[PATCH RFC v2 1/5] Allow SCSI LLDs to reserve block layer tags

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

blk_get_request() allocates a reserved tag if BLK_MQ_REQ_RESERVED has been
set in its third argument and a regular tag if that flag has not been set.
Make it possible for SCSI LLDs to mark a subset of the blk-mq tags as
'reserved'.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Yaniv Gardi <ygardi@codeaurora.org>
Cc: Subhash Jadavani <subhashj@codeaurora.org>
Cc: Stanley Chu <stanley.chu@mediatek.com>
Cc: Avri Altman <avri.altman@wdc.com>
Cc: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/scsi/scsi_lib.c  | 1 +
 include/scsi/scsi_host.h | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 2563b061f56b..b9ac7a93aafd 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1894,6 +1894,7 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
 		shost->tag_set.ops = &scsi_mq_ops_no_commit;
 	shost->tag_set.nr_hw_queues = shost->nr_hw_queues ? : 1;
 	shost->tag_set.queue_depth = shost->can_queue;
+	shost->tag_set.reserved_tags = shost->reserved_tags;
 	shost->tag_set.cmd_size = cmd_size;
 	shost->tag_set.numa_node = NUMA_NO_NODE;
 	shost->tag_set.flags = BLK_MQ_F_SHOULD_MERGE;
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index fccdf84ec7e2..abf1f0e59919 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -612,6 +612,8 @@ struct Scsi_Host {
 	 * is nr_hw_queues * can_queue.
 	 */
 	unsigned nr_hw_queues;
+	/* Number of tags that blk_mq_init_queue() should consider reserved. */
+	unsigned reserved_tags;
 	unsigned active_mode:2;
 	unsigned unchecked_isa_dma:1;
 
-- 
2.24.0.rc1.363.gb1bccd3e3d-goog