[PATCH 05/24] scsi: core: Introduce scsi_host_update_can_queue()

[Bart Van Assche <bvanassche@acm.org>]

The UFS host controller driver must submit a QUERY REQUEST command to the
UFS device to query the queue depth supported by the device (bQueueDepth).
If the same infrastructure is used for allocating a QUERY REQUEST command
as for SCSI commands then .can_queue must initially be set to a low value
and increased once the queue depth is known. Hence this patch that adds a
function that modifies .can_queue.

Cc: Hannes Reinecke <hare@suse.de>
Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/scsi/scsi.c      | 26 ++++++++++++++++++++++++++
 include/scsi/scsi_host.h |  2 ++
 2 files changed, 28 insertions(+)

diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 53daf923ad8e..602cc32f139f 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -198,6 +198,32 @@ void scsi_finish_command(struct scsi_cmnd *cmd)
 	scsi_io_completion(cmd, good_bytes);
 }
 
+/**
+ * scsi_host_update_can_queue - Modify @host->can_queue
+ *
+ * @host->__devices must be empty and I/O must have been quiesced before this
+ * function is called.
+ */
+int scsi_host_update_can_queue(struct Scsi_Host *host, int can_queue)
+{
+	struct blk_mq_tag_set prev_set;
+	int prev_can_queue, ret;
+
+	if (!list_empty(&host->__devices))
+		return -EINVAL;
+
+	prev_can_queue = host->can_queue;
+	prev_set = host->tag_set;
+	host->can_queue = can_queue;
+	ret = scsi_mq_setup_tags(host);
+	if (ret) {
+		host->can_queue = prev_can_queue;
+		return ret;
+	}
+	blk_mq_free_tag_set(&prev_set);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(scsi_host_update_can_queue);
 
 /*
  * 4096 is big enough for saturating fast SCSI LUNs.
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index 2c0f5ec1046e..7b6aa36eac8a 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -830,6 +830,8 @@ extern void scsi_block_requests(struct Scsi_Host *);
 extern int scsi_host_block(struct Scsi_Host *shost);
 extern int scsi_host_unblock(struct Scsi_Host *shost, int new_state);
 
+int scsi_host_update_can_queue(struct Scsi_Host *host, int can_queue);
+
 void scsi_host_busy_iter(struct Scsi_Host *,
 			 bool (*fn)(struct scsi_cmnd *, void *), void *priv);