[SCSI] iscsi: lower queue depth

From Wang Zhenyu:

High queue depth was a problem for some targets so make queue_depth adjustable

From Mike Christie

Make default queue_depth a little lower
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Alex Aizman <itn780@yahoo.com>
Signed-off-by: Dmitry Yusupov <dmitry_yus@yahoo.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>

drivers/scsi/iscsi_tcp.c

@@ -2401,6 +2401,15 @@ iscsi_queuecommand(struct scsi_cmnd *sc, void (*done)(struct scsi_cmnd *))
 	return 0;
 }
 
+static int
+iscsi_change_queue_depth(struct scsi_device *sdev, int depth)
+{
+	if (depth > ISCSI_MAX_CMD_PER_LUN)
+		depth = ISCSI_MAX_CMD_PER_LUN;
+	scsi_adjust_queue_depth(sdev, scsi_get_tag_type(sdev), depth);
+	return sdev->queue_depth;
+}
+
 static int
 iscsi_pool_init(struct iscsi_queue *q, int max, void ***items, int item_size)
 {
@@ -3260,13 +3269,14 @@ iscsi_r2tpool_free(struct iscsi_session *session)
 static struct scsi_host_template iscsi_sht = {
 	.name			= "iSCSI Initiator over TCP/IP, v."
 				  ISCSI_VERSION_STR,
-        .queuecommand           = iscsi_queuecommand,
+	.queuecommand           = iscsi_queuecommand,
+	.change_queue_depth	= iscsi_change_queue_depth,
 	.can_queue		= ISCSI_XMIT_CMDS_MAX - 1,
 	.sg_tablesize		= ISCSI_SG_TABLESIZE,
-	.cmd_per_lun		= ISCSI_CMD_PER_LUN,
-        .eh_abort_handler       = iscsi_eh_abort,
-        .eh_host_reset_handler	= iscsi_eh_host_reset,
-        .use_clustering         = DISABLE_CLUSTERING,
+	.cmd_per_lun		= ISCSI_DEF_CMD_PER_LUN,
+	.eh_abort_handler       = iscsi_eh_abort,
+	.eh_host_reset_handler	= iscsi_eh_host_reset,
+	.use_clustering         = DISABLE_CLUSTERING,
 	.proc_name		= "iscsi_tcp",
 	.this_id		= -1,
 };

drivers/scsi/iscsi_tcp.h

@@ -71,7 +71,8 @@
 #define ISCSI_MGMT_CMDS_MAX		32	/* must be power of 2 */
 #define ISCSI_MGMT_ITT_OFFSET		0xa00
 #define ISCSI_SG_TABLESIZE		SG_ALL
-#define ISCSI_CMD_PER_LUN		128
+#define ISCSI_DEF_CMD_PER_LUN		32
+#define ISCSI_MAX_CMD_PER_LUN		128
 #define ISCSI_TCP_MAX_CMD_LEN		16
 
 #define ITT_MASK			(0xfff)