BUFQ(9)                NetBSD Kernel Developer's Manual                BUFQ(9)

NAME
     bufq, bufq_init, bufq_register, bufq_unregister, bufq_state, bufq_alloc,
     bufq_drain, bufq_free, bufq_getstrategyname, bufq_move, bufq_put,
     bufq_get, bufq_peek, bufq_cancel -- device buffer queues

SYNOPSIS
     #include <sys/bufq.h>

     void
     bufq_init(void);

     int
     bufq_register(struct bufq_strat *);

     int
     bufq_unregister(struct bufq_strat *);

     int
     bufq_alloc(struct bufq_state **bufq, const char *strategy, int flags);

     void
     bufq_drain(struct bufq_state *bufq);

     void
     bufq_free(struct bufq_state *bufq);

     const char *
     bufq_getstrategyname(struct bufq_state *bufq);

     void
     bufq_move(struct bufq_state *dst, struct bufq_state *src);

     void
     bufq_put(struct bufq_state *bufq, struct buf *bp);

     struct buf *
     bufq_get(struct bufq_state *bufq);

     struct buf *
     bufq_peek(struct bufq_state *bufq);

     struct buf *
     bufq_cancel(struct bufq_state *bufq, struct buf *bp);

DESCRIPTION
     The bufq subsystem is a set of operations for the management of device
     buffer queues.  Various strategies for ordering of entries in the buffer
     queues can be provided by loadable kernel modules (see module(9)).  By
     default, the BUFQ_FCFS and BUFQ_DISKSORT strategies are included in the
     kernel; see options(4) for more details.

     The primary data type for using the operations is the bufq_state struc-
     ture, which is opaque for users.  Each buffer queue strategy module is
     defined by the bufq_strat structure, which is also opaque for users.

FUNCTIONS
     bufq_init(void)
              Initialize the bufq subsystem.  This routine must be called
              before any other bufq routines.

     bufq_register(bs)
              Registers the specified buffer queue strategy module so it can
              be used in subsequent operations.

     bufq_unregister(bs)
              Unregisters the specified buffer queue strategy module.  The
              routine will fail if any buffer queues for the specified strat-
              egy are in use (see bufq_alloc() below).

     bufq_alloc(bufq, strategy, flags)
              Allocate and initialize a bufq_state descriptor.

              The argument strategy specifies a buffer queue strategy to be
              used for this buffer queue.  The following special values can be
              used:

                    BUFQ_STRAT_ANY           Let bufq_alloc() select a strat-
                                             egy.
                    BUFQ_DISK_DEFAULT_STRAT  Let bufq_alloc() select a strat-
                                             egy, assuming it will be used for
                                             a normal disk device.

              Valid bits for the flags are:

                    BUFQ_SORT_RAWBLOCK  sort by b_rawblkno
                    BUFQ_SORT_CYLINDER  sort by b_cylinder and then by
                                        b_rawblkno
                    BUFQ_EXACT          Fail if a strategy specified by
                                        strategy is not available.  In that
                                        case, bufq_alloc returns ENOENT.  If
                                        this flag is not specified,
                                        bufq_alloc() will silently use one of
                                        available strategies.

              If a specific strategy is specified but not currently available,
              the bufq subsystem will attempt to auto-load the corresponding
              kernel module using module_autoload(9).

     bufq_drain(bufq)
              Drain a bufq_state descriptor.

     bufq_free(bufq)
              Destroy a bufq_state descriptor.

     bufq_getstrategyname(bufq)
              Get a strategy identifier of a buffer queue, the string returned
              will be NUL-terminated and it always will be a valid strategy
              name.

     bufq_move(dst, src)
              Move all requests from the buffer queue src to the buffer queue
              dst.

     bufq_put(bufq, bp)
              Put the buf bp in the queue.

     bufq_get(bufq)
              Get the next buf from the queue and remove it from the queue.
              Returns NULL if the queue is empty.

     bufq_peek(bufq)
              Get the next buf from the queue without removal.  The next buf
              will remain the same until bufq_get(), bufq_put(), or
              bufq_drain() is called.  Returns NULL if the queue is empty.

     bufq_cancel(bufq, bp)
              Cancel the buf bp issued earlier on the queue.  Returns NULL if
              the element can not be found on the queue or bp if it has been
              found and removed.  This operation can be computationally expen-
              sive if there are a lot of buffers queued.

CODE REFERENCES
     The actual code implementing the device buffer queues can be found in the
     file sys/kern/subr_bufq.c.  The code implementing specific buffer queue
     strategies can be found in the files sys/kern/bufq_*.c.

NOTES
     A list of currently available buffer queue strategies is available via
     the ``kern.bufq.strategies'' sysctl(7) variables.

HISTORY
     The bufq subsystem appeared in NetBSD 2.0.

AUTHORS
     The bufq subsystem was written by Jürgen Hannken-Illjes
     <hannken@NetBSD.org>.

NetBSD 7.0                     November 17, 2016                    NetBSD 7.0

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