ata: set ncq_prio_enabled iff device has support

We previously had a check to see if the device has support for
prioritized ncq commands and a check to see if a device flag
is set, through a sysfs variable, in order to send a prioritized
command.

This patch only allows the sysfs variable to be set if the device
supports prioritized commands enabling one check in ata_build_rw_tf
in order to determine whether or not to send a prioritized command.

This patch depends on ata: ATA Command Priority Disabled By Default

tj: Minor subject and formatting updates.
Signed-off-by: NAdam Manzanares <adam.manzanares@wdc.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

drivers/ata/libata-core.c

@@ -787,8 +787,7 @@ int ata_build_rw_tf(struct ata_taskfile *tf, struct ata_device *dev,
 		if (tf->flags & ATA_TFLAG_FUA)
 			tf->device |= 1 << 7;
 
-		if ((dev->flags & ATA_DFLAG_NCQ_PRIO) &&
-		    (dev->flags & ATA_DFLAG_NCQ_PRIO_ENABLE)) {
+		if (dev->flags & ATA_DFLAG_NCQ_PRIO_ENABLE) {
 			if (class == IOPRIO_CLASS_RT)
 				tf->hob_nsect |= ATA_PRIO_HIGH <<
 						 ATA_SHIFT_PRIO;

drivers/ata/libata-scsi.c

@@ -323,10 +323,16 @@ static ssize_t ata_ncq_prio_enable_store(struct device *device,
 		goto unlock;
 	}
 
-	if (input)
+	if (input) {
+		if (!(dev->flags & ATA_DFLAG_NCQ_PRIO)) {
+			rc = -EOPNOTSUPP;
+			goto unlock;
+		}
+
 		dev->flags |= ATA_DFLAG_NCQ_PRIO_ENABLE;
-	else
+	} else {
 		dev->flags &= ~ATA_DFLAG_NCQ_PRIO_ENABLE;
+	}
 
 unlock:
 	spin_unlock_irqrestore(ap->lock, flags);