[SCSI] scsi_debug: add ability to enable clustering

This adds a module parameter to enable clustering.

Without enabling clustering support, the transfer length for read and
write scsi commands is limited upto 8MB when page size is 4KB and
sg_tablesize is 2048 (= SCSI_MAX_SG_CHAIN_SEGMENTS).  I would like to
test commands with more than that transfer length.
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/scsi_debug.c

@@ -196,6 +196,7 @@ static unsigned int scsi_debug_unmap_max_blocks = DEF_UNMAP_MAX_BLOCKS;
 static unsigned int scsi_debug_unmap_max_desc = DEF_UNMAP_MAX_DESC;
 static unsigned int scsi_debug_write_same_length = DEF_WRITESAME_LENGTH;
 static bool scsi_debug_removable = DEF_REMOVABLE;
+static bool scsi_debug_clustering;
 
 static int scsi_debug_cmnd_count = 0;
 
@@ -2762,6 +2763,7 @@ static int schedule_resp(struct scsi_cmnd * cmnd,
  */
 module_param_named(add_host, scsi_debug_add_host, int, S_IRUGO | S_IWUSR);
 module_param_named(ato, scsi_debug_ato, int, S_IRUGO);
+module_param_named(clustering, scsi_debug_clustering, bool, S_IRUGO | S_IWUSR);
 module_param_named(delay, scsi_debug_delay, int, S_IRUGO | S_IWUSR);
 module_param_named(dev_size_mb, scsi_debug_dev_size_mb, int, S_IRUGO);
 module_param_named(dif, scsi_debug_dif, int, S_IRUGO);
@@ -2805,6 +2807,7 @@ MODULE_VERSION(SCSI_DEBUG_VERSION);
 
 MODULE_PARM_DESC(add_host, "0..127 hosts allowed(def=1)");
 MODULE_PARM_DESC(ato, "application tag ownership: 0=disk 1=host (def=1)");
+MODULE_PARM_DESC(clustering, "when set enables larger transfers (def=0)");
 MODULE_PARM_DESC(delay, "# of jiffies to delay response(def=1)");
 MODULE_PARM_DESC(dev_size_mb, "size in MB of ram shared by devs(def=8)");
 MODULE_PARM_DESC(dif, "data integrity field type: 0-3 (def=0)");
@@ -3952,6 +3955,8 @@ static int sdebug_driver_probe(struct device * dev)
 	sdbg_host = to_sdebug_host(dev);
 
 	sdebug_driver_template.can_queue = scsi_debug_max_queue;
+	if (scsi_debug_clustering)
+		sdebug_driver_template.use_clustering = ENABLE_CLUSTERING;
 	hpnt = scsi_host_alloc(&sdebug_driver_template, sizeof(sdbg_host));
 	if (NULL == hpnt) {
 		printk(KERN_ERR "%s: scsi_register failed\n", __func__);