scsi: core: Support allocating reserved commands

Quite some drivers are using management commands internally. These
commands typically use the same tag pool as regular SCSI commands. Tags
for these management commands are set aside before allocating the
block-mq tag bitmap for regular SCSI commands. The block layer already
supports this via the reserved tag mechanism. Add a new field
'nr_reserved_cmds' to the SCSI host template to instruct the block layer
to set aside a tag space for these management commands by using reserved
tags. Exclude reserved commands from .can_queue because .can_queue is
visible in sysfs.

[ bvanassche: modified patch title and patch description. Left out the
  following statements: "if (sht->nr_reserved_cmds)" and also
  "if (sdev->host->nr_reserved_cmds) flags |= BLK_MQ_REQ_RESERVED;". Moved
  nr_reserved_cmds declarations and statements close to the
  corresponding can_queue declarations and statements. See also
  https://lore.kernel.org/linux-scsi/20210503150333.130310-11-hare@suse.de/ ]

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://patch.msgid.link/20251031204029.2883185-2-bvanassche@acm.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

3 files changed, 23 insertions, 2 deletions

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index eb224a338fa2..8b7f5fafa9e0 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -436,6 +436,7 @@ struct Scsi_Host *scsi_host_alloc(const struct scsi_host_template *sht, int priv
 	shost->hostt = sht;
 	shost->this_id = sht->this_id;
 	shost->can_queue = sht->can_queue;
+	shost->nr_reserved_cmds = sht->nr_reserved_cmds;
 	shost->sg_tablesize = sht->sg_tablesize;
 	shost->sg_prot_tablesize = sht->sg_prot_tablesize;
 	shost->cmd_per_lun = sht->cmd_per_lun;
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index d7e42293b864..d52bbbe5a357 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -2083,7 +2083,8 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
 		tag_set->ops = &scsi_mq_ops_no_commit;
 	tag_set->nr_hw_queues = shost->nr_hw_queues ? : 1;
 	tag_set->nr_maps = shost->nr_maps ? : 1;
-	tag_set->queue_depth = shost->can_queue;
+	tag_set->queue_depth = shost->can_queue + shost->nr_reserved_cmds;
+	tag_set->reserved_tags = shost->nr_reserved_cmds;
 	tag_set->cmd_size = cmd_size;
 	tag_set->numa_node = dev_to_node(shost->dma_dev);
 	if (shost->hostt->tag_alloc_policy_rr)
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index f5a243261236..7b8f144ccf7d 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -375,11 +375,20 @@ struct scsi_host_template {
 	/*
 	 * This determines if we will use a non-interrupt driven
 	 * or an interrupt driven scheme.  It is set to the maximum number
-	 * of simultaneous commands a single hw queue in HBA will accept.
+	 * of simultaneous commands a single hw queue in HBA will accept
+	 * excluding internal commands.
 	 */
 	int can_queue;
 
 	/*
+	 * This determines how many commands the HBA will set aside
+	 * for internal commands. This number will be added to
+	 * @can_queue to calculate the maximum number of simultaneous
+	 * commands sent to the host.
+	 */
+	int nr_reserved_cmds;
+
+	/*
 	 * In many instances, especially where disconnect / reconnect are
 	 * supported, our host also has an ID on the SCSI bus.  If this is
 	 * the case, then it must be reserved.  Please set this_id to -1 if
@@ -611,7 +620,17 @@ struct Scsi_Host {
 	unsigned short max_cmd_len;
 
 	int this_id;
+
+	/*
+	 * Number of commands this host can handle at the same time.
+	 * This excludes reserved commands as specified by nr_reserved_cmds.
+	 */
 	int can_queue;
+	/*
+	 * Number of reserved commands to allocate, if any.
+	 */
+	unsigned int nr_reserved_cmds;
+
 	short cmd_per_lun;
 	short unsigned int sg_tablesize;
 	short unsigned int sg_prot_tablesize;