[PATCH v4 2/3] scsi: core: Support disabling fair tag sharing

[Bart Van Assche <bvanassche@xxxxxxx>]

Allow SCSI drivers to disable the block layer fair tag sharing algorithm.

Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Cc: Keith Busch <kbusch@xxxxxxxxxx>
Cc: Damien Le Moal <damien.lemoal@xxxxxxxxxxxxxxxxxx>
Cc: Yu Kuai <yukuai1@xxxxxxxxxxxxxxx>
Cc: Ed Tsai <ed.tsai@xxxxxxxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 drivers/scsi/hosts.c     | 1 +
 drivers/scsi/scsi_lib.c  | 2 ++
 include/scsi/scsi_host.h | 6 ++++++
 3 files changed, 9 insertions(+)

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index d7f51b84f3c7..872f87001374 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -442,6 +442,7 @@ struct Scsi_Host *scsi_host_alloc(const struct scsi_host_template *sht, int priv
 	shost->no_write_same = sht->no_write_same;
 	shost->host_tagset = sht->host_tagset;
 	shost->queuecommand_may_block = sht->queuecommand_may_block;
+	shost->disable_fair_tag_sharing = sht->disable_fair_tag_sharing;
 
 	if (shost_eh_deadline == -1 || !sht->eh_host_reset_handler)
 		shost->eh_deadline = -1;
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index c2f647a7c1b0..68683988c466 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1985,6 +1985,8 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
 		BLK_ALLOC_POLICY_TO_MQ_FLAG(shost->hostt->tag_alloc_policy);
 	if (shost->queuecommand_may_block)
 		tag_set->flags |= BLK_MQ_F_BLOCKING;
+	if (shost->disable_fair_tag_sharing)
+		tag_set->flags |= BLK_MQ_F_DISABLE_FAIR_TAG_SHARING;
 	tag_set->driver_data = shost;
 	if (shost->host_tagset)
 		tag_set->flags |= BLK_MQ_F_TAG_HCTX_SHARED;
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index 49f768d0ff37..2cd8a41b1f1c 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -461,6 +461,9 @@ struct scsi_host_template {
 	/* The queuecommand callback may block. See also BLK_MQ_F_BLOCKING. */
 	unsigned queuecommand_may_block:1;
 
+	/* See also BLK_MQ_F_DISABLE_FAIR_TAG_SHARING. */
+	unsigned disable_fair_tag_sharing:1;
+
 	/*
 	 * Countdown for host blocking with no commands outstanding.
 	 */
@@ -659,6 +662,9 @@ struct Scsi_Host {
 	/* The queuecommand callback may block. See also BLK_MQ_F_BLOCKING. */
 	unsigned queuecommand_may_block:1;
 
+	/* See also BLK_MQ_F_DISABLE_FAIR_TAG_SHARING. */
+	unsigned disable_fair_tag_sharing:1;
+
 	/* Host responded with short (<36 bytes) INQUIRY result */
 	unsigned short_inquiry:1;