.\"     $NetBSD: bufq.9,v 1.22 2016/11/18 08:29:43 wiz Exp $
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Juergen Hannken-Illjes.
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.Dd November 17, 2016
.Dt BUFQ 9
.Os
.Sh NAME
.Nm bufq ,
.Nm bufq_init ,
.Nm bufq_register ,
.Nm bufq_unregister ,
.Nm bufq_state ,
.Nm bufq_alloc ,
.Nm bufq_drain ,
.Nm bufq_free ,
.Nm bufq_getstrategyname ,
.Nm bufq_move ,
.Nm bufq_put ,
.Nm bufq_get ,
.Nm bufq_peek ,
.Nm bufq_cancel
.Nd device buffer queues
.Sh SYNOPSIS
.In sys/bufq.h
.Ft void
.Fn bufq_init "void"
.Ft int
.Fn bufq_register "struct bufq_strat *"
.Ft int
.Fn bufq_unregister "struct bufq_strat *"
.Ft int
.Fn bufq_alloc "struct bufq_state **bufq" "const char *strategy" "int flags"
.Ft void
.Fn bufq_drain "struct bufq_state *bufq"
.Ft void
.Fn bufq_free "struct bufq_state *bufq"
.Ft "const char *"
.Fn bufq_getstrategyname "struct bufq_state *bufq"
.Ft void
.Fn bufq_move "struct bufq_state *dst" "struct bufq_state *src"
.Ft void
.Fn bufq_put "struct bufq_state *bufq" "struct buf *bp"
.Ft "struct buf *"
.Fn bufq_get "struct bufq_state *bufq"
.Ft "struct buf *"
.Fn bufq_peek "struct bufq_state *bufq"
.Ft "struct buf *"
.Fn bufq_cancel "struct bufq_state *bufq" "struct buf *bp"
.Sh DESCRIPTION
The
.Nm
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
.Xr module 9 ) .
By default, the
.Dv BUFQ_FCFS
and
.Dv BUFQ_DISKSORT
strategies are included in
the kernel; see
.Xr options 4
for more details.
.Pp
The primary data type for using the operations is the
.Em bufq_state
structure, which is opaque for users.
Each buffer queue strategy module is defined by the
.Em bufq_strat
structure, which is also opaque for users.
.Sh FUNCTIONS
.Bl -tag -width compact
.It Fn bufq_init "void"
Initialize the
.Nm
subsystem.
This routine must be called before any other
.Nm
routines.
.It Fn bufq_register "bs"
Registers the specified buffer queue strategy module so it can be used
in subsequent operations.
.It Fn bufq_unregister "bs"
Unregisters the specified buffer queue strategy module.
The routine will fail if any buffer queues for the specified strategy
are in use (see
.Fn bufq_alloc
below).
.It Fn bufq_alloc "bufq" "strategy" "flags"
Allocate and initialize a
.Em bufq_state
descriptor.
.Pp
The argument
.Fa strategy
specifies a buffer queue strategy to be used for this buffer queue.
The following special values can be used:
.Pp
.Bl -tag -offset indent -width BUFQ_DISK_DEFAULT_STRAT -compact
.It Dv BUFQ_STRAT_ANY
Let
.Fn bufq_alloc
select a strategy.
.It Dv BUFQ_DISK_DEFAULT_STRAT
Let
.Fn bufq_alloc
select a strategy, assuming it will be used for a normal disk device.
.El
.Pp
Valid bits for the
.Fa flags
are:
.Pp
.Bl -tag -offset indent -width BUFQ_SORT_RAWBLOCK -compact
.It Dv BUFQ_SORT_RAWBLOCK
sort by
.Em b_rawblkno
.It Dv BUFQ_SORT_CYLINDER
sort by
.Em b_cylinder
and then by
.Em b_rawblkno
.It Dv BUFQ_EXACT
Fail if a strategy specified by
.Fa strategy
is not available.
In that case,
.Fa bufq_alloc
returns
.Dv ENOENT .
If this flag is not specified,
.Fn bufq_alloc
will silently use one of available strategies.
.El
.Pp
If a specific strategy is specified but not currently available, the
.Nm bufq
subsystem will attempt to auto-load the corresponding kernel module using
.Xr module_autoload 9 .
.It Fn bufq_drain "bufq"
Drain a
.Em bufq_state
descriptor.
.It Fn bufq_free "bufq"
Destroy a
.Em bufq_state
descriptor.
.It Fn 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.
.It Fn bufq_move "dst" "src"
Move all requests from the buffer queue
.Fa src
to the buffer queue
.Fa dst .
.It Fn bufq_put "bufq" "bp"
Put the buf
.Fa bp
in the queue.
.It Fn bufq_get "bufq"
Get the next buf from the queue and remove it from the queue.
Returns
.Dv NULL
if the queue is empty.
.It Fn bufq_peek "bufq"
Get the next buf from the queue without removal.
The next buf will remain the same until
.Fn bufq_get ,
.Fn bufq_put ,
or
.Fn bufq_drain
is called.
Returns
.Dv NULL
if the queue is empty.
.It Fn bufq_cancel "bufq" "bp"
Cancel the buf
.Fa bp
issued earlier on the queue.
Returns
.Dv NULL
if the element can not be found on the queue or
.Fa bp
if it has been found and removed.
This operation can be computationally expensive if there are
a lot of buffers queued.
.El
.Sh CODE REFERENCES
The actual code implementing the device buffer queues can be found
in the file
.Pa sys/kern/subr_bufq.c .
The code implementing specific buffer queue strategies can be found in
the files
.Pa sys/kern/bufq_*.c .
.Sh NOTES
A list of currently available buffer queue strategies is available via
the
.Dq kern.bufq.strategies
.Xr sysctl 7
variables.
.Sh HISTORY
The
.Nm
subsystem appeared in
.Nx 2.0 .
.Sh AUTHORS
The
.Nm
subsystem was written by
.An J\(:urgen Hannken-Illjes
.Aq hannken@NetBSD.org .