leds: triggers: Allow to switch the trigger to "panic" on a kernel panic

This commit adds a new led_cdev flag LED_PANIC_INDICATOR, which
allows to mark a specific LED to be switched to the "panic"
trigger, on a kernel panic.

This is useful to allow the user to assign a regular trigger
to a given LED, and still blink that LED on a kernel panic.

Signed-off-by: Ezequiel Garcia <[e-mail]>
Reviewed-by: Matthias Brugger <[e-mail]>
Acked-by: Pavel Machek <[e-mail]>
Signed-off-by: Jacek Anaszewski <[e-mail]>

drivers/leds/led-triggers.c

···
  * Nests outside led_cdev->trigger_lock
  */
 static DECLARE_RWSEM(triggers_list_lock);
-static LIST_HEAD(trigger_list);
+LIST_HEAD(trigger_list);
 
  /* Used by LED Class */
 

drivers/leds/leds.h

···
 
 extern struct rw_semaphore leds_list_lock;
 extern struct list_head leds_list;
+extern struct list_head trigger_list;
 
 #endif	/* __LEDS_H_INCLUDED */

drivers/leds/trigger/Kconfig

···
 	depends on LEDS_TRIGGERS
 	help
 	  This allows LEDs to be configured to blink on a kernel panic.
+	  Enabling this option will allow to mark certain LEDs as panic indicators,
+	  allowing to blink them on a kernel panic, even if they are set to
+	  a different trigger.
 	  If unsure, say Y.
 
 endif # LEDS_TRIGGERS

drivers/leds/trigger/ledtrig-panic.c

···
 
 #include <linux/kernel.h>
 #include <linux/init.h>
+#include <linux/notifier.h>
 #include <linux/leds.h>
+#include "../leds.h"
 
 static struct led_trigger *trigger;
+
+/*
+ * This is called in a special context by the atomic panic
+ * notifier. This means the trigger can be changed without
+ * worrying about locking.
+ */
+static void led_trigger_set_panic(struct led_classdev *led_cdev)
+{
+	struct led_trigger *trig;
+
+	list_for_each_entry(trig, &trigger_list, next_trig) {
+		if (strcmp("panic", trig->name))
+			continue;
+		if (led_cdev->trigger)
+			list_del(&led_cdev->trig_list);
+		list_add_tail(&led_cdev->trig_list, &trig->led_cdevs);
+
+		/* Avoid the delayed blink path */
+		led_cdev->blink_delay_on = 0;
+		led_cdev->blink_delay_off = 0;
+
+		led_cdev->trigger = trig;
+		if (trig->activate)
+			trig->activate(led_cdev);
+		break;
+	}
+}
+
+static int led_trigger_panic_notifier(struct notifier_block *nb,
+				      unsigned long code, void *unused)
+{
+	struct led_classdev *led_cdev;
+
+	list_for_each_entry(led_cdev, &leds_list, node)
+		if (led_cdev->flags & LED_PANIC_INDICATOR)
+			led_trigger_set_panic(led_cdev);
+	return NOTIFY_DONE;
+}
+
+static struct notifier_block led_trigger_panic_nb = {
+	.notifier_call = led_trigger_panic_notifier,
+};
 
 static long led_panic_blink(int state)
 {
···
 
 static int __init ledtrig_panic_init(void)
 {
+	atomic_notifier_chain_register(&panic_notifier_list,
+				       &led_trigger_panic_nb);
+
 	led_trigger_register_simple("panic", &trigger);
 	panic_blink = led_panic_blink;
 	return 0;

include/linux/leds.h

···
 #define LED_SYSFS_DISABLE	(1 << 22)
 #define LED_DEV_CAP_FLASH	(1 << 23)
 #define LED_HW_PLUGGABLE	(1 << 24)
+#define LED_PANIC_INDICATOR	(1 << 25)
 
 	/* Set LED brightness level
 	 * Must not sleep. Use brightness_set_blocking for drivers