[SCSI] libsas: introduce scmd_dbg() to quiet false positive "timeout" messages

libsas sometimes short circuits timeouts to force commands into error
recovery.  It is misleading to log that the command timed-out in
sas_scsi_timed_out() when in fact it was just queued for error handling.
It's also redundant in the case of a true timeout as libata eh will
detect and report timeouts via it's AC_ERR_TIMEOUT facility.

Given that some environments consider "timeout" errors to be indicative
of impending device failure demote the sas_scsi_timed_out() timeout
message to be disabled by default.  This parallels ata_scsi_timed_out().

[jejb: checkpatch fix]
Reported-by: NXun Ni <xun.ni@intel.com>
Tested-by: NNelson Cheng <nelson.cheng@intel.com>
Acked-by: NLukasz Dorau <lukasz.dorau@intel.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>

drivers/scsi/libsas/sas_scsi_host.c

@@ -862,7 +862,7 @@ void sas_scsi_recover_host(struct Scsi_Host *shost)
 
 enum blk_eh_timer_return sas_scsi_timed_out(struct scsi_cmnd *cmd)
 {
-	scmd_printk(KERN_DEBUG, cmd, "command %p timed out\n", cmd);
+	scmd_dbg(cmd, "command %p timed out\n", cmd);
 
 	return BLK_EH_NOT_HANDLED;
 }

include/scsi/scsi_device.h

@@ -235,12 +235,24 @@ struct scsi_dh_data {
 #define sdev_printk(prefix, sdev, fmt, a...)	\
 	dev_printk(prefix, &(sdev)->sdev_gendev, fmt, ##a)
 
+#define sdev_dbg(sdev, fmt, a...) \
+	dev_dbg(&(sdev)->sdev_gendev, fmt, ##a)
+
 #define scmd_printk(prefix, scmd, fmt, a...)				\
         (scmd)->request->rq_disk ?					\
 	sdev_printk(prefix, (scmd)->device, "[%s] " fmt,		\
 		    (scmd)->request->rq_disk->disk_name, ##a) :		\
 	sdev_printk(prefix, (scmd)->device, fmt, ##a)
 
+#define scmd_dbg(scmd, fmt, a...)					   \
+	do {								   \
+		if ((scmd)->request->rq_disk)				   \
+			sdev_dbg((scmd)->device, "[%s] " fmt,		   \
+				 (scmd)->request->rq_disk->disk_name, ##a);\
+		else							   \
+			sdev_dbg((scmd)->device, fmt, ##a);		   \
+	} while (0)
+
 enum scsi_target_state {
 	STARGET_CREATED = 1,
 	STARGET_RUNNING,