[PATCH 3/4] scsi: Support scsi_devices without a device wide limit

[Mike Christie <michael.christie@oracle.com>]

For virtio-scsi, we export a wide variety of non-scsi devices like
NVMe (local and RDMA/TCP based) drives and block based devices using
ublk. And then it's common to have multiple high perf devices im a LVM
volume. The problem for these setups, is we can easily hit the 4096
scsi_device queue depth limit so we end up throttling IO in the guest
when the real device can handle more IO.

In these situations we don't have a device wide limit that maps to
cmd_per_lun. We have per hw queue limits or on the host we are doing
more dynamic throttling. To allow for these types of devices, this
patch allows drivers to set SCSI_UNLIMITED_CMD_PER_LUN for the
cmd_per_lun. When set, we will then only be limited by the per hw
queue limits.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
---
 drivers/scsi/hosts.c     |  5 +++--
 drivers/scsi/scsi_scan.c | 25 ++++++++++++++-----------
 include/scsi/scsi_host.h |  4 ++++
 3 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index e047747d4ecf..c93c59e847c5 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -238,8 +238,9 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
 	}
 
 	/* Use min_t(int, ...) in case shost->can_queue exceeds SHRT_MAX */
-	shost->cmd_per_lun = min_t(int, shost->cmd_per_lun,
-				   shost->can_queue);
+	if (shost->cmd_per_lun != SCSI_UNLIMITED_CMD_PER_LUN)
+		shost->cmd_per_lun = min_t(int, shost->cmd_per_lun,
+					   shost->can_queue);
 
 	error = scsi_init_sense_cache(shost);
 	if (error)
diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
index 7b11bc7de0e3..ecc3638c1909 100644
--- a/drivers/scsi/scsi_scan.c
+++ b/drivers/scsi/scsi_scan.c
@@ -352,18 +352,20 @@ static struct scsi_device *scsi_alloc_sdev(struct scsi_target *starget,
 	if (scsi_device_is_pseudo_dev(sdev))
 		return sdev;
 
-	depth = sdev->host->cmd_per_lun ?: 1;
+	if (sdev->host->cmd_per_lun != SCSI_UNLIMITED_CMD_PER_LUN) {
+		depth = sdev->host->cmd_per_lun ?: 1;
 
-	/*
-	 * Use .can_queue as budget map's depth because we have to
-	 * support adjusting queue depth from sysfs. Meantime use
-	 * default device queue depth to figure out sbitmap shift
-	 * since we use this queue depth most of times.
-	 */
-	if (scsi_realloc_sdev_budget_map(sdev, depth))
-		goto out_device_destroy;
+		/*
+		 * Use .can_queue as budget map's depth because we have to
+		 * support adjusting queue depth from sysfs. Meantime use
+		 * default device queue depth to figure out sbitmap shift
+		 * since we use this queue depth most of times.
+		 */
+		if (scsi_realloc_sdev_budget_map(sdev, depth))
+			goto out_device_destroy;
 
-	scsi_change_queue_depth(sdev, depth);
+		scsi_change_queue_depth(sdev, depth);
+	}
 
 	if (shost->hostt->sdev_init) {
 		ret = shost->hostt->sdev_init(sdev);
@@ -1108,7 +1110,8 @@ static int scsi_add_lun(struct scsi_device *sdev, unsigned char *inq_result,
 	 * Set up budget map again since memory consumption of the map depends
 	 * on actual queue depth.
 	 */
-	if (hostt->sdev_configure)
+	if (hostt->sdev_configure &&
+	    sdev->host->cmd_per_lun != SCSI_UNLIMITED_CMD_PER_LUN)
 		scsi_realloc_sdev_budget_map(sdev, sdev->queue_depth);
 
 	if (sdev->scsi_level >= SCSI_3)
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index 7c747b566bc3..7555898dba25 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -443,6 +443,7 @@ struct scsi_host_template {
 	 */
 #define SCSI_DEFAULT_MAX_SECTORS	1024
 
+#define SCSI_UNLIMITED_CMD_PER_LUN	-1
 	/*
 	 * True if this host adapter can make good use of linked commands.
 	 * This will allow more than one command to be queued to a given
@@ -451,6 +452,9 @@ struct scsi_host_template {
 	 * command block per lun, 2 for two, etc.  Do not set this to 0.
 	 * You should make sure that the host adapter will do the right thing
 	 * before you try setting this above 1.
+	 *
+	 * Adapters that do not have a device limit can set this to
+	 * SCSI_UNLIMITED_CMD_PER_LUN.
 	 */
 	short cmd_per_lun;
 
-- 
2.47.1