leds: led-triggers: Improvements for default trigger

Accept "default" written to sysfs trigger attr.
If the text "default" is written to the LED's sysfs 'trigger' attr, then
call led_trigger_set_default() to set the LED to its default trigger.

If the default trigger is set to "none", then led_trigger_set_default()
will remove a trigger. This is in contrast to the default trigger being
unset, in which case led_trigger_set_default() does nothing.

Signed-off-by: Craig McQueen <[e-mail]>
Reviewed-by: Jacek Anaszewski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

Documentation/ABI/testing/sysfs-class-led

···
 		/sys/class/leds/<led> once a given trigger is selected. For
 		their documentation see `sysfs-class-led-trigger-*`.
 
+		Writing "none" removes the trigger for this LED.
+
+		Writing "default" sets the trigger to the LED's default trigger
+		(which would often be configured in the device tree for the
+		hardware).
+
 What:		/sys/class/leds/<led>/inverted
 Date:		January 2011
 KernelVersion:	2.6.38

drivers/leds/led-triggers.c

···
 		goto unlock;
 	}
 
+	if (sysfs_streq(buf, "default")) {
+		led_trigger_set_default(led_cdev);
+		goto unlock;
+	}
+
 	down_read(&triggers_list_lock);
 	list_for_each_entry(trig, &trigger_list, next_trig) {
 		if (sysfs_streq(buf, trig->name) && trigger_relevant(led_cdev, trig)) {
···
 	struct led_trigger *trig;
 	int len = led_trigger_snprintf(buf, size, "%s",
 				       led_cdev->trigger ? "none" : "[none]");
+
+	if (led_cdev->default_trigger)
+		len += led_trigger_snprintf(buf + len, size - len, " default");
 
 	list_for_each_entry(trig, &trigger_list, next_trig) {
 		bool hit;
···
 
 	if (!led_cdev->default_trigger)
 		return;
+
+	if (!strcmp(led_cdev->default_trigger, "none")) {
+		led_trigger_remove(led_cdev);
+		return;
+	}
 
 	down_read(&triggers_list_lock);
 	down_write(&led_cdev->trigger_lock);