leds: trigger: Store brightness set by led_trigger_event()

If a simple trigger is assigned to a LED, then the LED may be off until
the next led_trigger_event() call. This may be an issue for simple
triggers with rare led_trigger_event() calls, e.g. power supply
charging indicators (drivers/power/supply/power_supply_leds.c).
Therefore persist the brightness value of the last led_trigger_event()
call and use this value if the trigger is assigned to a LED.
In addition add a getter for the trigger brightness value.

Signed-off-by: Heiner Kallweit <[e-mail]>
Reviewed-by: Takashi Iwai <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/leds/led-triggers.c

···
 		spin_unlock(&trig->leddev_list_lock);
 		led_cdev->trigger = trig;
 
+		ret = 0;
 		if (trig->activate)
 			ret = trig->activate(led_cdev);
 		else
-			ret = 0;
-
+			led_set_brightness(led_cdev, trig->brightness);
 		if (ret)
 			goto err_activate;
 
···
 
 	if (!trig)
 		return;
+
+	trig->brightness = brightness;
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(led_cdev, &trig->led_cdevs, trig_list)

include/linux/leds.h

···
 	int		(*activate)(struct led_classdev *led_cdev);
 	void		(*deactivate)(struct led_classdev *led_cdev);
 
+	/* Brightness set by led_trigger_event */
+	enum led_brightness brightness;
+
 	/* LED-private triggers have this set */
 	struct led_hw_trigger_type *trigger_type;
 
···
 	return led_cdev->trigger_data;
 }
 
+static inline enum led_brightness
+led_trigger_get_brightness(const struct led_trigger *trigger)
+{
+	return trigger ? trigger->brightness : LED_OFF;
+}
+
 #define module_led_trigger(__led_trigger) \
 	module_driver(__led_trigger, led_trigger_register, \
 		      led_trigger_unregister)
···
 static inline void *led_get_trigger_data(struct led_classdev *led_cdev)
 {
 	return NULL;
+}
+
+static inline enum led_brightness
+led_trigger_get_brightness(const struct led_trigger *trigger)
+{
+	return LED_OFF;
 }
 
 #endif /* CONFIG_LEDS_TRIGGERS */