scsi: add a CONFIG_SCSI_MQ_DEFAULT option

Add a Kconfig option to enable the blk-mq path for SCSI by default
to ease testing and deployment in setups that know they benefit
from blk-mq.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NMartin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: NRobert Elliott <elliott@hp.com>
Tested-by: NRobert Elliott <elliott@hp.com>

drivers/scsi/Kconfig

@@ -45,6 +45,17 @@ config SCSI_NETLINK
 	default	n
 	select NET
 
+config SCSI_MQ_DEFAULT
+	bool "SCSI: use blk-mq I/O path by default"
+	depends on SCSI
+	---help---
+	  This option enables the new blk-mq based I/O path for SCSI
+	  devices by default.  With the option the scsi_mod.use_blk_mq
+	  module/boot option defaults to Y, without it to N, but it can
+	  still be overriden either way.
+
+	  If unsure say N.
+
 config SCSI_PROC_FS
 	bool "legacy /proc/scsi/ support"
 	depends on SCSI && PROC_FS

drivers/scsi/scsi.c

@@ -1367,7 +1367,11 @@ MODULE_LICENSE("GPL");
 module_param(scsi_logging_level, int, S_IRUGO|S_IWUSR);
 MODULE_PARM_DESC(scsi_logging_level, "a bit mask of logging levels");
 
+#ifdef CONFIG_SCSI_MQ_DEFAULT
+bool scsi_use_blk_mq = true;
+#else
 bool scsi_use_blk_mq = false;
+#endif
 module_param_named(use_blk_mq, scsi_use_blk_mq, bool, S_IWUSR | S_IRUGO);
 
 static int __init init_scsi(void)