leds: Introduce ordered workqueue for LEDs events instead of system_wq

This allows to setup ordered workqueue for LEDs events. This may be
useful, because default 'system_wq' does not guarantee execution order
of each work_struct, thus for several brightness update requests (for
multiple LEDs), real brightness switch could be in random order.

Yes, for sysfs-based LEDs we have flush_work() call inside
brightness_store() operation, but it's blocking call, so userspace
caller can be blocked at a long time, which means LEDs animation stream
can be broken.

Ordered workqueue has the same behaviour as system_wq + flush_work(),
but all scheduled works are async and userspace caller is not blocked,
which it better for userspace animation scheduling.

Signed-off-by: Alexey Romanov <[e-mail]>
Signed-off-by: Dmitry Rokosov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[Lee: Couple of style fix-ups]
Signed-off-by: Lee Jones <[e-mail]>

drivers/leds/led-class.c

···
 static DEFINE_MUTEX(leds_lookup_lock);
 static LIST_HEAD(leds_lookup_list);
 
+static struct workqueue_struct *leds_wq;
+
 static ssize_t brightness_show(struct device *dev,
 		struct device_attribute *attr, char *buf)
 {
···
 	if (state == LED_OFF)
 		led_trigger_remove(led_cdev);
 	led_set_brightness(led_cdev, state);
-	flush_work(&led_cdev->set_brightness_work);
 
 	ret = size;
 unlock:
···
 
 	led_update_brightness(led_cdev);
 
+	led_cdev->wq = leds_wq;
+
 	led_init_core(led_cdev);
 
 #ifdef CONFIG_LEDS_TRIGGERS
···
 
 static int __init leds_init(void)
 {
+	leds_wq = alloc_ordered_workqueue("leds", 0);
+	if (!leds_wq) {
+		pr_err("Failed to create LEDs ordered workqueue\n");
+		return -ENOMEM;
+	}
+
 	return class_register(&leds_class);
 }
 
 static void __exit leds_exit(void)
 {
 	class_unregister(&leds_class);
+	destroy_workqueue(leds_wq);
 }
 
 subsys_initcall(leds_init);

drivers/leds/led-core.c

···
 		led_cdev->delayed_delay_on = delay_on;
 		led_cdev->delayed_delay_off = delay_off;
 		set_bit(LED_SET_BLINK, &led_cdev->work_flags);
-		schedule_work(&led_cdev->set_brightness_work);
+		queue_work(led_cdev->wq, &led_cdev->set_brightness_work);
 		return;
 	}
 
···
 		 */
 		if (!brightness) {
 			set_bit(LED_BLINK_DISABLE, &led_cdev->work_flags);
-			schedule_work(&led_cdev->set_brightness_work);
+			queue_work(led_cdev->wq, &led_cdev->set_brightness_work);
 		} else {
 			set_bit(LED_BLINK_BRIGHTNESS_CHANGE,
 				&led_cdev->work_flags);
···
 		set_bit(LED_SET_BRIGHTNESS_OFF, &led_cdev->work_flags);
 	}
 
-	schedule_work(&led_cdev->set_brightness_work);
+	queue_work(led_cdev->wq, &led_cdev->set_brightness_work);
 }
 EXPORT_SYMBOL_GPL(led_set_brightness_nopm);
 

include/linux/leds.h

···
 	int			 new_blink_brightness;
 	void			(*flash_resume)(struct led_classdev *led_cdev);
 
+	struct workqueue_struct *wq;			/* LED workqueue */
 	struct work_struct	set_brightness_work;
 	int			delayed_set_value;
 	unsigned long		delayed_delay_on;