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     drbr, drbr_free, drbr_enqueue, drbr_dequeue, drbr_dequeue_cond,
     drbr_flush, drbr_empty, drbr_inuse, drbr_stats_update, — network driver
     interface to buf_ring


     #include <sys/param.h>
     #include <net/if.h>
     #include <net/if_var.h>

     drbr_free(struct buf_ring *br, struct malloc_type *type);

     drbr_enqueue(struct ifnet *ifp, struct buf_ring *br, struct mbuf *m);

     struct mbuf *
     drbr_dequeue(struct ifnet *ifp, struct buf_ring *br);

     struct mbuf *
     drbr_dequeue_cond(struct ifnet *ifp, struct buf_ring *br,
         int (*func) (struct mbuf *, void *), void *arg);

     drbr_flush(struct ifnet *ifp, struct buf_ring *br);

     drbr_empty(struct ifnet *ifp, struct buf_ring *br);

     drbr_inuse(struct ifnet *ifp, struct buf_ring *br);

     drbr_stats_update(struct ifnet *ifp, int len, int mflags);


     The drbr functions provide an API to network drivers for using
     buf_ring(9) for enqueueing and dequeueing packets.  This is meant as a
     replacement for the IFQ interface for packet queuing.  It allows a packet
     to be enqueued with a single atomic and packet dequeue to be done without
     any per-packet atomics as it is protected by the driver's tx queue lock.
     If INVARIANTS is enabled, drbr_dequeue() will assert that the tx queue
     lock is held when it is called.

     The drbr_free() function frees all the enqueued mbufs and then frees the

     The drbr_enqueue() function is used to enqueue an mbuf to a buf_ring,
     falling back to the ifnet's IFQ if ALTQ(4) is enabled.

     The drbr_dequeue() function is used to dequeue an mbuf from a buf_ring
     or, if ALTQ(4) is enabled, from the ifnet's IFQ.

     The drbr_dequeue_cond() function is used to conditionally dequeue an mbuf
     from a buf_ring based on whether func returns TRUE or FALSE.

     The drbr_flush() function frees all mbufs enqueued in the buf_ring and
     the ifnet's IFQ.

     The drbr_empty() function returns TRUE if there are no mbufs enqueued,
     FALSE otherwise.

     The drbr_inuse() function returns the number of mbufs enqueued.  Note to
     users that this is intrinsically racy as there is no guarantee that there
     will not be more mbufs when drbr_dequeue() is actually called.  Provided
     the tx queue lock is held there will not be less.

     The drbr_stats_update() function updates the number of bytes and the
     number of multicast packets sent.


     The drbr_enqueue() function returns ENOBUFS if there are no slots
     available in the buf_ring and 0 on success.

     The drbr_dequeue() and drbr_dequeue_cond() functions return an mbuf on
     success and NULL if the buf_ring is empty.


     These functions were introduced in FreeBSD 8.0.

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