scsi: scsi_debug: Support host tagset

When host_max_queue is set (> 0), set the Scsi_Host.host_tagset such that
blk-mq will use a hostwide tagset over all SCSI host submission queues.

This means that we may expose all submission queues and always use the hwq
chosen by blk-mq.

And since if sdebug_host_max_queue is set, sdebug_max_queue is fixed to the
same value, we can simplify how sdebug_driver_template.can_queue is set.
Signed-off-by: John Garry <john.garry@huawei.com>
Tested-by: Douglas Gilbert <dgilbert@interlog.com>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/scsi/scsi_debug.c

@@ -4696,19 +4696,14 @@ static int resp_rwp_zone(struct scsi_cmnd *scp, struct sdebug_dev_info *devip)
 static struct sdebug_queue *get_queue(struct scsi_cmnd *cmnd)
 {
 	u16 hwq;
+	u32 tag = blk_mq_unique_tag(cmnd->request);
 
-	if (sdebug_host_max_queue) {
-		/* Provide a simple method to choose the hwq */
-		hwq = smp_processor_id() % submit_queues;
-	} else {
-		u32 tag = blk_mq_unique_tag(cmnd->request);
+	hwq = blk_mq_unique_tag_to_hwq(tag);
 
-		hwq = blk_mq_unique_tag_to_hwq(tag);
+	pr_debug("tag=%#x, hwq=%d\n", tag, hwq);
+	if (WARN_ON_ONCE(hwq >= submit_queues))
+		hwq = 0;
 
-		pr_debug("tag=%#x, hwq=%d\n", tag, hwq);
-		if (WARN_ON_ONCE(hwq >= submit_queues))
-			hwq = 0;
-	}
 	return sdebug_q_arr + hwq;
 }
 
@@ -7347,10 +7342,7 @@ static int sdebug_driver_probe(struct device *dev)
 
 	sdbg_host = to_sdebug_host(dev);
 
-	if (sdebug_host_max_queue)
-		sdebug_driver_template.can_queue = sdebug_host_max_queue;
-	else
-		sdebug_driver_template.can_queue = sdebug_max_queue;
+	sdebug_driver_template.can_queue = sdebug_max_queue;
 	if (!sdebug_clustering)
 		sdebug_driver_template.dma_boundary = PAGE_SIZE - 1;
 
@@ -7367,11 +7359,11 @@ static int sdebug_driver_probe(struct device *dev)
 	}
 	/*
 	 * Decide whether to tell scsi subsystem that we want mq. The
-	 * following should give the same answer for each host. If the host
-	 * has a limit of hostwide max commands, then do not set.
+	 * following should give the same answer for each host.
 	 */
-	if (!sdebug_host_max_queue)
-		hpnt->nr_hw_queues = submit_queues;
+	hpnt->nr_hw_queues = submit_queues;
+	if (sdebug_host_max_queue)
+		hpnt->host_tagset = 1;
 
 	sdbg_host->shost = hpnt;
 	*((struct sdebug_host_info **)hpnt->hostdata) = sdbg_host;