[PATCH v2 1/3] leds: triggers: Allow to switch the trigger to "panic" on a kernel panic

[Ezequiel Garcia]

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 <ezequiel@xxxxxxxxxxxxxxxxxxxx>
---
 drivers/leds/led-triggers.c | 57 +++++++++++++++++++++++++++++++++++++++++++++
 include/linux/leds.h        |  1 +
 2 files changed, 58 insertions(+)

diff --git a/drivers/leds/led-triggers.c b/drivers/leds/led-triggers.c
index 2181581795d3..d27020daf711 100644
--- a/drivers/leds/led-triggers.c
+++ b/drivers/leds/led-triggers.c
@@ -148,6 +148,48 @@ void led_trigger_remove(struct led_classdev *led_cdev)
 }
 EXPORT_SYMBOL_GPL(led_trigger_remove);
 
+/*
+ * This is a 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,
+};
+
 void led_trigger_set_default(struct led_classdev *led_cdev)
 {
 	struct led_trigger *trig;
@@ -356,6 +398,21 @@ void led_trigger_unregister_simple(struct led_trigger *trig)
 }
 EXPORT_SYMBOL_GPL(led_trigger_unregister_simple);
 
+static int __init leds_trigger_init(void)
+{
+	atomic_notifier_chain_register(&panic_notifier_list,
+				       &led_trigger_panic_nb);
+	return 0;
+}
+
+static void __exit leds_trigger_exit(void)
+{
+	atomic_notifier_chain_unregister(&panic_notifier_list,
+					 &led_trigger_panic_nb);
+}
+module_init(leds_trigger_init);
+module_exit(leds_trigger_exit);
+
 MODULE_AUTHOR("Richard Purdie");
 MODULE_LICENSE("GPL");
 MODULE_DESCRIPTION("LED Triggers Core");
diff --git a/include/linux/leds.h b/include/linux/leds.h
index f203a8f89d30..49adf9c6e326 100644
--- a/include/linux/leds.h
+++ b/include/linux/leds.h
@@ -50,6 +50,7 @@ struct led_classdev {
 #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
-- 
2.7.0