the 2.4
series kernels a packet interface ioctl has been added to the uniform cdrom
layer (see the diagram in Section 9.2 above). [11]

 The default action of the IDE subsystem in Linux is to claim all ATA devices
for its built-in drivers. In the case of an ATAPI cd writer, it will be
claimed by the built-in ide-cd driver. Once this has happened, the SCSI
subsystem is unable to get control over an ATAPI device. The ide-scsi (pseudo
lower level SCSI) driver can only register ATAPI devices in the SCSI
subsystem that have not already been claimed by the IDE subsystem.

 Notice the built-in qualification in the previous paragraph. If both the
ide-cd and ide-scsi drivers are modules then the first one loaded will claim
the ATAPI cd devices (e.g. cd/dvd readers and writers). Furthermore you can
switch the controlling driver module by rmmod-ing one and modprobe-ing the
other.

 Probably the most flexible way to instruct the IDE core driver that you want
the cd writer at /dev/hdd accessible to cdrecord is to use the kernel boot
option: "hdd=ide-scsi". This will cause the ide-cd driver to bypass /dev/hdd
(irrespective of whether ide-cd driver is built-in or a module). As long as
the ide-scsi driver is built-in or a module then it will "capture" the cd
writer at /dev/hdd (with the IDE core driver loading the ide-scsi module if
required).

 The ide-cd driver module can be instructed to ignore certain ATA devices
with the following syntax:
modprobe ide-cd ignore='hdc hdd'                                             
In this case the ide-cd driver will ignore the devices at /dev/hdc and /dev/
hdd . This effect can also be accomplished by placing a line like this:
"options ide-cd ignore=hdd" in the /etc/modules.conf file.

 A new option added in the lk 2.4 series is of the form "hdd=scsi". This
option seems to have a similar function to the "hdd=ide-scsi" option
discussed above. Furthermore "hdd=scsi" can only be used if both the SCSI
mid-level and the ide-scsi drivers are built into the kernel (otherwise "BAD
OPTION" is reported by the ide_setup function).

 To find out whether an ATAPI cd device is "owned" by the SCSI subsystem, the
output of cat /proc/scsi/scsi can be checked. Another technique is to observe
the "drive name:" line of cat /proc/sys/dev/cdrom/info for "sr" entries. The
following output is from my system:
$ cat /proc/sys/dev/cdrom/info                                               
CD-ROM information, Id: cdrom.c 3.12 2000/10/18                              
                                                                             
drive name:             sr1     sr0                                          
drive speed:            16      0                                            
drive # of slots:       1       1                                            
Can close tray:         1       1                                            
Can open tray:          1       1                                            
Can lock tray:          1       1                                            
Can change speed:       1       1                                            
Can select disk:        0       0                                            
Can read multisession:  1       1                                            
Can read MCN:           1       1                                            
Reports media changed:  1       1                                            
Can play audio:         1       1                                            
Can write CD-R:         1       0                                            
Can write CD-RW:        1       0                                            
Can read DVD:           0       1                                            
Can write DVD-R:        0       0                                            
Can write DVD-RAM:      0       0                                            

 Once an ATAPI cd writer at /dev/hdd has been registered by the SCSI
subsystem, then cdroms should be mounted via the "scd" device name and cd
players should also use the "scd" device. Strangely the hdparm command should
still use the  /dev/hdd device file (or the "echo ... > /proc/ide/hdd/
settings" method described in this section). [12]
-----------------------------------------------------------------------------

9.3. Tape driver (st)

 The tape driver interface is documented in the file /usr/src/linux/drivers/
scsi/README.st and on the st(4) man page (type man st). The file README.st
also documents the different parameters and options of the driver together
with the basic mechanisms used in the driver.

 The tape driver is usually accessed via the mt command (see man mt). mtx is
an associated program for controlling tape autoloaders (see  
mtx.sourceforge.net).

 The st driver detects those SCSI devices whose peripheral device type is
"Sequential-access" (code number 1) unless they appear on the driver's
"reject_list". [Currently the OnStream tape drives (described in a following
section) are the only entry in this reject_list.]

 The st driver is capable of recognizing 32 tape drives. There are 8 device
file names for each tape drive: a rewind and non-rewind variant for each of 4
modes (numbered 0 to 3). See the tape device file name examples in Section
3.2 on device names. Any number of tape drives (up to the overall limit of
32) can be added after the st driver is loaded.

 ATAPI tape drives can be controlled by this driver with help from the
ide-scsi pseudo adapter driver. The discussion in Section 9.2.4 also applies
for ATAPI tape drives (and ATAPI floppies).
-----------------------------------------------------------------------------

9.3.1. st boot parameters

 
st=xxx[,yyy] where xxx is one of the following:                              
buffer_kbs:<n>                                                               
write_threshold_kbs:<n>                                                      
max_buffers:<n>                                                              
max_sg_segs:<n>                                                              
                                                                             
(The old boot parameters st=aa[,bb[,cc[,dd]]] supported but deprecated)      

 The default driver buffer size (buffer_kbs) is 32 (i.e. 32 KB). The default
asynchronous write threshold (write_threshold_kbs) is 30 (i.e. 30 KB). The
default number of buffers allocated at initialization (max_buffers) is 4. The
default number of scatter/gather segments to use (max_sg_segs) is 32.
-----------------------------------------------------------------------------

9.3.2. st module parameters

 
buffer_kbs=<n>                                                               
write_threshold_kbs=<n>                                                      
max_buffers=<n>                                                              
max_sg_segs=<n>                                                              
-----------------------------------------------------------------------------

9.3.3. st proc interface

 None.
-----------------------------------------------------------------------------

9.3.4. osst driver for OnStream devices

 There is an auxiliary tape driver for tape drives manufactured by OnStream.
It is an additional upper level driver and can co-exist with the st driver.
Its driver name is "osst" (as is its module name).

 The OnStream SC-x0 SCSI tape drives can not be driven by the standard st
driver, but instead need this special osst driver and use the /dev/osst<x>
char device nodes (major 206). [Where <x> follows the same naming scheme as
st devices outlined in Section 3.2.] Via usb-storage and ide-scsi, you may be
able to drive the USB-x0 and DI-x0 drives as well. Note that there is also a
second generation of OnStream tape drives (ADR-x0) that supports the standard
SCSI-2 commands for tapes (QIC-157) and can be driven by the standard driver
st. For more information, you may have a look at the kernel source file /usr/
src/linux/drivers/scsi/README.osst. More info on the OnStream driver may be
found on  linux1.onstream.nl/test/.
-----------------------------------------------------------------------------

9.4. Generic driver (sg)

 All types of SCSI devices are accessible via the sg driver. This means
devices such as CDROM drives can be accessed both via the sr and sg drivers.
Other SCSI devices such as scanners can only be accessed via the sg driver.
The sg driver is capable of recognizing 256 SCSI devices. Any number of
devices (up to the overall limit of 256) can be added after the sg driver is
loaded.

 See reference W4 for the SCSI Generic (sg) driver documentation (also found
there is the sg_utils package). For SCSI standards see reference W1 and for a
book on the subject of SCSI programming and pass through mechanisms see
reference B3.

 The sg driver in lk 2.4 is "version 3" which adds an additional interface
structure and some new ioctl()s. The most interesting new ioctl() is SG_IO
which sends a SCSI command and waits for its response. See the Linux
Documentation Project site:  www.tldp.org/HOWTO/SCSI-Generic-HOWTO/ for a
full description of the sg driver. A (possibly later) version of this
document can be found at  www.torque.net/sg/p/sg_v3_ho.html.

 The abbreviation "sg" is used within the kernel to refer both to the SCSI
generic driver and the scatter-gather capability offered by many modern IO
devices (usually associated with DMA). The context usually makes it clear
which one is being referred to. As an example, note the contorted sg ioctl()
named SG_GET_SG_TABLESIZE where the second "SG" refers to scatter gather.

 The public interface for sg is found in the file:  /usr/src/linux/include/
scsi/sg.h. Depending on the distribution this may or may not contain the same
information as  /usr/include/scsi/sg.h which is controlled by the GNU library
maintainers. If these 2 files are not the same use the former header file.
Those writing applications based on sg should see its documentation for more
on this matter.

 The sg driver registers all SCSI devices (with a current maximum of 256) as
they are seen. Each newly registered SCSI device gets allocated the next
available minor device number. At least initially this will be the same
sequence that devices are displayed in mid level's  cat /proc/scsi/scsi. The
sg devices device mapping can be seen with cat /proc/scsi/sg/devices or  cat
/proc/scsi/sg/device_strs. Differences between cat /proc/scsi/scsi and sg
orderings will appear when a low level driver is removed (e.g. rmmod aha1542)
or when a device is removed with remove-single-device. The sg driver will
leave remaining SCSI device mapping to minor device numbers unchanged. This
potentially leaves a "hole" in the sg mapping. An example follows:
$ cat /proc/scsi/scsi                                                        
Attached devices:                                                            
Host: scsi0 Channel: 00 Id: 00 Lun: 00                                       
  Vendor: IBM      Model: DNES-309170W     Rev: SA30                         
  Type:   Direct-Access            ANSI SCSI revision: 03                    
Host: scsi1 Channel: 00 Id: 02 Lun: 00                                       
  Vendor: PIONEER  Model: DVD-ROM DVD-303  Rev: 1.10                         
  Type:   CD-ROM                   ANSI SCSI revision: 02                    
Host: scsi1 Channel: 00 Id: 06 Lun: 00                                       
  Vendor: YAMAHA   Model: CRW4416S         Rev: 1.0g                         
  Type:   CD-ROM                   ANSI SCSI revision: 02                    
                                                                             
$ cat /proc/scsi/sg/device_strs                                              
IBM             DNES-309170W            SA30                                 
PIONEER         DVD-ROM DVD-303         1.10                                 
YAMAHA          CRW4416S                1.0g                                 
                                                                             
$ echo "scsi remove-single-device 1 0 2 0" > /proc/scsi/scsi                 
                                                                             
$ cat /proc/scsi/scsi                                                        
Attached devices:                                                            
Host: scsi0 Channel: 00 Id: 00 Lun: 00                                       
  Vendor: IBM      Model: DNES-309170W     Rev: SA30                         
  Type:   Direct-Access            ANSI SCSI revision: 03                    
Host: scsi1 Channel: 00 Id: 06 Lun: 00                                       
  Vendor: YAMAHA   Model: CRW4416S         Rev: 1.0g                         
  Type:   CD-ROM                   ANSI SCSI revision: 02                    
                                                                             
$ cat /proc/scsi/sg/device_strs                                              
IBM             DNES-309170W            SA30                                 
<no active device>                                                           
YAMAHA          CRW4416S                1.0g                                 
Notice how the sg driver maintains the row positions of the remaining devices
in the "device_strs" output. So when the Pioneer dvd player is removed, a
hole opens up in the sg device mapping which is not reflected in the cat /
proc/scsi/scsi output. That "hole" corresponds to the device name  /dev/sg1.

 The new sg_io_hdr interface includes a data transfer residual count field
called "resid". Only some lower level adapters support this feature and those
that don't always yield zero in this field. At the time of writing the
advansys, aha152x and the sym53c8xx drivers support this feature.
-----------------------------------------------------------------------------

9.4.1. sg boot parameters

 The sg driver maintains a reserved buffer for each open file descriptor. The
purpose is to guarantee applications that data transfers up to the size of
the reserved buffer will not fail for lack of kernel memory. This is
important for applications like cdrecord that cannot easily recover (the CDR)
from a ENOMEM error.

 In the absence of the boot parameter 'sg_def_reserved_size' or the sg module
parameter 'def_reserved_size', then each time a sg file descriptor is opened
the reserved buffer size is inherited from SG_DEF_RESERVED_SIZE which is
defined in include/linux/sg.h.

 The SG_DEF_RESERVED_SIZE define value can be overridden by this kernel boot
option:
    sg_def_reserved_size=<n>                                                 
-----------------------------------------------------------------------------

9.4.2. sg module parameters

 When the sg module is loaded the SG_DEF_RESERVED_SIZE define value can be
overridden by supplying this option:
    def_reserved_size=<n>                                                    
-----------------------------------------------------------------------------

9.4.3. sg proc interface

 All the following files are readable by all and produce ASCII output when
read. The file 'def_reserved_size' is also writable by root. The ASCII output
has been formatted in such a way as to be human and machine readable (and
hence a compromise). Use Unix commands of the form cat device_hdrs devices to
see the output of tables.
/proc/scsi/sg/debug       [internal state of sg driver]                      
/proc/scsi/sg/def_reserved_size                                              
                          [like boot/module load parameter]                  
/proc/scsi/sg/devices     [table of numeric device data]                     
/proc/scsi/sg/device_hdr  [column headers for sg/devices]                    
/proc/scsi/sg/device_strs [table of strings from INQUIRY]                    
/proc/scsi/sg/hosts       [table of numeric host data]                       
/proc/scsi/sg/host_hdr    [column headers for sg/hosts]                      
/proc/scsi/sg/host_strs   [table of string ids for hosts]                    
/proc/scsi/sg/version     [sg version number and date]                       
All the above files are owned by root and readable by all while
def_reserved_size is writable by root. For the devices and device_strs files
the first row output corresponds to /dev/sg0 (sg minor device number 0). The
second row output corresponds to /dev/sg1, etc. For the hosts and host_strs
files the first row output corresponds to host (adapter number) 0, etc. For
numeric tables a missing device or host is indicated by a row of "-1" values.
For string tables a missing device or host is indicated by a row containing "
<no active device/host>".
-----------------------------------------------------------------------------

Chapter 10. Lower Level drivers

 There are too many SCSI low level drivers to detail in this document. As an
alternative to giving any superficial overview here, the reader is given
suggestions of places to look for further information.

 The source directory for the SCSI subsystem in the Linux kernel is a good
place to start: /usr/src/linux/drivers/scsi. Several drivers have information
in a "readme" file: README.<driver_name> . Others have extensive information
at the top of their ".c" file This information often includes a version
number, change logs and kernel boot time and module load time options. Often
the latter information can be found in the installation guides of the various
Linux distributions. Sometimes the driver maintainer will have a web site
containing the most recent bug fix information. Official maintainers are
listed in the /usr/src/linux/MAINTAINERS file. If there is nothing there,
look in the relevant ".c" file in the SCSI subsystem directory. Some old
drivers have no active maintainers. In such cases posting to the linux-scsi
newsgroup may help [see N1 ].

 For an overview of the drivers supplied with the kernel source tree, use one
of the kernel configuration programs (e.g. cd /usr/src/linux; make menuconfig
). The help information associated with each selection can be found together
in one (large) flat file at /usr/src/linux/Documentation/Configure.help.
Drivers can be obtained from other places. It is unlikely that a SCSI driver
made for the lk 2.2 series (or before) will build or operate successfully in
the lk 2.4 series. [From a programmatic viewpoint there are not a lot of
things that need changing.] Drivers may even be only available in binary
form, in which case make sure that you trust the provider and follow their
instructions closely.

 Lower level drivers can support either of 2 error handling strategies. The
older one is considered obsolete while the newer one is often called
"new_eh". The advantage of "new_eh" is that it uses a separate kernel thread
per host (named "scsi_eh_<n>" where <n> is the host number) to facilitate
error recovery. Both error handling strategies were also available in the lk
2.2 series in which very few adapter drivers used "new_eh". In the lk 2.4
series, more drivers are using it and the plan for the forthcoming lk 2.5
development series is to drop mid level support for the older, obsolete error
strategy.

 Drew Eckhardt's SCSI-HOWTO document [see reference W7 ] goes into much more
detail about lower level (adapter) drivers than this document. Since that
SCSI-HOWTO is 5 years old, many things have changed and more drivers have
been added.

 There is a lower level driver called scsi_debug that simulates one or more
"direct access" devices (i.e. disk(s)) using the computer's memory. From lk
2.4.17 it acts as a "ram disk". While there are many ram disk implementations
available in Linux (e.g. ramfs), scsi_debug may help to isolate a defective
scsi driver in a problematic installation. See scsi_debug.c for further
information.
-----------------------------------------------------------------------------

10.1. Pseudo drivers

 SCSI can be viewed as a command set and a set of hardware buses that convey
that command set. Those hardware buses can be further divided into those used
exclusively for SCSI (e.g. ultra wide), those shared with other protocols
(e.g. USB, IEEE 1394) and those buses not defined by the various SCSI
standards. In the final category there are several interesting examples
including ATAPI CD writers and PC parallel bus ZIP drives. Such devices use
the SCSI command set (or something very close to it) over a foreign bus.

 This section briefly outlines various pseudo lower level drivers which
essentially communicate with other Linux subsystems in order to send the SCSI
command set to devices controlled by those other subsystems. This raises some
ownership issues that often confuse users and result in many questions to the
maintainers.

IDE-SCSI. From configuration point of view, ide-scsi will grab and try to
control every ATA (a.k.a. IDE) device which doesn't have a "native" driver
attached (such as ide-cd, ide-tape, etc). So for example, if both ide-cd and
ide-scsi are compiled into the kernel in a system which has an ATAPI cdrom,
ide-cd will get to control it. If only ide-scsi is compiled in, it will get
the device. There are some kernel boot time parameters to control which
driver gets which device.

 The preferences of the IDE subsystem can be overridden with one of these
kernel boot time parameters (of which the first is most interesting for this
subsystem):

��*� hdx=ide-scsi
   
��*� hdx=ide-cdrom
   
��*� hdx=ide-floppy
   

[The term hdx is used to refer to one of the IDE/ATA devices in {hda, hdb,
hdc ...}.] In the 2.4 series "hdx=scsi" was added but it is not very useful,
see see Section 9.2.4.

 When the driver is running, the device will be accessible using the SCSI
device (/dev/sda, /dev/sr0 , etc), and not through the corresponding  /dev/
hdx device. Still, the /dev/hdx device will be available, but only for
configuration.

 All the generic IDE configuration parameters (DMA on/off, 32-bit I/O,
unmasking irq's, etc) are available by using the  /dev/hdx device, for
example to enable DMA:
        hdparm -d1 /dev/hdx                                                  
[13] Using cat /proc/ide/hdx/settings will show the available settings. All
the generic IDE driver settings will be available there, as well as the
following "ide-scsi specific" settings:

��*�bios_cyl
   
��*�bios_head
   
��*�bios_sect
   
��*�transform
   
��*�log
   

The first three choose the virtual geometry that the drive will return to the
sd driver, in case it's a disk drive (ZIP, etc). "transform" will configure/
enable/disable the SCSI to ATAPI CDB transformation layer:

��*�bit 0: Enable(1)/Disable(0) transformation for commands not originated
    from the sg driver.
   
��*�bit 1: Enable/Disable transformation for commands issued using the sg
    driver.
   

"log" will log debugging information. This is useful also to debug user-space
programs using the sg driver, as it will list the CDB traffic on the bus --
each issued command, along with its completion status. To enable/disable a
specific settings, use something like:
        echo "log:1" > /proc/ide/hdx/settings                                
To turn off the "using_dma" flag use:
        echo "using_dma:0" > /proc/ide/hdx/settings                          

PPA + IMM. Iomega ZIP drives come in a variety of flavours including parallel
port, SCSI, and ATAPI. The parallel port versions (both old and new) are
driven by ppa and imm respectively.

 The parallel port ZIP drives are actually SCSI devices which tunnel SCSI
commands over the parallel port using interfaces called VPI0 (older-style)
and VPI2 (newer-style). The ppa driver is the VPI0 host implementation and
the imm driver is the VPI2 host implementation.

 The way it works is that the HBA is a chip inside the ZIP drive, so that the
host adapter and the peripheral are in the same actual case.

PPSCSI. The new, not-yet-integrated, architecture for devices that use SCSI
over a parallel port cable is ppscsi. The ppscsi module provides the boiler
plate code and makes it easy to write implementations for different
interfaces.

 Each ppscsi protocol module registers itself with the ppscsi module, passing
in a list of entry points for the various things that are common to all
protocol drivers.

[ppscsi]

  The structure of the PPSCSI drivers.

 The plan is that the ppscsi architecture will absorb both the ppa and imm
drivers and protocol modules; only vpi0 has been written so far. See  
www.torque.net/parport/ppscsi.html.

USB. USB classifies a group of devices as "mass storage" (e.g. disks) and
interacts with these using the SCSI command set. The module name is
"usb-storage". See  www.one-eyed-alien.net/~mdharm/linux-usb.

 There is also the usb/microtek driver for controlling X6 USB scanners from
Microtek. When configured, the SANE application uses the sg driver to send
SCSI commands over USB to control this scanner.

I2O. See kernel source file /usr/src/linux/drivers/i2o/io2_scsi.c .

IEEE 1394. Support for IEEE 1394 devices that use the SBP-2 protocol is now
available (lk 2.4.7). See the IEEE 1394 paragraph in this section for more
information.

iSCSI. An IETF draft is taking shape for iSCSI. This sends the SCSI command
set over a TCP network connection. iSCSI seems to be gaining popularity
quickly and there are several implementations for Linux taking shape. One
implementation is at  sourceforge.net/projects/intel-iscsi/. Use your
favourite search engine to find other projects.
-----------------------------------------------------------------------------

Chapter 11. Raw devices

 A raw device can be bound to an existing block device (e.g. a disk) and be
used to perform "raw" IO with that existing block device. Such "raw" IO
bypasses the caching that is normally associated with block devices. Hence a
raw device offers a more "direct" route to the physical device and allows an
application more control over the timing of IO to that physical device. This
makes raw devices suitable for complex applications like Database Management
Systems that typically do their own caching.

 Raw devices are character devices (major number 162). The first minor number
(i.e. 0) is reserved as a control interface and is usually found at /dev/
rawctl. A utility called raw (see man raw) can be used to bind a raw device
to an existing block device. These "existing block devices" may be disks or
cdroms/dvds whose underlying interface can be anything supported by Linux
(e.g. IDE/ATA or SCSI).

 A sequence of commands listing the raw devices and then binding a SCSI disk
partition followed by binding the whole disk looks like this on my system:
$ ls -lR /dev/raw*                                                           
crw-r--r--    1 root     root     162,   0 Dec  6 06:54 /dev/rawctl          
                                                                             
/dev/raw:                                                                    
total 0                                                                      
crw-r--r--    1 root     root     162,   1 Dec  6 06:54 raw1                 
crw-r--r--    1 root     root     162,   2 Dec  6 06:54 raw2                 
crw-r--r--    1 root     root     162,   3 Dec  6 06:54 raw3                 
crw-r--r--    1 root     root     162,   4 Dec  6 06:54 raw4                 
$                                                                            
$ raw -qa                                                                    
$                                                                            
$ raw /dev/raw/raw1 /dev/sda3                                                
/dev/raw/raw1:  bound to major 8, minor 3                                    
$ raw /dev/raw/raw2 /dev/sda                                                 
/dev/raw/raw2:  bound to major 8, minor 0                                    
$ raw -qa                                                                    
/dev/raw/raw1:  bound to major 8, minor 3                                    
/dev/raw/raw2:  bound to major 8, minor 0                                    

 The normal array of system calls for character devices are available on raw
devices. The size of the transfer for read(2) and write(2) must be an
integral multiple of the physical device's block size. For a disk this will
be its sector size which is normally 512 bytes. The data buffer given to read
() and write() system calls must be aligned to the block size. The lseek(2)
call needs to align its file read/write offset to a block boundary as well.
The pread(3) call (see man pread) combines a read() and an lseek() and can be
useful with raw devices (ditto with pwrite() ). Care should be taken with
offsets greater than 2 GB (or perhaps 4 GB) on 32 bit architectures where the
"off_t" type is 32 bits long. One solution is to use the _llseek() call (see 
man llseek).

 Unix utilities such as recent versions of dd and lmdd (from the lmbench
suite of programs) can be used to move data to and from "raw" devices as they
meet the above-mentioned block alignment requirements. Recent versions of the
sg_dd command in the sg_utils package can access both raw and sg devices.

 Also note that if the physical device has an odd number of sectors (as shown
by blockdev --getsize /dev/raw/raw*), the last sector will not be accessible
using raw IO.

Warning If a block device is being accessed via a bound raw device and also  
        via its normal block interface then there is no cache coherency      
        between the two access mechanisms. For example if /dev/sda1 was both 
        mounted and being accessed via a bound raw device then there could be
        data inconsistencies.                                                
-----------------------------------------------------------------------------

Chapter 12. Devfs pseudo file system

 The main documentation for devfs can be found at: reference W5. The devfs
name conventions for the SCSI subsystem are outlined in Section 3.3. Devfs is
selected by the kernel build option CONFIG_DEVFS_FS and whether it is mounted
at boot time (as /dev) or not is controlled by the kernel build option
CONFIG_DEVFS_MOUNT. The latter option can be overridden by the kernel boot
time options "devfs=mount" or "devfs=nomount", whichever is appropriate.

 The devfs SCSI node names with their default permissions are:
disc        rw-------   whole disk including mbr                             
part1       rw-------   first partition {...p1}                              
...                                                                          
part15      rw-------   15th partition {...p15}                              
cd          rw-rw-rw-   cd or dvd devices                                    
mt          rw-rw-rw-   tape mode 0 with rewind {...m0}                      
mtl         rw-rw-rw-   tape mode 1 with rewind {...m1}                      
mtm         rw-rw-rw-   tape mode 2 with rewind {...m2}                      
mta         rw-rw-rw-   tape mode 3 with rewind {...m3}                      
mtn         rw-rw-rw-   tape mode 0 with no rewind {...m0n}                  
mtln        rw-rw-rw-   tape mode 1 with no rewind {...m1n}                  
mtmn        rw-rw-rw-   tape mode 2 with no rewind {...m2n}                  
mtan        rw-rw-rw-   tape mode 3 with no rewind {...m3n}                  
generic     rw-r-----                                                        

 These node names are only present if the corresponding device (or
sub-entities of the device (e.g. partitions)) and driver are present. For
example if there is no sg driver present then there is no "generic" device
name. The strings that appear above in braces are appended to the abridged
"c0b0t0u0" notations outlined below as appropriate.

 The devfs file names that are block or character special files will be
called the primary device names in this description. The devfs daemon, called
devfsd, introduces many symbolic links to those primary device names. This is
done both for backward compatibility and convenience. These symbolic links
will be called secondary device names.

 The secondary device names are controlled by the devfsd configuration file
usually found in /etc/devfsd.conf . Following is a list of secondary device
names when the default devfsd.conf file is used:
Secondary name       slink to this primary device name                       
--------------------------------------------------------------               
/dev/sda             /dev/scsi/host0/bus0/target2/lun0/disc                  
/dev/sda1            /dev/scsi/host0/bus0/target2/lun0/part1                 
/dev/sd/c0b0t2u0     /dev/scsi/host0/bus0/target2/lun0/disc                  
/dev/sd/c0b0t2u0p1   /dev/scsi/host0/bus0/target2/lun0/part1                 
/dev/sr0             /dev/scsi/host0/bus0/target4/lun0/cd                    
/dev/sr/c0b0t4u0     /dev/scsi/host0/bus0/target4/lun0/cd                    
/dev/st0             /dev/scsi/host1/bus0/target0/lun0/mt                    
/dev/nst0a           /dev/scsi/host1/bus0/target0/lun0/mtan                  
/dev/st/c1b0t0u0m0   /dev/scsi/host1/bus0/target0/lun0/mt                    
/dev/st/c1b0t0u0m3n  /dev/scsi/host1/bus0/target0/lun0/mtan                  
/dev/sg0             /dev/scsi/host0/bus0/target2/lun0/generic               
/dev/sg1             /dev/scsi/host0/bus0/target4/lun0/generic               
/dev/sg2             /dev/scsi/host1/bus0/target0/lun0/generic               
/dev/sg/c0b0t2u0     /dev/scsi/host0/bus0/target2/lun0/generic               
/dev/sg/c0b0t4u0     /dev/scsi/host0/bus0/target4/lun0/generic               
/dev/sg/c1b0t0u0     /dev/scsi/host1/bus0/target0/lun0/generic               

 Note that the more common /dev/scd0 variant for SCSI cdroms is not
supported. There are also /dev/discs, /dev/cdroms and /dev/tapes directories
that contain symbolic links to all devices (i.e. not just SCSI devices) that
fall into that categorization:
Secondary name       slink to this primary device                            
------------------------------------------------------------                 
/dev/discs/disc0     /dev/ide/host0/bus0/target0/lun0      *                 
/dev/discs/disc1     /dev/scsi/host0/bus0/target2/lun0     *                 
/dev/cdroms/cdrom0   /dev/ide/host0/bus1/target1/lun0/cd                     
/dev/cdroms/cdrom1   /dev/scsi/host0/bus0/target4/lun0/cd                    
/dev/tapes/tape0     /dev/scsi/host1/bus0/target0/lun0     *                 

 Those entries marked with "*" are directories containing the primary
devices. Note that IDE/ATA devices are listed before SCSI devices. These
secondary device names mimic the same persistence rules as the primary device
names. So when a SCSI device (?), or its lower level driver or its upper
level driver are removed then so are the primary and secondary device names
associated with it.

 When devfs is mounted as /dev, the old "/dev/sda6" type can still be used in
some contexts. This may be convenient if typing is required at the kernel
boot time prompt. For example if a user wants to change the root partition on
a "devfs" machine then any of the following examples may be used as a kernel
boot time option:
   root=/dev/sda6                                                            
   root=/dev/scsi/host0/bus0/target0/lun0/part6                              
   root=/dev/sd/c0b0t0u0p6                                                   

 There are many device scanning programs that expect to see the pre-devfs
device names present and it will some time before they become devfs aware.
Also some programs rely on a open of /dev/sg0 (for example) to load the sg
driver (assuming it is a module and not already loaded). This can be arranged
by an entry in /etc/devfsd.conf file of:
        LOOKUP          sg.*            MODLOAD                              
and the following in /etc/modules.devfs :
        probeall        /dev/sg         scsi-hosts sg                        
        alias           /dev/sg*        /dev/sg                              

 The sg device permissions can be changed with this entry in the /etc/
devfsd.conf file:
  REGISTER scsi/host.*/bus.*/target.*/lun.*/generic                          
                              PERMISSIONS 0.0 rw-rw-rw-                      
See "man devfsd" for more information.

 An application can determine whether devfs is active by the presence or
otherwise of the file /dev/.devfsd.

 A feature of a /dev directory based on a persistent file system (e.g. ext2)
is the ability to associate permissions with a device file name and keep them
from one boot to the next. As noted above the default action of devfs is to
assign device file name permissions anew each time a machine is booted. The
PERMISSIONS action in the  /etc/devfsd.conf can be used to assert permissions
but this may be considered a little awkward. The devfs document (W5)
describes a method for getting the best of both worlds. This technique relies
on the recently added feature in lk 2.4 to mount the same file system at
multiple points.
-----------------------------------------------------------------------------

Appendix A. Common bus types (SCSI and other)

 A very good overview of the various bus types touched on in this appendix
(both SCSI and others) can be found at  www.pctechguide.com/04disk2.htm.

SCSI. The original SCSI 1 standard (ANSI specification X3.131-1986)
introduced an 8 bit parallel bus that was able to do asynchronous transfers
at 1.5 MegaBytes/sec and synchronous transfers up to 5 MB/sec. SCSI commands
are sent at the asynchronous rate. SCSI data is transferred either at the
asynchronous rate (worst case) or a negotiated synchronous rate (with 5 MB/
sec being the best case).

FAST SCSI. The SCSI 2 standard raised the maximum synchronous speed to 10 MB/
sec. SCSI 2 defined several parallel buses: single ended (as used by SCSI 1)
and a new differential bus. The differential bus has better noise immunity
and its maximum bus length is 25 metres (compared with single ended's 6
metres). Tagged queuing of commands was also added by SCSI 2.

WIDE SCSI. The SCSI 2 standard also increased the width of the bus allowing
16 and 32 bit "wide" variants. Very little use has been made of the 32 bit
width so "wide" usually refers to a 16 bit wide data path. The maximum number
of SCSI devices that can connect to a parallel SCSI bus is directly related
to the bus width hence "wide" buses allow a maximum of 16 SCSI devices to be
connected. [At least one of those devices must be the SCSI "initiator" which
is usually a host adapter.]

ULTRA SCSI. Traditionally synchronous buses are clocked either on the rising
or falling edge of the clock (which is normally a square wave). A recent
trend has been to clock on both edges and thus double the available
bandwidth. This is how ULTRA SCSI doubles the SCSI 2 "fast" speed to 20 MB/
sec.

ULTRA WIDE SCSI. The same "ultra" technique applied to a (16 bit) wide SCSI
parallel bus yields a bandwidth of 40 MB/sec.

ULTRA 2 WIDE SCSI. This variant introduces a new "low voltage" differential
signalling (LVD) that allows the synchronous clock speed to be doubled
yielding 80 MB/sec when using a (16 bit) wide bus. In this case the maximum
SCSI bus length is 12 metres. To be backward compatible with ULTRA WIDE this
variant can fall back to "single ended" operation. This leads to the
abbreviation LVD/SE being used by adapter manufacturers. One shortcoming of
this approach is that the presence of one UW device on a U2W bus will cause
all other U2W devices to communicate at the slower (i.e. UW) rate. Some
adapters overcome this by having separate LVD and SE physical buses on the
same logical SCSI bus.

ULTRA 160 SCSI. ULTRA 160 doubles parallel SCSI bus bandwidth yet again. It
uses a 16 bit wide data path, LVD signalling (see previous entry) and double
transition clocking that increases the maximum synchronous bandwidth to 160
MB/sec. Additional features include cyclic redundancy codes (CRC) to improve
data integrity (compared with a parity bit) and domain validation which
adjusts transfer rates if the error rate is too high.

ULTRA 320 SCSI. Shortly ULTRA 320 adapters will be available (disks with that
interface are already on the market). This is also a 16 bit wide LVD bus that
can fall back to slower speeds for compatibility with older devices. It
extends the features of Ultra 160 by doubling the clock speed. Packetized
SCSI which sends commands and status at full bus speed (rather than 5 MB/sec)
is included. Other improvements include "quick arbitration and selection",
"read and write data streaming" and CRC protection for command blocks as well
as data (Ultra 160 had CRC protection for data only). Note that adapter cards
using 64 bit PCI (or better: PCI-X) are required to stop the PCI bus being a
bottleneck at these speeds. More information can be found at www.scsita.org.
Recently an Ultra 320 HBA vendor claimed up to 105,000 IO operations per
second which implies per command SCSI bus overhead is less than 10
microseconds. There is a draft ULTRA 640 standard but that may be overtaken
by Serial Attached SCSI.

Serial Attached SCSI (SAS). Serial Attached SCSI (SAS) uses the same
transport technology as Serial ATA (sATA) and extends it. [sATA is described
below.] SAS can use external expanders to control up to 16000 devices from a
single HBA. The data transfer is full duplex and 1.5 Gbps or 3.0 Gbps "phys"s
can be aggregated to increase bandwidth. Cable lengths can be up to 6 meters.
SAS disks are dual ported. sATA disks can be connected to a SAS expanders
(but SAS disks can't be connected to a sATA HBA). SAS was demonstrated at
CeBit recently but won't reach the market until 2004.

FC-AL. This stands for Fibre Channel - Arbitrated Loop and may involve dual 2
Gigabit per second single mode fibre optic links spanning 10 kilometres with
throughput of up to 400 MegaBytes per second. Often associated with storage
area networks (SANs). Up to 126 devices can be attached to a loop which in
turn can be extended to 16 million devices in public loop mode. The
transmission medium isn't necessarily fibre optic cable: copper (in the form
of co-axial cable) can also be used at lower speeds and for shorter
distances.

SRP/InfiniBand. SRP (SCSI RDMA Protocol) [ SRP_draft] is a SCSI transport for
InfiniBand [ Infiniband_trade_association], a high-performance interconnect
running at 10 and 30 Gbps. SRP driver source is available at 
infiniband.sourceforge.net/Storage/SRP .

IEEE 1394. This standard also goes by the name of "Fire Wire" [trademarked by
Apple] and "iLink" [trademarked by Sony]. It is a serial bus that can run at
up to 400 Megabits/sec (IEEE 1394a). A newer standard, IEEE 1394b, ups this
to 800 Megabits/sec (with extensions to 1.6 and 3.2 Gigabits/sec) with cable
runs up to 100 metres. It has a similar but more general architecture than
USB. The IEEE 1394 standard allows for the SCSI command set to be carried
over a 1394 bus. There is a "sbp2" driver now available for the Linux IEEE
1394 stack. This sbp2 driver is also a SCSI subsystem lower level driver (so
it is functionally similar to the ide-scsi driver). So IEEE 1394 devices that
use the SBP-2 protocol (e.g. disks, cd-rw/dvd drives, MO drives and scanners)
can be accessed via the SCSI subsystem. See  Linux1394.sourceforge.net for
more information. The sbp2 driver is now in lk 2.4.7 .

iSCSI. This is a new IETF standard for sending the SCSI command set over a
TCP connection (or several of them). This permits SCSI devices (targets such
as disks) to be network appliances, accessed locally (or potentially at a
great distance) by a host machine.

NON SCSI buses. The following buses are not defined by the SCSI standards but
are of interest because they either can carry the SCSI command set, are in
some way related to the Linux SCSI subsystem or supply a similar
functionality to SCSI products.

IDE/ATA (ATAPI). IDE is the most used disk type on PC systems today. The
acronym stands for Integrated Drive Electronics and as the name suggests it
places the bulk of the IO "intelligence" on the disk controller card rather
than spreading it between the device (most often a disk) and a controller
(HBA) as SCSI does. IDE grew out of the ST506 and ESDI standards in the
1980s. EIDE (extended IDE) is a related acronym. The modern standards that
refer to this bus architecture are known as ATA and can be found at  
www.t13.org. The ATA Packet Interface (ATAPI) extends the disk oriented
command set to support CDROM and tape drives. The ATAPI command set closely
resembles the SCSI command set. The most recent ATA technology is outlined in
the next paragraph.

ATA 133. The ATA standards used by IDE devices have also been marching
through the adjectives (e.g. fast and ultra) and the numbers (e.g. 2, 33, 66,
100 and 133). The most recent addition is ATA 133 which supports burst rates
of 133 MB/sec and up to 2 devices per bus. [PCs typically have 2 and often 4
ATA buses.] ATA 66, 100 and 133 need a special cable. ATA cables are
relatively short precluding IDE devices being external to the computer. Cable
lengths have previously been limited to 18 inches although 1 metre long
cables have now appeared. Coincidently 133 MB/sec in also the maximum
throughput of the normal PCI bus found in most PCs. The are higher speed (and
wider) versions of PCI but they are relatively rare.

Serial ATA (sATA). Serial ATA uses 2 differential pairs to exchange data with
a sATA disk less than 1 metre away at 1.5 Gigabits per second. One pair takes
data to the disk and the other returns data from the disk. Data rates up to
150 Megabytes per second are possible (data transfer is half duplex). sATA is
a point to point connection, not a bus, so ATA's master and slave strapping
disappears. sATA cabling is less bulky and the form factor of its plugs and
sockets are smaller than parallel ATA (and the SCSI Parallel interface). sATA
devices are beginning to appear on the market. sATA-2 is a draft standard
that doubles the serial data rate to 3 Gigabits per second.

USB. Universal Serial Bus (USB) has a bandwidth of between 1.5 and 12
Megabits/sec (the latter speed with USB 1.1). Up to 127 devices can be
connected using a series of hubs each of which connects up to 7 devices (with
a 5 metre limit). USB supplies 5 volts at 0.5 amps to power small devices.
USB is "plug and play", hot pluggable and supports isochronous data transfers
(required for audio and video devices that need guaranteed minimum
bandwidth).

PC Parallel port. The original PC parallel port was uni-directional (towards
the printer) and was capable of about 10 KB/sec. The IEEE 1284 standard in
1994 introduced 5 modes of data transfer:

��*�Compatibility mode (forward direction)
   
��*�Nibble mode (reverse direction)
   
��*�Byte mode (reverse direction)
   
��*�EPP mode (bi-directional)
   
��*�ECP mode (bi-directional)
   

Enhanced Parallel Port (EPP) achieves transfer speeds of between 500 KB/sec
and 2 MB/sec and is targeted at CD-ROMs, tapes and hard drives. Extended
Capability Port (ECP) includes run length encoding and support for DMA. ECP
is targeted at fast printers and scanners.

I2O. "The I2O (Intelligent Input/Output) specification defines a standard
architecture for intelligent I/O that is independent of both the specific
device being controlled and the host operating system (OS)" [from 
www.i2osig.org]. It defines a "split driver" model in which the OS Services
Module (OSM) sits between the host OS device interface and the I2O
communications layer while the Hardware Device Module (HDM) sits between the
I2O communications layer and the hardware. The HDM may well run on a
dedicated processor (IOP).
-----------------------------------------------------------------------------

Appendix B. Changes between lk 2.2 and (during) 2.4

 Significant work has been done to change the single SCSI command queue used
in lk 2.2 to one command queue per device. To make the SCSI subsystem more
SMP friendly the granularity of the locks is much finer grained. In lk 2.2
the whole subsystem essentially used one lock.

 Even though it is not part of the SCSI subsystem, the inclusion of devfs
solves many SCSI device addressing problems that existed in the past.
Associated with devfs but very useful even in its absence is the "scsihosts"
kernel boot time (and module load time) option. This option allows users to
have some control over the ordering of multiple SCSI hosts.

 This appendix is difficult to maintain since features and drivers that have
proven useful in lk 2.4 (and its development tree) have tended to be back
ported into the higher release numbers of the lk 2.2 series.

 Currently (lk 2.4.2) support for MO devices is broken. Old DOS file systems
with a block size of 2048 bytes also have been reported as broken. The
problem seems to arise with media that have a physical block size larger than
the 1 KB logical block size used by the block subsystem. Only the sd driver
has this problem (luckily not the sr driver in which 2048 byte sectors are
the norm).
-----------------------------------------------------------------------------

B.1. Mid level changes

 
SCSI_IOCTL_GET_IDLUN    {ioctl, changed}                                     
-----------------------------------------------------------------------------

B.2. sd changes

 
HDIO_GETGEO_BIG         {ioctl, new}                                         
                                                                             
-----------------------------------------------------------------------------

B.3. sr changes

 No sr changes reported. As a related matter, the "hdx=scsi" kernel boot
option has been added. See Section 9.2.4 for more details.
-----------------------------------------------------------------------------

B.4. st changes

 No interface changes. In lk 2.2 the maximum number of extra tape devices
that could be added after boot time was limited to 3. This limitation has
been removed (leaving a maximum of 32 tape devices as noted earlier).

 A variant st driver called "osst" to handle early model OnStream tape drives
has been added in lk 2.4 .
-----------------------------------------------------------------------------

B.5. sg changes

 The main change is the addition of a new interface structure called
"sg_io_hdr". The existing interface structure (called "sg_header") was found
to be inflexible requiring the concatenation of raw data together with
meta-data in the read() and write() commands.
sg_io_hdr          {new interface structure}                                 
SG_IO              {new ioctl}                                               
direct IO          {present but commented out, see ALLOW_DIO}                
procfs output      {new information in /proc/scsi/sg directory}              
boot/module parameters  {new}                                                

 Up to 64 bytes of sense data can be obtained from the sg_io_hdr interface
structure. Also a residual count associated with the data transfer is
available (if the lower level driver supports it, if not the residual count
will be 0).
-----------------------------------------------------------------------------

B.6. Changes during the lk 2.4 series

 Even though the lk 2.4 production series is meant to be "stable" there have
been a significant number of changes as well as bug fixes. The following list
does not include changes to the lower level (adapter) drivers. Each item of
the list is prefixed by the kernel version that it was introduced. [14]

��*� [2.4.4] added the SCSI_IOCTL_GET_PCI ioctl(),
   
��*� [2.4.7] the "lun" bits (3 bits representing lun values 0 through 7 in
    the SCSI 1 and SCSI 2 standards) are no longer masked into the second
    byte of SCSI commands if the INQUIRY for that devices shows a SCSI level
    greater than SCSI_2,
   
��*� [2.4.7] the max_scsi_luns kernel (and module scsi_mod) option previously
    could be 1 to 7. Now the upper value can be large. [The scan algorithms
    are still doing a sequential scan rather than using REPORT_LUNS.]
   
��*� [2.4.7] both scsi_unregister_host() and scsi_unregister_module() now
    return an int (previously they were void functions). They return 0 for
    success, -1 for failure (typically busy),
   
��*� [2.4.7] the upper level drivers now report the correct scsi device name
    when they are attached. [The log messages that started with "Detected
    ..." previously sometimes reported the wrong device (e.g. sdc rather than
    sdb).] Kernel boot up messages will now show SCSI devices as "Attached
    ...",
   
��*� [2.4.7] 'max_sectors' was added to the Scsi_Host structure,
   
��*� [2.4.8] some mid level logic was altered to retry commands if the sense
    buffer indicates that logical unit is becoming ready [ASC=4, ASQ=1],
   
��*� [2.4.9] a major st update,
   
��*� [2.4.9] mid level changed to retry commands if lower level (adapter)
    driver returned DID_RESET,
   
��*� [2.4.10] original result (including SCSI status) saved when mid level
    issues a REQUEST SENSE so it can be restored afterwards,
   
��*� [2.4.10] added BLKGETSIZE64, BLKBSZSET and BLKBSZGET ioctls to sd + sr,
   
��*� [2.4.10] sg update that fixes generic_unplug_device() race + bumps
    access_count on opens (and decrements on releases),
   
��*� [2.4.11] added MODULE_LICENSE macro in most drivers, mostly
    MODULE_LICENSE("GPL"),
   
��*� [2.4.11] scsi_pid bumped for each command (why?),
   
��*� [2.4.11] st update to bump access_count. Now all upper level drivers
    increment access_count on opens and decrement it on releases,
   
��*� [2.4.13] scatterlist structure grows (alt_address is removed, page and
    offset added),
   
��*� [2.4.13] don't probe luns > 7 for target <= SCSI_2 ,
   
��*� [2.4.14] fine tuning (bug fixes) associated with scatterlist structure
    changes [it broke st ?],
   
��*� [2.4.15] 16 byte SCSI commands permitted [MAX_COMMAND_SIZE changes from
    12 to 16]. HBA driver must set Scsi_Host::max_cmd_len to 16 for mid level
    to forward 16 byte SCSI commands,
   
��*� [2.4.15] BLKGETSIZE + BLKGETSIZE64 ioctl() implementations moved out of
    SCSI subsystem (and into block subsystem),
   
��*� [2.4.15] large st update,
   
��*� [2.4.15] lk 2.5.0 forks off so lk2.4.15==lk2.5.0 .
   
��*� [2.4.17] add generic_unplug_device() call to scsi_wait_req(). This stops
    long waits in SCSI_IOCTL_SEND_COMMAND.
   
��*� [2.4.17] fix device scanning bug where, in some cases, the scsi_level
    (i.e. SCSI standard adherence) was misplaced.
   
��*� [2.4.17] major sg driver update, add mmap()-ed IO
   
��*� [2.4.18] permit upper level driver "init()" functions (e.g. sd_init() )
    to fail gracefully. [Add Scsi_Device::detected and scsi_unregister_module
    () .]
   
��*� [2.4.18] Fix for clustering (SCSI commands) on MO devices.
   
��*� [2.4.18] st driver update (compression algorithms).
   
��*� [2.4.18] update Documentation/scsi.txt and scsi-generic.txt .
   
��*� [2.4.18] Revamp scsi_debug driver .
   
��*� [2.4.19] Scsi reservation and reset capability added. Reservations allow
    multiple machines to share the same device (via a reserve/release
    mechanism). Scsi reset (via sg) is needed to "break" a reservation held
    by a non-responding machine.
   
��*� [2.4.19] Introduce BLIST_LARGELUN to handle LUNs larger than 7 despite
    reporting SCSI 2.
   
��*� [2.4.19] Change sd and sr so RECOVERED_ERROR is not treated as a hard
    error. Send warning to log/console.
   
��*� [2.4.19] Zero out sg's buffers before use. [Sg version upgraded from
    3.1.22 to 3.1.24 but this is not reflected in sg.h (supeficial).]
   
��*� [2.4.20] Support for highmem I/O added. Used by aic7xxx, 3w-xxxx, esp,
    megaraid, qlogicfc and sym53c8xx_2 LLDs.
   
��*� [2.4.20] "blocking_open" boot time, module load time parameter added to
    st.
   
��*� [2.4.21] Give new HBAs a new host number (higher than any previously
    used) unless there is a "scsihosts" match. Host numbering sequence
    "holes" are only re-used if there is a "scsihosts" match.
   
��*� [2.4.21] stop the SCSI status RECOVERED ERROR being treated as an error
    by the mid level (complements a change in 2.4.19).
   
��*� [2.4.21] use the TEST_UNIT_READY command (rather than START_STOP) to
    determine if removable media has changed (in sd driver).
   
��*� [2.4.21] major work on ide-scsi driver.
   
��*� [2.4.21] add aic79xx driver for Adaptec Ultra 320 controllers.
   
��*� [2.4.22] Extend timeout of SEND DIAGNOSTIC command to 2 hours. This is
    for foreground extended self tests.
   
��*� [2.4.26] Add 'scsi_allow_ghost_devices' kernel boot time and scsi_mod
    module option.
   
��*� [2.4.27] SATA support via "libata" library introduced. SATA disks appear
    with SCSI subsystem names (e.g. "/dev/sdb) and respond to SCSI commands
    (via a command translation facility).
   

-----------------------------------------------------------------------------
Appendix C. Troubleshooting

 Many SCSI problems are caused by cabling and (lack of, or inappropriate)
termination. This often results in repeated SCSI bus resets, parity or CRC
errors and sometimes reduced transfer speeds. There is a good SCSI
termination tutorial at this site: [http://www.scsita.org/aboutscsi/
SCSI_Termination_Tutorial.html]  www.scsita.org/aboutscsi/
SCSI_Termination_Tutorial.html. There is other useful SCSI information at
that site (see W9).

 There is also a SCSI "faq" site (see W10) that addresses many configuration
and troubleshooting issues. Although the main focus of this site is Windows
(and its ASPI interface), much is relevant to SCSI in Linux and other Unix
implementations.

 When it looks like something has partially locked up the system, the ps
command can be useful for finding out what may be causing the problem. The
following options may be useful for identifying what part of the kernel may
be causing the problem. This information could be forwarded to the
maintainers.
ps -eo cmd,wchan                                                             
ps -eo fname,tty,pid,stat,pcpu,wchan                                         
ps -eo pid,stat,pcpu,nwchan,wchan=WIDE-WCHAN-COLUMN -o args                  
The most interesting option for finding the location of the "hang" is
"wchan". If this is a kernel address then ps will use /proc/ksyms to find the
nearest symbolic location. The "nwchan" option outputs the numerical address
of the "hang".

 If the system is not responding to keystrokes, then <Alt+ScrollLock> in text
mode should output a stack trace while <Ctrl+ScrollLock> should output a list
of all processes. If the log is still working, the output will be sent there
as well as appearing on the console.

 If the kernel has been built with the CONFIG_MAGIC_SYSRQ, then in text mode
<Alt+SysRq+H> will list available commands. Of these <Alt+SysRq+S> is useful
for doing an emergency sync while <Alt+SysRq+U> will remount file systems in
read only mode. After that <Alt+SysRq+B> to reboot the machine might be your
next move.
-----------------------------------------------------------------------------

Appendix D. Performance, Test and Debugging tools

scu. The SCSI Command Utility (SCU) implements various SCSI commands
necessary for normal maintenance and diagnostics of SCSI peripherals. Some of
its features include: formatting, scanning for (and reassigning) bad blocks,
downloading new firmware, executing diagnostics and obtaining performance
information. It is available on several Unix platforms (and NT), however it
is only currently available in binary form. See [http://www.bit-net.com/
~rmiller/scu.html]  www.bit-net.com/~rmiller/scu.html for more details.

dd. Very useful for testing the streaming performance of disks and cdroms/
dvds. See man dd for more details. Here is an example for timing how long a
disk takes to read 1 GB (10**9 bytes) starting from block 0:
$ time dd if=/dev/sda of=/dev/null bs=512 count=1953126                      
If the raw device /dev/raw/raw1 is bound to /dev/sda then the above line is
equivalent to:
$ time dd if=/dev/raw/raw1 of=/dev/null bs=512 count=1953126                 
This may be slower than expected since one 512 byte sector is being read at a
time. Changing the last 2 arguments to "bs=8k count=122071" should give
better timings for the "raw" dd.

dt. The Data Test (DT) program is modelled on dd's syntax but dt can do a lot
more than sequential copies. It is a comprehensive data test program for SCSI
devices such as disks, tapes and cdrom/dvds. It is available on several Unix
platforms (and NT), and its source is available (unlike its stable mate "scu"
discussed earlier). See [http://www.bit-net.com/~rmiller/dt.html]  
www.bit-net.com/~rmiller/dt.html for more details.

lmdd. This command is part of the lmbench suite of programs and is a variant
of the dd command. It has been tailored for IO measurements and outputs
timing and throughput numbers on completion. Hence the time command and a
calculator are not needed.

blockdev. Fetches the sector size, the number of sectors and read ahead
status of a block device (typically a disk). Can also be used to flush
buffers and reread the partition table. See man blockdev.

sg_dd. This command is part of the sg_utils package (see W4) and is another
variant of the dd command in which either the input and/or output file is a
sg or a raw device. The block size argument ("bs") must match that of the
physical device in question. The "skip" and "seek" arguments can be up to 2**
31 - 1 on a 32 bit architecture allowing 1TB disks to be accessed (2G * 512).
The Linux system command llseek() is used to seek with a 64 bit file read/
write offset. The lmdd does not handle the > 2GB case and the dd command gets
creative with multiple relative seeks. sg_dd has a "bpt" (blocks per
transfer) argument that controls the number of blocks read or written in each
IO transaction.

 There are other programs in the sg_utils package to scan the SCSI bus (
sg_scan and sg_map), to measure SCSI bus throughput (sg_rbuf and sg_turs ),
show data from the SCSI inquiry command (sg_inq) and spin up (or down) media
(sg_start).

dd_rescue + scsiinfo. This dd variant is designed to rescue damaged media
such as SCSI (or IDE) disks and CDROMs (see W6). The  scsiinfo utility for
displaying and changing mode page information is also at that site.

sard. This utility is modelled on System V Release 4's sar -d for producing
IO statistics for mounted devices and partitions. It has been developed by
Stephen Tweedie and includes the sard utility and a required kernel patch
which expands the output of /proc/partitions . It can be found at [ftp://
ftp.uk.linux.org/pub/linux/sct/fs/profiling]  ftp.uk.linux.org/pub/linux/sct/
fs/profiling. It collects statistics at a relatively low level (e.g. SCSI mid
level) compared to programs like vmstat (see "man vmstat").
-----------------------------------------------------------------------------

Appendix E. Compile options and System calls including ioctls

 The compile options in this appendix are those which a system administrator
might conceivably want to change. Naturally the defaults are chosen so the
vast majority of users will not need to modify anything. In some cases
setting kernel build time options, kernel boot time parameters or module load
parameters has the same effect as changing a driver compile time option.

 System calls act as the interface between application programs and the
kernel and its drivers. In the case of the layered driver architecture that
the SCSI subsystem uses, the upper layer drivers handle most of the system
calls.

 The SCSI subsystem has a "bubble down" ioctl structure. First the upper
level driver associated with the open file descriptor attempts to decode the
ioctl. If it doesn't recognize it then the ioctl is passed down to the mid
level. If the mid level doesn't recognize it then the ioctl is passed down to
the lower level driver associated with the file descriptor. If the lower
level driver doesn't recognize it then a EINVAL error is generated.

 Some ioctls are dispatched to related subsystems.
-----------------------------------------------------------------------------

E.1. Mid level

 The following header files in the kernel source are relevant to the mid
level:
        /usr/src/linux/include/scsi/scsi.h                                   
        /usr/src/linux/include/scsi/scsi_ioctl.h                             

 These files are meant for applications to use (other than parts in a
__KERNEL__ conditional compilation block). They may also be found in /usr/
include/scsi directory but it is best not to trust these versions as they are
maintained with the glibc library and may lag the kernel version being used.
Usually in Linux systems /usr/include/linux can be relied upon to be a
symbolic link to the kernel source's include area (typically /usr/src/linux/
include/linux). This symbolic link can be used to include the correct
scsi_ioctl.h using the following trick:  #include <linux/../scsi/scsi_ioctl.h
>

 This include file: /usr/src/linux/drivers/scsi/scsi.h is the key internal
header file for the SCSI subsystem. As such it will not be discussed here
other than to point out it has the same file name (but it's in a different
directory) as the include file mentioned at the beginning of this section.
This sometimes causes confusion.

 The mid level drivers/scsi/scsi_scan.c file maintains an array of known SCSI
devices with idiosyncrasies . [This was known as the "black list" but that
was considered to judgmental.] The array is called "device_list". The various
value are:

��*�BLIST_NOLUN only probe lun 0
   
��*�BLIST_FORCELUN force all 8 luns to be probed
   
��*�BLIST_BORKEN passes through broken flag to lower level driver
   
��*�BLIST_KEY sends magical MODE SENSE (pc=0x2e) to unlock device
   
��*�BLIST_SINGLELUN only allow IO on one lun at a time
   
��*�BLIST_NOTQ disable tagged queuing
   
��*�BLIST_SPARSELUN keep going after lun not found
   
��*�BLIST_MAX5LUN only probe up to lun 5
   
��*�BLIST_ISDISK override INQUIRY's type with disk (direct access) type
   
��*�BLIST_ISROM override INQUIRY's type with ROM
   

-----------------------------------------------------------------------------
E.1.1. Mid level compile options

 None.
-----------------------------------------------------------------------------

E.1.2. Mid level ioctls

 See the following files:
/usr/src/linux/include/scsi/scsi.h                                           

 Note that the SCSI status constants defined in include/scsi/scsi.h are
shifted 1 bit right from the values in the SCSI standards:
scsi.h constant      value     SCSI 2 standard value                         
----------------------------------------------------                         
CHECK_CONDITION       0x1           0x2                                      
CHECK_GOOD            0x2           0x4                                      
BUSY                  0x4           0x8                                      
....                                                                         

 Summary of ioctl()s follow:
SCSI_IOCTL_SEND_COMMAND                                                       
  This interface is deprecated - users should use                             
  the scsi generic (sg) interface instead, as this                            
  is a more flexible approach to performing                                   
  generic SCSI commands on a device.                                          
                                                                              
  The structure that we are passed should look like:                          
                                                                              
  struct sdata {                                                              
   unsigned int inlen;     [i] Length of data written to device               
   unsigned int outlen;    [i] Length of data read from device                
   unsigned char cmd[x];   [i] SCSI command (6 <= x <= 16)                    
                           [o] Data read from device starts here              
                           [o] On error, sense buffer starts here             
   unsigned char wdata[y]; [i] Data written to device starts here             
  };                                                                          
  Notes:                                                                      
    -  The SCSI command length is determined by examining                     
       the 1st byte of the given command. There is no way                     
       to override this.                                                      
    -  Data transfers are limited to PAGE_SIZE (4K on                         
       i386, 8K on alpha).                                                    
    -  The length (x + y) must be at least OMAX_SB_LEN                        
       bytes long to accommodate the sense buffer when                        
       an error occurs. The sense buffer is truncated to                      
       OMAX_SB_LEN (16) bytes so that old code will not                       
       be surprised.                                                          
    -  If a Unix error occurs (e.g. ENOMEM) then the user                     
       will receive a negative return and the Unix error                      
       code in 'errno'. If the SCSI command succeeds then                     
       0 is returned. Positive numbers returned are the                       
       compacted SCSI error codes (4 bytes in one int)                        
       where the lowest byte is the SCSI status. See the                      
       drivers/scsi/scsi.h file for more information on this.                 
                                                                              
SCSI_IOCTL_GET_IDLUN                                                          
  This ioctl takes a pointer to a "struct scsi_idlun" object                  
  as its third argument. The "struct scsi_idlun" definition                   
  is found in <scsi/scsi.h>. It gets populated with scsi                      
  host, channel, device id and lun data for the given device.                 
  Unfortunately that header file "hides" that structure                       
  behind a "#ifdef __KERNEL__" block. To use this, that                       
  structure needs to be replicated in the user's program.                     
  Something like:                                                             
  typedef struct my_scsi_idlun {                                              
      int four_in_one;    /* 4 separate bytes of info                         
                             compacted into 1 int */                          
      int host_unique_id; /* distinguishes adapter cards from                 
                             same supplier */                                 
  } My_scsi_idlun;                                                            
      "four_in_one" is made up as follows:                                    
      (scsi_device_id | (lun << 8) | (channel << 16) |                        
      (host << 24))                                                           
  These 4 components are assumed (or masked) to be 1 byte each.               
                                                                              
SCSI_IOCTL_GET_BUS_NUMBER                                                     
  In lk 2.2 and earlier this ioctl was needed to get the                      
  host number. During lk 2.3 development the                                  
  SCSI_IOCTL_GET_IDLUN ioctl was changed to include this                      
  information. Hence this ioctl is only needed for                            
  backward compatibility.                                                     
SCSI_IOCTL_TAGGED_ENABLE                                                      
  Probably a remnant of the past when the mid level                           
  addressed such issues. Now this functionality is                            
  controlled by the lower level drivers. Best ignored.                        
SCSI_IOCTL_TAGGED_DISABLE                                                     
  See comment for SCSI_IOCTL_TAGGED_ENABLE.                                   
SCSI_IOCTL_PROBE_HOST                                                         
  This ioctl expects its 3rd argument to be a pointer to                      
  a union that looks like this:                                               
  union probe_host {                                                          
    unsigned int length;  /* [i] max length of                                
                                 output ASCII string */                       
    char str[length];     /* [o] N.B. may need '\0'                           
                                 appended */                                  
  };                                                                          
  The host associated with the device's fd either has a                       
  host dependent information string or failing that its                       
  name, output into the given structure. Note that the                        
  output starts at the beginning of given structure                           
  (overwriting the input length). N.B. A trailing '\0'                        
  may need to be put on the output string if it has been                      
  truncated by the input length. A return value of 1                          
  indicates the host is present, 0 indicates that the                         
  host isn't present (how can that happen?) and a                             
  negative value indicates an error.                                          
                                                                              
SCSI_IOCTL_DOORLOCK                                                           
SCSI_IOCTL_DOORUNLOCK                                                         
SCSI_IOCTL_TEST_UNIT_READY                                                    
  Returns 0 if the unit (device) is ready, a positive                         
  number if it is not or a negative number when there                         
  is an OS error.                                                             
                                                                              
SCSI_IOCTL_START_UNIT                                                         
SCSI_IOCTL_STOP_UNIT                                                          
SCSI_EMULATED_HOST          {same as SG_EMULATED_HOST <new>}                  
                                                                              
SCSI_IOCTL_GET_PCI                                                            
  Yields the PCI slot name (pci_dev::slot_name) associated with the lower     
  level (adapter) driver that controls the current device. Up to 8 characters 
  are output to the locations pointed to by 'arg'. If the current device      
  is not controlled by a PCI device then errno is set to ENXIO.               
  [This ioctl() was introduced in lk 2.4.4]                                   
-----------------------------------------------------------------------------

E.2. sd driver

 
-----------------------------------------------------------------------------

E.2.1. sd compile options

 
MAX_RETRIES     {5}                                                          
SD_TIMEOUT      {30 seconds}                                                 
SD_MOD_TIMEOUT  {75 seconds}                                                 
-----------------------------------------------------------------------------

E.2.2. sd ioctls and user interface

 The relevant files to see:
include/linux/hdreg.h                                                        
include/linux/genhd.h                                                        
include/linux/fs.h                                                           

 A list of ioctl()s follow:
HDIO_GETGEO_BIG                                                              
HDIO_GETGEO     [retrieve disk geometry]                                     
BLKGETSIZE      [number of sectors in device]                                
BLKROSET        [set read only flag]                                         
BLKROGET        [get read only flag]                                         
BLKRASET        [set read ahead value]                                       
BLKRAGET        [get read ahead value]                                       
BLKFLSBUF       [instructs SCSI subsystem to flush buffers]                  
BLKSSZGET       [get device block size]                                      
BLKPG           [partition table manipulation]                               
BLKELVGET       [get elevator parameters]                                    
BLKELVSET       [set elevator parameters]                                    
BLKRRPART       [reread the partition table]                                 
                                                                             
open()    (all flags ignored)                                                
close()                                                                      
ioctl()   (see list above)                                                   
-----------------------------------------------------------------------------

E.3. sr driver

 
-----------------------------------------------------------------------------

E.3.1. sr compile options

 None.
-----------------------------------------------------------------------------

E.3.2. sr ioctls and user interface

 See the following files:
/usr/src/linux/include/linux/cdrom.h                                         
/usr/src/linux/drivers/cdrom/cdrom.c [revision history section]              
/usr/src/linux/Documentation/cdrom/cdrom-standard.tex                        

 Some of the following ioctls are described in cdrom-standard.tex :
CDROMCLOSETRAY                                                               
CDROM_SET_OPTIONS                                                            
CDROM_CLEAR_OPTIONS                                                          
CDROM_SELECT_SPEED                                                           
CDROM_SELECT_DISC                                                            
CDROM_MEDIA_CHANGED                                                          
CDROM_DRIVE_STATUS                                                           
CDROM_CHANGER_NSLOTS                                                         
CDROM_LOCKDOOR                                                               
CDROM_DEBUG                                                                  
CDROM_GET_CAPABILITY                                                         
DVD_READ_STRUCT                                                              
DVD_WRITE_STRUCT                                                             
DVD_AUTH                                                                     
CDROM_SEND_PACKET                                                            
CDROM_NEXT_WRITABLE                                                          
CDROM_LAST_WRITTEN                                                           

 The O_NONBLOCK flag on the open() of scd devices is important. Without it
the open() will wait until there is media in the device before returning.
open()          O_NONBLOCK                                                   
close()                                                                      
read()                                                                       
write()                                                                      
ioctl()                                                                      
-----------------------------------------------------------------------------

E.4. st driver

 
-----------------------------------------------------------------------------

E.4.1. st compile options

 Most of the following compile options can be overridden with boot/module
parameters and/or runtime configuration (i.e. ioctls).

 The following parameters are defined in linux/drivers/scsi/st_options.h
ST_NOWAIT                    {0}                                             
ST_IN_FILE_POS               {0}                                             
ST_RECOVERED_WRITE_FATAL     {0}                                             
ST_DEFAULT_BLOCK             {0}                                             
ST_BUFFER_BLOCKS             {32}                                            
ST_WRITE_THRESHOLD_BLOCKS    {30}                                            
ST_MAX_BUFFERS               {4}                                             
ST_MAX_SG                    {16}                                            
ST_FIRST_SG                  {8}                                             
ST_FIRST_ORDER               {5}                                             
ST_TWO_FM                    {0}                                             
ST_BUFFER_WRITES             {1}                                             
ST_ASYNC_WRITES              {1}                                             
ST_READ_AHEAD                {1}                                             
ST_AUTO_LOCK                 {0}                                             
ST_FAST_MTEOM                {0}                                             
ST_SCSI2LOGICAL              {0}                                             
ST_SYSV                      {0}                                             

 The following parameters are defined in linux/drivers/scsi/st.c
ST_TIMEOUT                   {900*HZ}                                        
ST_LONG_TIMEOUT              {14000*HZ}                                      
-----------------------------------------------------------------------------

E.4.2. st ioctls and user interface

 The Linux tape interface is defined in /usr/src/linux/include/linux/mtio.h .

 The following ioctl()s are listed in alphabetical order with a brief
explanation to the right. [See st documentation (especially man 4 st) for
more details.]
MTIOCTOP   [execute tape commands and set drive/driver options]              
MTIOCGET   [get the status of the drive]                                     
MTIOCPOS   [get the current tape location]                                   
                                                                             
open()   O_RDONLY, O_RDWR                                                    
close()                                                                      
read()                                                                       
write()                                                                      
ioctl()                                                                      
-----------------------------------------------------------------------------

E.5. sg driver

 The following header files in the kernel source are relevant to the sg
driver:
        /usr/src/linux/include/scsi/sg.h                                     

 As pointed out in Section E.1 this is best included in applications by
using:
    #include <linux/../scsi/sg.h>                                            
-----------------------------------------------------------------------------

E.5.1. sg compile options

 Here are some defines from the sg.h file that the user could conceivably
want to change. The current default values are shown in braces on the right:
SG_SCATTER_SZ           {32768}                                              
SG_DEF_RESERVED_SIZE    {SG_SCATTER_SZ}                                      
SG_DEF_FORCE_LOW_DMA    {0}                                                  
SG_DEF_FORCE_PACK_ID    {0}                                                  
SG_DEF_KEP_ORPHAN       {0}                                                  
SG_MAX_QUEUE            {16}                                                 
SG_DEFAULT_RETRIES      {1}  # i.e. don't retry                              
SG_BIG_BUFF             {SG_DEF_RESERVED_SIZE}                               
SG_DEFAULT_TIMEOUT      {60 seconds}                                         
SG_DEF_COMMAND_Q        {0 *}                                                
SG_DEF_UNDERRUN_FLAG    {0}                                                  
                                                                             
* The per file descriptor copy of this flips to 1 (thus                      
  allowing command queuing) as soon as a write() based                       
  on the newer sg_io_hdr structure is detected.                              
-----------------------------------------------------------------------------

E.5.2. sg ioctls and user interface

 The following ioctl()s are listed in alphabetical order with a brief
explanation to the right. [See sg documentation for more details.]
SG_EMULATED_HOST     [indicate if adapter is ide-scsi]                       
SG_GET_COMMAND_Q     [command queuing flag state]                            
SG_GET_KEEP_ORPHAN   [interrupted SG_IO keep orphan flag state]              
SG_GET_LOW_DMA       ["low dma flag" (<= 16 MB on i386) state]               
SG_GET_NUM_WAITING   [number of responses waiting to be read()]              
SG_GET_PACK_ID       [pack_id of next to read() response                     
                      (-1 if none)]                                          
SG_GET_REQUEST_TABLE [yields array of requests being processed]              
SG_GET_RESERVED_SIZE [current size of reserved buffer]                       
SG_GET_SCSI_ID       [a little more info than the mid level's                
                      SCSI_IOCTL_GET_IDLUN ioctl]                            
SG_GET_SG_TABLESIZE  [max entries in host's scatter gather table]            
SG_GET_TIMEOUT       [yields timeout (unit: jiffies                          
                      (10ms on i386))]                                       
SG_GET_TRANSFORM     [state of ide-scsi's transform flag]                    
SG_IO                [send given SCSI command and wait for                   
                      response]                                              
SG_NEXT_CMD_LEN      [change command length of next command]                 
SG_SCSI_RESET        [send a SCSI bus, device or host reset]                 
SG_SET_COMMAND_Q     [set command queuing state {old=0, new=1}]              
SG_SET_DEBUG         [set debug level {0}]                                   
SG_SET_KEEP_ORPHAN   [set SG_IO's keep orphan flag {0}]                      
SG_SET_FORCE_LOW_DMA [force DMA buffer low (<= 16 MB on i386)                
                      {0}]                                                   
SG_SET_FORCE_PACK_ID [so read() can fetch by pack_id {0}]                    
SG_SET_RESERVED_SIZE [change default buffer size                             
                      {SG_DEF_RESERVED_SIZE}]                                
SG_SET_TIMEOUT       [change current timeout {60 secs} ]                     
SG_SET_TRANSFORM     [set ide-scsi's ATAPI transform flag {0}]               
                                                                             
open()    [recognized oflags: O_RDONLY, O_RDWR, O_EXCL,                      
           O_NONBLOCK]                                                       
close()                                                                      
read()                                                                       
write()                                                                      
ioctl()                                                                      
poll()    [used when in O_NONBLOCK mode]                                     
fasync()  [enables generation of SIGIO signal for read()]                    
-----------------------------------------------------------------------------

Appendix F. References, Credits and Corrections

WEB. The following references are found on the web. Please alert the author
if any of these links become stale.

  [W1] SCSI (draft) standards, resources: www.t10.org

  [W2] Eric Youngdale is the chief architect of the Linux SCSI subsystem:  
www.andante.org/scsi.html

  [W4] The author's scsi generic (sg) site is:  www.torque.net/sg. The Linux
Documentation Project's site includes the  www.tldp.org/HOWTO/
SCSI-Generic-HOWTO/ . A (possibly later) version of that document can be
found at  www.torque.net/sg/p/sg_v3_ho . The sg_utils and sg3_utils packages,
as tarballs and as binary and source rpms can also be found on this page.
These packages and others available for the sg driver are discussed at  
www.torque.net/sg/u_index.html.

  [W5] Richard Gooch's devfs site:  www.atnf.csiro.au/~rgooch/linux/docs/
devfs.html

  [W6] Kurt Garloff's site (including the scsidev and the scsiinfo
utilities):  www.garloff.de/kurt/linux/. Kurt also has the damaged media
rescue program dd_rescue at this site:  www.garloff.de/kurt/linux/ddrescue

  [W7] Drew Eckhardt's SCSI-HOWTO from 1996 (in ASCII):  metalab.unc.edu/pub/
Linux/docs/HOWTO/unmaintained/SCSI-HOWTO

  [W8] Linux Documentation Project (LDP):  tldp.org

  [W9] SCSI Trade Association site has a lot of useful information:  
www.scsita.org

  [W10] SCSI FAQ site - useful source of information and links:  
www.scsifaq.org

NEWSGROUPS. The following entries are actually reflectors rather than
newsgroups. Various web locations archive their contents (e.g. 
marc.theaimsgroup.com).

  [N1] Linux SCSI reflector: < linux-scsi@vger.kernel.org >. This is a
relatively low volume (circa 200 postings per month) Linux SCSI specific
group that many of the SCSI subsystem maintainers monitor.

  [N2] Linux kernel reflector: < linux-kernel@vger.kernel.org >. This is a
relatively high volume (circa 5000 postings per month) group for all aspects
of the Linux kernel. The Linux SCSI reflector should be tried first.

BOOKS. Here are some books that the author found useful.

  [B1] "Linux Device Drivers" Second edition by Alessandro Rubini and
Jonathan Corbet [O'Reilly 2001 ISBN 0-596-00008-1] This is a solid text on
Linux device drivers including some information on the SCSI subsystem. It
covers the block subsystem well and has many char device driver examples. It
has been updated for the Linux 2.4 series kernels and also includes
information on the Linux 2.2 and 2.0 series. This book is highly recommended.
The authors and the publisher have unselfishly made this book available under
the GNU Free Documentation License (version 1.1). It can be found in html at 
www.xml.com/ldd/chapter/book .

  [B2] "Running Linux" 3rd edition by M. Welsh, M. K. Dalheimer & L. Kaufman
[O'Reilly 1999 ISBN 1-56592-469-X] This is a classic Linux tome which
includes some SCSI configuration info.

  [B3] "The Programmer's Guide to SCSI" by Brian Sawert [Addison Wesley 1998
ISBN 0-201-18538-5] This book covers many SCSI topics, including the pass
through mechanisms of Linux (sg) and ASPI/ASPI32 as used by Windows.

CREDITS. The author is grateful for the following contributions:

��*�Kai M�kisara (st) <Kai.Makisara at metla dot fi>
   
��*�Jens Axboe (sr) <axboe at suse dot de>
   
��*�Richard Gooch (devfs) <rgooch at atnf dot csiro dot au>
   
��*�Tim Waugh (ppa, imm, ppscsi + docbook) <twaugh at redhat dot com>
   
��*�Gadi Oxman (ide-scsi) <gadio at netvision dot net dot il>
   

CORRECTIONS and SUGGESTIONS. Please send any corrections or suggestions to
the author at <dgilbert at interlog dot com> or <dougg at torque dot net> .

Notes

[1]  SCSI standards allow for multiple initiators to be present on a single  
     bus. This is not well supported in Linux although there are patches     
     around that improve this situation.                                     
[2]  If 15 partitions is too limiting then the Logical Volume Manager (LVM)  
     might be considered. See /usr/src/linux/Documentation/LVM-HOWTO . LVM   
     will also allow a logical partition to span multiple block devices.     
[3]  One slight wrinkle with grub is that /etc/grub.conf is a symbolic link  
     to /boot/grub/grub.conf. This can be useful to know when /boot is a     
     separate partition.                                                     
[4]  There is a sequencing issue here if the root file system is on the SCSI 
     device controlled by the lower level (adapter) driver to be loaded since
     it contains the /etc/modules.conf file. Also there is the issue of how  
     the boot loader obtains the initial kernel image from a SCSI device     
     (often from the (master) boot record). The latter is usually taken care 
     of by the system's or adapter card's BIOS.                              
[5]  An example of using mkinitrd: assume the root partition is on a SCSI    
     disk connected to a controller from Adaptec that requires the aic7xxx   
     driver. After building a kernel with the aic7xxx driver specified as a  
     module then load the image into the normal place (probably in the /boot 
     directory). Next make sure a line like "alias scsi_hostadapter aic7xxx" 
     is in the /etc/modules.conf file. Then from the /boot directory execute 
     a line like mkinitrd /boot/initrd-2.4.5.img 2.4.5 (this assumes lk 2.4.5
     is being build). This should result in the file initrd-2.4.5.img being  
     created. The /etc/lilo.conf should then have a section added looking    
     something like this:                                                    
     image=/boot/vmlinuz-2.4.5                                               
         label=linux                                                         
         initrd=/boot/initrd-2.4.5.img                                       
         read-only                                                           
         root=/dev/sda7                                                      
     The following should also be selected in the kernel configuration:      
     CONFIG_BLK_DEV_RAM=y                                                    
     CONFIG_BLK_DEV_RAM_SIZE=4096                                            
     CONFIG_BLK_DEV_INITRD=y                                                 
     See also Documentation/initrd.txt.                                      
[6]  PCI adapters are much "safer" for initialization code than the older ISA
     adapters. Hence the order of initialization of PCI adapters is unlikely 
     to lead to lockups. In this case the order of initialization (and thus  
     SCSI adapter numbers) of built in drivers may be modified by changing   
     the order of entries in the SCSI subsystem Makefile ( /usr/src/linux/   
     drivers/scsi/Makefile). Beware: some adapters may be recognized by more 
     than one lower level driver (e.g. those based on NCR chipsets).         
[7]  Either comma or colon can be delimiters for "scsihosts". This means that
     "scsihosts=advansys,imm,,ide-scsi" is also valid. Also if a machine's   
     boot sequence involves an "initrd" stage (look in /etc/grub.conf or /etc
     /lilo.conf to find out if this is the case), then the mkinitrd command  
     should be run after a change to the "scsihosts" boot time parameter.    
     This will generate a new initrd image that needs to be put in the       
     correct place (most probably in the /boot directory).                   
[8]  Using "scsihosts" can lead to a situation in which the computer's BIOS  
     finds the boot track (and hence boot time parameters set in lilo or     
     grub) on one disk while the kernel finds the root partition on another  
     disk. This can be quite confusing when it is unplanned. Hence after     
     changing (or adding) "scsihosts" in lilo or grub's configuration, it may
     be wise to boot the machine to see which disks are accessed.            
[9]  The parsing of "add-single-device" and "remove-single-device" is rather 
     inflexible. Hence it is best to stay close to the demonstrated syntax   
     with no extra spaces (and no tabs).                                     
[10] ATA is the modern name for what was previously known as IDE and/or EIDE.
     Note that the subsystem that controls ATA devices in Linux is called the
     "IDE" subsystem for historical reasons.                                 
[11] Other ATA devices such as tapes and floppies often use the ATAPI        
     interface. However, the vast majority of ATA disks do not use the ATAPI 
     interface.                                                              
[12] In the linux 2.4 kernel series there has been an increase in problems   
     when the ide-scsi driver is used so that cdrecord can control ATAPI     
     (IDE) cd writers. The problem may be related to the aggressive manner in
     which the IDE subsystem attempts to optimize the speed of data transfers
     on devices it controls. Some people experiencing timeouts and machine   
     lockups have found that reducing the DMA setting via the hdparm command 
     has fixed the problem. If the cd writer is connected to /dev/hdd then   
     users have reported success with one of these two commands:             
         hdparm -d0 -c1 /dev/hdd                                             
         hdparm -d 1 -X 34 /dev/hdd                                          
     The first one turns off DMA completely while the second one sets it in  
     "multiword DMA mode 2". Cd writers do not need the types of speeds that 
     modern disks utilize. Even burning at "x16" implies a sustained transfer
     rate of 16 times 150 KB/sec which is approximately 2.4 MB/sec, not      
     really that fast. There has also been a report that moving a cd writer  
     off a high speed IDE controller (Promise) and back to the motherboard's 
     lower speed IDE controllers has fixed a random IDE bus reset problem.   
     Another report suggests reducing (or turning off) the DMA on the IDE    
     hard disk can also stop lockups.                                        
[13] It has been reported that in some distributions the attempt to use the  
     hdparm command fails. In this case use the "echo ... > /proc/ide/hdx/   
     settings" form.                                                         
[14] This list has been compiled from the official 2.4 series kernels        
     released at [http://www.kernel.org] www.kernel.org. Distributions are   
     free to tailor the official kernels and this may impact what is         
     supported (or changed) in the SCSI subsystem. For example this machine  
     reports this kernel: "2.4.18-27.8.0". So that is roughly based on the   
     official 2.4.18 kernel which the vendor has "modded" 27 times for the   
     "8.0" level of their distribution. As an example of the type of changes,
     the aic7xxx driver in the official 2.4.18 does not support Adaptec's    
     Ultra 320 series of PCI adapters; however that vendor's version does.   





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