[PATCH V8 12/13] scsi: make sure sdev->queue_depth is <= max(shost->can_queue, 1024)

[Ming Lei <ming.lei@redhat.com>]

Limit scsi device's queue depth is less than max(host->can_queue, 1024)
in scsi_change_queue_depth(), and 1024 is big enough for saturating
current fast SCSI LUN(SSD, or raid volume on multiple SSDs). Also
single hw queue depth is usually enough for saturating single LUN
because per-core performance is often considered in storage design.

We need this patch for replacing sdev->device_busy with sbitmap which
has to be pre-allocated with reasonable max depth.

Cc: Omar Sandoval <osandov@fb.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Cc: Ewan D. Milne <emilne@redhat.com>
Tested-by: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/scsi/scsi.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 24619c3bebd5..a28d48c850cf 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -214,6 +214,15 @@ void scsi_finish_command(struct scsi_cmnd *cmd)
 	scsi_io_completion(cmd, good_bytes);
 }
 
+
+/*
+ * 1024 is big enough for saturating the fast scsi LUN now
+ */
+static int scsi_device_max_queue_depth(struct scsi_device *sdev)
+{
+	return max_t(int, sdev->host->can_queue, 1024);
+}
+
 /**
  * scsi_change_queue_depth - change a device's queue depth
  * @sdev: SCSI Device in question
@@ -223,6 +232,8 @@ void scsi_finish_command(struct scsi_cmnd *cmd)
  */
 int scsi_change_queue_depth(struct scsi_device *sdev, int depth)
 {
+	depth = min_t(int, depth, scsi_device_max_queue_depth(sdev));
+
 	if (depth > 0) {
 		sdev->queue_depth = depth;
 		wmb();
-- 
2.29.2