illumos: manual page: bioclone.9f

BIOCLONE(9F) Kernel Functions for Drivers BIOCLONE(9F)

NAME

bioclone - clone another buffer

SYNOPSIS

#include <sys/ddi.h>
#include <sys/sunddi.h>

struct buf *bioclone(struct buf *bp, off_t off, size_t len, dev_t dev,
daddr_t blkno, int (*iodone) (struct buf *), struct buf *bp_mem,
int sleepflag);

INTERFACE LEVEL

illumos DDI specific (illumos DDI).

PARAMETERS

bp
Pointer to the buf(9S) structure describing the original I/O
request.

off
Offset within original I/O request where new I/O request
should start.

len
Length of the I/O request.

dev
Device number.

blkno
Block number on device.

iodone
Specific biodone(9F) routine.

bp_mem
Pointer to a buffer structure to be filled in or NULL.

sleepflag
Determines whether caller can sleep for memory. Possible
flags are KM_SLEEP to allow sleeping until memory is
available, or KM_NOSLEEP to return NULL immediately if
memory is not available.

DESCRIPTION

The bioclone() function returns an initialized buffer to perform I/O to a
portion of another buffer. The new buffer will be set up to perform I/O
to the range within the original I/O request specified by the parameters
off and len. An offset 0 starts the new I/O request at the same address
as the original request. off + len must not exceed b_bcount, the length
of the original request. The device number dev specifies the device to
which the buffer is to perform I/O. blkno is the block number on device.
It will be assigned to the b_blkno field of the cloned buffer structure.
iodone lets the driver identify a specific biodone(9F) routine to be
called by the driver when the I/O is complete. bp_mem determines from
where the space for the buffer should be allocated. If bp_mem is NULL,
bioclone() will allocate a new buffer using getrbuf(9F). If sleepflag is
set to KM_SLEEP, the driver may sleep until space is freed up. If
sleepflag is set to KM_NOSLEEP, the driver will not sleep. In either
case, a pointer to the allocated space is returned or NULL to indicate
that no space was available. After the transfer is completed, the buffer
has to be freed using freerbuf(9F). If bp_mem is not NULL, it will be
used as the space for the buffer structure. The driver has to ensure that
bp_mem is initialized properly either using getrbuf(9F) or bioinit(9F).

If the original buffer is mapped into the kernel virtual address space
using bp_mapin(9F) before calling bioclone(), a clone buffer will share
the kernel mapping of the original buffer. An additional bp_mapin() to
get a kernel mapping for the clone buffer is not necessary.

The driver has to ensure that the original buffer is not freed while any
of the clone buffers is still performing I/O. The biodone() function has
to be called on all clone buffers before it is called on the original
buffer.

RETURN VALUES

The bioclone() function returns a pointer to the initialized buffer
header, or NULL if no space is available.

CONTEXT

The bioclone() function can be called from user, interrupt, or kernel
context. Drivers must not allow bioclone() to sleep if called from an
interrupt routine.

EXAMPLES

Example 1: Using bioclone() for Disk Striping

A device driver can use bioclone() for disk striping. For each disk in
the stripe, a clone buffer is created which performs I/O to a portion of
the original buffer.

static int
stripe_strategy(struct buf *bp)
{
...
bp_orig = bp;
bp_1 = bioclone(bp_orig, 0, size_1, dev_1, blkno_1,
stripe_done, NULL, KM_SLEEP);
fragment++;
...
bp_n = bioclone(bp_orig, offset_n, size_n, dev_n,
blkno_n, stripe_done, NULL, KM_SLEEP);
fragment++;
/* submit bp_1 ... bp_n to device */
xxstrategy(bp_x);
return (0);
}

static uint_t
xxintr(caddr_t arg)
{
...
/*
* get bp of completed subrequest. biodone(9F) will
* call stripe_done()
*/
biodone(bp);
return (0);
}

static int
stripe_done(struct buf *bp)
{
...
freerbuf(bp);
fragment--;
if (fragment == 0) {
/* get bp_orig */
biodone(bp_orig);
}
return (0);
}