leds: Add support to leds with readable status

Some led hardware allows drivers to query the led state, and this patch
adds a hook to let the led class take advantage of that information when
available.

Without this functionality, when access to the led hardware is not
exclusive (i.e. firmware or hardware might change its state behind the
kernel's back), reality goes out of sync with the led class' idea of what
the led is doing, which is annoying at best.

Behaviour for drivers that do not or cannot read the led status is
unchanged.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>

drivers/leds/led-class.c

@@ -24,6 +24,12 @@
 
 static struct class *leds_class;
 
+static void led_update_brightness(struct led_classdev *led_cdev)
+{
+	if (led_cdev->brightness_get)
+		led_cdev->brightness = led_cdev->brightness_get(led_cdev);
+}
+
 static ssize_t led_brightness_show(struct device *dev, 
 		struct device_attribute *attr, char *buf)
 {
@@ -31,6 +37,7 @@ static ssize_t led_brightness_show(struct device *dev,
 	ssize_t ret = 0;
 
 	/* no lock needed for this */
+	led_update_brightness(led_cdev);
 	sprintf(buf, "%u\n", led_cdev->brightness);
 	ret = strlen(buf) + 1;
 
@@ -113,6 +120,8 @@ int led_classdev_register(struct device *parent, struct led_classdev *led_cdev)
 	list_add_tail(&led_cdev->node, &leds_list);
 	up_write(&leds_list_lock);
 
+	led_update_brightness(led_cdev);
+
 #ifdef CONFIG_LEDS_TRIGGERS
 	init_rwsem(&led_cdev->trigger_lock);
 

include/linux/leds.h

@@ -37,6 +37,8 @@ struct led_classdev {
 	/* Set LED brightness level */
 	void		(*brightness_set)(struct led_classdev *led_cdev,
 					  enum led_brightness brightness);
+	/* Get LED brightness level */
+	enum led_brightness (*brightness_get)(struct led_classdev *led_cdev);
 
 	/* Activate hardware accelerated blink */
 	int		(*blink_set)(struct led_classdev *led_cdev,