command queueing cmd_per_lun, queue_depth and tags

[Patrick Mansfield <patmans@us.ibm.com>]

Currently cmd_per_lun is the default queue depth for both tagged and
untagged queueing. That is, if the LLDD does not modify queue_depth in its
slave_configure, queue_depth will be set to cmd_per_lun, no matter what
the command queueing/tag support. 

If we don't allow queueing in the LLDD, and cmd_per_lun is supposed to be
the default depth for untagged support, shouldn't it always be 1, and
hence go away? 

Similarily, why do some LLDD's use a cmd_per_lun of 3, or (like
ncr53c8xx_slave_configure) set queue_depth for !tagged_supported to
anything other than 1?

I know (at least) FCP can always do simple queueing, but I don't think
scsi core should allow multiple commands to be sent if the device does not
have CMDQUE (or BQUE).

It seems more sensible to have cmd_per_lun be more like it was in 2.4,
where it was (generally) the default queue_depth for all devices on an
initiator (all devices on a scsi_host).

Also we don't even check the BQUE in the INQUIRY result (byte 6, bit 7).
Should this be changed? That is set tagged_supported if BQUE is set, like
we do for CMDQUE (byte 7 bit 1). And also set simple_tags if
tagged_supported is set.

That is something like this patch to always set queue_depth to 1 by
default, then later if BQUE or CMDQUE are set, set queue_depth to
cmd_per_lun. Of course slave_configure can still override this, but this
change means queue_depth is 1 if you call scsi_deactivate_tcq().

Using this patch might also require changes in some LLDD's, like ipr.

diff -uprN -X /home/patman/dontdiff /home/linux/views/linux-2.6.17-rc1/drivers/scsi/scsi.c linux-2.6.17-rc1/drivers/scsi/scsi.c
--- /home/linux/views/linux-2.6.17-rc1/drivers/scsi/scsi.c	2006-04-03 11:35:58.000000000 -0700
+++ linux-2.6.17-rc1/drivers/scsi/scsi.c	2006-04-07 10:00:15.000000000 -0700
@@ -866,9 +866,7 @@ EXPORT_SYMBOL(scsi_finish_command);
  * Arguments:	sdev	- SCSI Device in question
  * 		tagged	- Do we use tagged queueing (non-0) or do we treat
  * 			  this device as an untagged device (0)
- * 		tags	- Number of tags allowed if tagged queueing enabled,
- * 			  or number of commands the low level driver can
- * 			  queue up in non-tagged mode (as per cmd_per_lun).
+ * 		tags	- Number of tags allowed if tagged queueing enabled
  *
  * Returns:	Nothing
  *
@@ -883,6 +881,8 @@ void scsi_adjust_queue_depth(struct scsi
 {
 	unsigned long flags;
 
+	if (!tagged)
+		tags = 1;
 	/*
 	 * refuse to set tagged depth to an unworkable size
 	 */
@@ -913,7 +913,6 @@ void scsi_adjust_queue_depth(struct scsi
 				    "disabled\n");
 		case 0:
 			sdev->ordered_tags = sdev->simple_tags = 0;
-			sdev->queue_depth = tags;
 			break;
 	}
  out:
@@ -935,8 +934,7 @@ EXPORT_SYMBOL(scsi_adjust_queue_depth);
  *
  * Returns:	0 - No change needed
  * 		>0 - Adjust queue depth to this new depth
- * 		-1 - Drop back to untagged operation using host->cmd_per_lun
- * 			as the untagged command depth
+ * 		-1 - Drop back to untagged operation
  *
  * Lock Status:	None held on entry
  *
@@ -960,7 +958,7 @@ int scsi_track_queue_full(struct scsi_de
 		return 0;
 	if (sdev->last_queue_full_depth < 8) {
 		/* Drop back to untagged */
-		scsi_adjust_queue_depth(sdev, 0, sdev->host->cmd_per_lun);
+		scsi_adjust_queue_depth(sdev, 0, 1);
 		return -1;
 	}
 	
diff -uprN -X /home/patman/dontdiff /home/linux/views/linux-2.6.17-rc1/drivers/scsi/scsi_scan.c linux-2.6.17-rc1/drivers/scsi/scsi_scan.c
--- /home/linux/views/linux-2.6.17-rc1/drivers/scsi/scsi_scan.c	2006-04-03 11:35:58.000000000 -0700
+++ linux-2.6.17-rc1/drivers/scsi/scsi_scan.c	2006-04-07 09:47:47.000000000 -0700
@@ -256,7 +256,7 @@ static struct scsi_device *scsi_alloc_sd
 	}
 
 	sdev->request_queue->queuedata = sdev;
-	scsi_adjust_queue_depth(sdev, 0, sdev->host->cmd_per_lun);
+	scsi_adjust_queue_depth(sdev, 0, 1);
 
 	scsi_sysfs_device_initialize(sdev);
 
@@ -719,9 +719,12 @@ static int scsi_add_lun(struct scsi_devi
 	 * End sysfs code.
 	 */
 
-	if ((sdev->scsi_level >= SCSI_2) && (inq_result[7] & 2) &&
-	    !(*bflags & BLIST_NOTQ))
+	if ((sdev->scsi_level >= SCSI_2) && ((inq_result[6] & 0x10) ||
+	     (inq_result[7] & 2)) && !(*bflags & BLIST_NOTQ)) {
 		sdev->tagged_supported = 1;
+		scsi_adjust_queue_depth(sdev, MSG_SIMPLE_TAG,
+					sdev->host->cmd_per_lun);
+	}
 	/*
 	 * Some devices (Texel CD ROM drives) have handshaking problems
 	 * when used with the Seagate controllers. borken is initialized

-- Patrick Mansfield