[PATCH RFC v3 03/22] scsi: core: Implement reserved command handling

[John Garry]

From: Hannes Reinecke <hare@xxxxxxx>

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@xxxxxxx>
#jpg: Set tag_set->queue_depth = shost->can_queue, and not
= shost->can_queue + shost->nr_reserved_cmds;
Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
---
 drivers/scsi/hosts.c     |  3 +++
 drivers/scsi/scsi_lib.c  |  2 ++
 include/scsi/scsi_host.h | 15 ++++++++++++++-
 3 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index 12346e2297fd..db89afc37bc9 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -489,6 +489,9 @@ 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 39d4fd124375..a8c4e7c037ae 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1978,6 +1978,8 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
 	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->reserved_tags = shost->nr_reserved_cmds;
+
 	tag_set->cmd_size = cmd_size;
 	tag_set->numa_node = dev_to_node(shost->dma_dev);
 	tag_set->flags = BLK_MQ_F_SHOULD_MERGE;
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index 750ccf126377..91678c77398e 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -360,10 +360,17 @@ 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
+	 * including reserved commands.
 	 */
 	int can_queue;
 
+	/*
+	 * This determines how many commands the HBA will set aside
+	 * for reserved commands.
+	 */
+	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
@@ -611,6 +618,12 @@ struct Scsi_Host {
 	 */
 	unsigned nr_hw_queues;
 	unsigned nr_maps;
+
+	/*
+	 * Number of reserved commands to allocate, if any.
+	 */
+	unsigned int nr_reserved_cmds;
+
 	unsigned active_mode:2;
 
 	/*
-- 
2.35.3