libata: Introduce ncq_prio_supported sysfs sttribute

Currently, the only way a user can determine if a SATA device supports
NCQ priority is to try to enable the use of this feature using the
ncq_prio_enable sysfs device attribute. If enabling the feature fails,
it is because the device does not support NCQ priority. Otherwise, the
feature is enabled and success indicates that the device supports NCQ
priority.

Improve this odd interface by introducing the read-only
ncq_prio_supported sysfs device attribute to indicate if a SATA device
supports NCQ priority. The value of this attribute reflects the status
of device flag ATA_DFLAG_NCQ_PRIO, which is set only for devices
supporting NCQ priority.

Add this new sysfs attribute to the device attributes group of libahci
and libata-sata.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Link: https://lore.kernel.org/r/20210816014456.2191776-10-damien.lemoal@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/ata/libahci.c

@@ -125,6 +125,7 @@ EXPORT_SYMBOL_GPL(ahci_shost_attrs);
 struct device_attribute *ahci_sdev_attrs[] = {
 	&dev_attr_sw_activity,
 	&dev_attr_unload_heads,
+	&dev_attr_ncq_prio_supported,
 	&dev_attr_ncq_prio_enable,
 	NULL
 };

drivers/ata/libata-sata.c

@@ -834,6 +834,30 @@ DEVICE_ATTR(link_power_management_policy, S_IRUGO | S_IWUSR,
 	    ata_scsi_lpm_show, ata_scsi_lpm_store);
 EXPORT_SYMBOL_GPL(dev_attr_link_power_management_policy);
 
+static ssize_t ata_ncq_prio_supported_show(struct device *device,
+					   struct device_attribute *attr,
+					   char *buf)
+{
+	struct scsi_device *sdev = to_scsi_device(device);
+	struct ata_port *ap = ata_shost_to_port(sdev->host);
+	struct ata_device *dev;
+	bool ncq_prio_supported;
+	int rc = 0;
+
+	spin_lock_irq(ap->lock);
+	dev = ata_scsi_find_dev(ap, sdev);
+	if (!dev)
+		rc = -ENODEV;
+	else
+		ncq_prio_supported = dev->flags & ATA_DFLAG_NCQ_PRIO;
+	spin_unlock_irq(ap->lock);
+
+	return rc ? rc : sysfs_emit(buf, "%u\n", ncq_prio_supported);
+}
+
+DEVICE_ATTR(ncq_prio_supported, S_IRUGO, ata_ncq_prio_supported_show, NULL);
+EXPORT_SYMBOL_GPL(dev_attr_ncq_prio_supported);
+
 static ssize_t ata_ncq_prio_enable_show(struct device *device,
 					struct device_attribute *attr,
 					char *buf)
@@ -901,6 +925,7 @@ EXPORT_SYMBOL_GPL(dev_attr_ncq_prio_enable);
 struct device_attribute *ata_ncq_sdev_attrs[] = {
 	&dev_attr_unload_heads,
 	&dev_attr_ncq_prio_enable,
+	&dev_attr_ncq_prio_supported,
 	NULL
 };
 EXPORT_SYMBOL_GPL(ata_ncq_sdev_attrs);

include/linux/libata.h

@@ -539,6 +539,7 @@ typedef void (*ata_postreset_fn_t)(struct ata_link *link, unsigned int *classes)
 extern struct device_attribute dev_attr_unload_heads;
 #ifdef CONFIG_SATA_HOST
 extern struct device_attribute dev_attr_link_power_management_policy;
+extern struct device_attribute dev_attr_ncq_prio_supported;
 extern struct device_attribute dev_attr_ncq_prio_enable;
 extern struct device_attribute dev_attr_em_message_type;
 extern struct device_attribute dev_attr_em_message;