[PATCH 10/18] scsi: implement reserved command handling

[Hannes Reinecke <hare@suse.de>]

Quite some drivers are using management commands internally, which
typically use the same hardware tag pool (ie they are being allocated
from the same hardware resources) as the 'normal' I/O commands.
These commands are set aside before allocating the block-mq tag bitmap,
so they'll never show up as busy in the tag map.
The block-layer, OTOH, already has 'reserved_tags' to handle precisely
this situation.
So this patch adds 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.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: John Garry <john.garry@huawei.com>
---
 drivers/scsi/hosts.c      |  3 +++
 drivers/scsi/scsi_lib.c   | 10 +++++++++-
 drivers/scsi/scsi_sysfs.c |  2 ++
 include/scsi/scsi_host.h  | 22 +++++++++++++++++++++-
 4 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index 2f162603876f..661ed7696562 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -469,6 +469,9 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
 	if (sht->virt_boundary_mask)
 		shost->virt_boundary_mask = sht->virt_boundary_mask;
 
+	if (sht->nr_reserved_cmds)
+		shost->nr_reserved_cmds = sht->nr_reserved_cmds;
+
 	device_initialize(&shost->shost_gendev);
 	dev_set_name(&shost->shost_gendev, "host%d", shost->host_no);
 	shost->shost_gendev.bus = &scsi_bus_type;
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index f83b04e49bae..3c83b0fabefb 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1971,7 +1971,8 @@ 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 = NUMA_NO_NODE;
 	tag_set->flags = BLK_MQ_F_SHOULD_MERGE;
@@ -1996,6 +1997,9 @@ void scsi_mq_destroy_tags(struct Scsi_Host *shost)
  * @flags: BLK_MQ_REQ_* flags, e.g. BLK_MQ_REQ_NOWAIT.
  *
  * Allocates a SCSI command for internal LLDD use.
+ * If 'nr_reserved_commands' is specified by the host the
+ * command will be allocated from the reserved tag pool;
+ * otherwise the normal tag pool will be used.
  */
 struct scsi_cmnd *scsi_get_internal_cmd(struct scsi_device *sdev,
 	unsigned int op, blk_mq_req_flags_t flags)
@@ -2005,6 +2009,10 @@ struct scsi_cmnd *scsi_get_internal_cmd(struct scsi_device *sdev,
 
 	WARN_ON_ONCE(((op & REQ_OP_MASK) != REQ_OP_SCSI_IN) &&
 		     ((op & REQ_OP_MASK) != REQ_OP_SCSI_OUT));
+
+	if (sdev->host->nr_reserved_cmds)
+		flags |= BLK_MQ_REQ_RESERVED;
+
 	rq = blk_mq_alloc_request(sdev->request_queue, op, flags);
 	if (IS_ERR(rq))
 		return NULL;
diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
index d5260d1b7b38..f4119999a402 100644
--- a/drivers/scsi/scsi_sysfs.c
+++ b/drivers/scsi/scsi_sysfs.c
@@ -371,6 +371,7 @@ static DEVICE_ATTR(eh_deadline, S_IRUGO | S_IWUSR, show_shost_eh_deadline, store
 shost_rd_attr(unique_id, "%u\n");
 shost_rd_attr(cmd_per_lun, "%hd\n");
 shost_rd_attr(can_queue, "%d\n");
+shost_rd_attr(nr_reserved_cmds, "%d\n");
 shost_rd_attr(sg_tablesize, "%hu\n");
 shost_rd_attr(sg_prot_tablesize, "%hu\n");
 shost_rd_attr(unchecked_isa_dma, "%d\n");
@@ -422,6 +423,7 @@ static struct attribute *scsi_sysfs_shost_attrs[] = {
 	&dev_attr_host_reset.attr,
 	&dev_attr_eh_deadline.attr,
 	&dev_attr_nr_hw_queues.attr,
+	&dev_attr_nr_reserved_cmds.attr,
 	NULL
 };
 
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index f115150559ca..0831b33ee186 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -367,10 +367,19 @@ 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
@@ -614,6 +623,11 @@ 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;
 	short cmd_per_lun;
 	short unsigned int sg_tablesize;
@@ -632,6 +646,12 @@ struct Scsi_Host {
 	 */
 	unsigned nr_hw_queues;
 	unsigned nr_maps;
+
+	/*
+	 * Number of reserved commands to allocate, if any.
+	 */
+	unsigned nr_reserved_cmds;
+
 	unsigned active_mode:2;
 	unsigned unchecked_isa_dma:1;
 
-- 
2.29.2