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block SG_IO: add SG_FLAG_Q_AT_HEAD flag

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After the SG_IO ioctl was copied into the block layer and
later into the bsg driver, subtle differences emerged.

One difference is the way injected commands are queued through
the block layer (i.e. this is not SCSI device queueing nor SATA
NCQ). Summarizing:
  - SG_IO on block layer device: blk_exec*(at_head=false)
  - sg device SG_IO: at_head=true
  - bsg device SG_IO: at_head=true

Some time ago Boaz Harrosh introduced a sg v4 flag called
BSG_FLAG_Q_AT_TAIL to override the bsg driver default. A
recent patch titled: "sg: add SG_FLAG_Q_AT_TAIL flag"
allowed the sg driver default to be overridden. This patch
allows a SG_IO ioctl sent to a block layer device to have
its default overridden.

ChangeLog:
    - introduce SG_FLAG_Q_AT_HEAD flag in sg.h to cause
      commands that are injected via a block layer
      device SG_IO ioctl to set at_head=true
    - make comments clearer about queueing in sg.h since the
      header is used both by the sg device and block layer
      device implementations of the SG_IO ioctl.
    - introduce BSG_FLAG_Q_AT_HEAD in bsg.h for compatibility
      (it does nothing) and update comments.

Signed-off-by: Douglas Gilbert <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Mike Christie <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Douglas Gilbert 2014-07-01 10:48:05 -06:00 • committed by Jens Axboe
commit d15156138d
3 changed files with 13 additions and 6 deletions
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5
block/scsi_ioctl.c
View file

@ -290,6 +290,7 @@ static int sg_io(struct request_queue *q, struct gendisk *bd_disk,
unsigned long start_time;
ssize_t ret = 0;
int writing = 0;
int at_head = 0;
struct request *rq;
char sense[SCSI_SENSE_BUFFERSIZE];
struct bio *bio;
@ -313,6 +314,8 @@ static int sg_io(struct request_queue *q, struct gendisk *bd_disk,
case SG_DXFER_FROM_DEV:
break;
}
if (hdr->flags & SG_FLAG_Q_AT_HEAD)
at_head = 1;
rq = blk_get_request(q, writing ? WRITE : READ, GFP_KERNEL);
if (!rq)
@ -369,7 +372,7 @@ static int sg_io(struct request_queue *q, struct gendisk *bd_disk,
* (if he doesn't check that is his problem).
* N.B. a non-zero SCSI status is _not_ necessarily an error.
*/
blk_execute_rq(q, bd_disk, rq, 0);
blk_execute_rq(q, bd_disk, rq, at_head);
hdr->duration = jiffies_to_msecs(jiffies - start_time);