[SCSI] sym53c8xx: use a queue depth of 1 for untagged devices

sym53c8xx uses a command queue depth of 2 for untagged devices,
without good reason.  This _mostly_ seems to work ok, but it has
caused me some subtle problems.  For example, I have an application
where one thread sends write commands to a tape drive, and another
thread sends log sense polling commands.  With a queue depth of
2, the polling commands end up being starved for long periods of
time while multiple write commands are serviced (this may also be
related to the fact the the sg driver queues commands in LIFO order).
This problem is fixed by changing the queue depth to 1 for untagged
devices.  I have tested this change extensively with many different
tape drives, medium changers, and disk drives (disk drives of course
use tagged commands and are therefore unaffected by this patch).

Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

diff --git a/drivers/scsi/sym53c8xx_2/sym_glue.c b/drivers/scsi/sym53c8xx_2/sym_glue.c
index a0d5aa6aad79f5cabc11fcc41e55a552bdcf9495..23e782015880173a47684b166bff6196ea864963 100644 (file)
--- a/drivers/scsi/sym53c8xx_2/sym_glue.c
+++ b/drivers/scsi/sym53c8xx_2/sym_glue.c
@@ -792,9 +792,9 @@ static int sym53c8xx_slave_configure(struct scsi_device *sdev)
 
        /*
         *  Select queue depth from driver setup.
-        *  Donnot use more than configured by user.
-        *  Use at least 2.
-        *  Donnot use more than our maximum.
+        *  Do not use more than configured by user.
+        *  Use at least 1.
+        *  Do not use more than our maximum.
         */
        reqtags = sym_driver_setup.max_tag;
        if (reqtags > tp->usrtags)
@@ -803,7 +803,7 @@ static int sym53c8xx_slave_configure(struct scsi_device *sdev)
                reqtags = 0;
        if (reqtags > SYM_CONF_MAX_TAG)
                reqtags = SYM_CONF_MAX_TAG;
-       depth_to_use = reqtags ? reqtags : 2;
+       depth_to_use = reqtags ? reqtags : 1;
        scsi_adjust_queue_depth(sdev,
                                sdev->tagged_supported ? MSG_SIMPLE_TAG : 0,
                                depth_to_use);