watchdog: report fw_version in sysfs

This synchronizes the information reported by ioctl and sysfs.
The mismatch is confusing because "wdctl" from util-linux uses the ioctl
when used with root privileges and sysfs without.

The file is called "fw_version" instead of "firmware_version" as
"firmware_version" is already used as custom attribute by single drivers.
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20221216-watchdog-sysfs-v2-1-6189311103a9@weissschuh.netSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

Documentation/ABI/testing/sysfs-class-watchdog

@@ -6,6 +6,13 @@ Description:
 		device at boot. It is equivalent to WDIOC_GETBOOTSTATUS of
 		ioctl interface.
 
+What:		/sys/class/watchdog/watchdogn/fw_version
+Date:		April 2023
+Contact:	Thomas Weißschuh
+Description:
+		It is a read only file. It contains firmware version of
+		watchdog device.
+
 What:		/sys/class/watchdog/watchdogn/identity
 Date:		August 2015
 Contact:	Wim Van Sebroeck <wim@iguana.be>

drivers/watchdog/watchdog_dev.c

@@ -547,6 +547,15 @@ static ssize_t pretimeout_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(pretimeout);
 
+static ssize_t fw_version_show(struct device *dev, struct device_attribute *attr,
+			       char *buf)
+{
+	struct watchdog_device *wdd = dev_get_drvdata(dev);
+
+	return sysfs_emit(buf, "%d\n", wdd->info->firmware_version);
+}
+static DEVICE_ATTR_RO(fw_version);
+
 static ssize_t identity_show(struct device *dev, struct device_attribute *attr,
 				char *buf)
 {
@@ -618,6 +627,7 @@ static umode_t wdt_is_visible(struct kobject *kobj, struct attribute *attr,
 }
 static struct attribute *wdt_attrs[] = {
 	&dev_attr_state.attr,
+	&dev_attr_fw_version.attr,
 	&dev_attr_identity.attr,
 	&dev_attr_timeout.attr,
 	&dev_attr_min_timeout.attr,