backlight: Add backlight device (un)registration notification

Some firmware drivers, ie acpi-video want to get themselves out of the
way (in some cases) when their also is a raw backlight device available.

Due to module loading ordering being unknown, acpi-video cannot be certain
that the backlight_device_registered(BACKLIGHT_RAW) it does for this is
the final verdict wrt there being a BACKLIGHT_RAW device.

By adding notification acpi-video can listen for backlight devices showing
up after it has loaded, and unregister its backlight device if desired.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

2 files changed, 47 insertions, 0 deletions

diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
index bd2172c2d650..428089009cd5 100644
--- a/drivers/video/backlight/backlight.c
+++ b/drivers/video/backlight/backlight.c
@@ -23,6 +23,7 @@
 
 static struct list_head backlight_dev_list;
 static struct mutex backlight_dev_list_mutex;
+static struct blocking_notifier_head backlight_notifier;
 
 static const char *const backlight_types[] = {
         [BACKLIGHT_RAW] = "raw",
@@ -370,6 +371,9 @@ struct backlight_device *backlight_device_register(const char *name,
         list_add(&new_bd->entry, &backlight_dev_list);
         mutex_unlock(&backlight_dev_list_mutex);
 
+        blocking_notifier_call_chain(&backlight_notifier,
+                                     BACKLIGHT_REGISTERED, new_bd);
+
         return new_bd;
 }
 EXPORT_SYMBOL(backlight_device_register);
@@ -413,6 +417,10 @@ void backlight_device_unregister(struct backlight_device *bd)
                 pmac_backlight = NULL;
         mutex_unlock(&pmac_backlight_mutex);
 #endif
+
+        blocking_notifier_call_chain(&backlight_notifier,
+                                     BACKLIGHT_UNREGISTERED, bd);
+
         mutex_lock(&bd->ops_lock);
         bd->ops = NULL;
         mutex_unlock(&bd->ops_lock);
@@ -438,6 +446,36 @@ static int devm_backlight_device_match(struct device *dev, void *res,
 }
 
 /**
+ * backlight_register_notifier - get notified of backlight (un)registration
+ * @nb: notifier block with the notifier to call on backlight (un)registration
+ *
+ * @return 0 on success, otherwise a negative error code
+ *
+ * Register a notifier to get notified when backlight devices get registered
+ * or unregistered.
+ */
+int backlight_register_notifier(struct notifier_block *nb)
+{
+        return blocking_notifier_chain_register(&backlight_notifier, nb);
+}
+EXPORT_SYMBOL(backlight_register_notifier);
+
+/**
+ * backlight_unregister_notifier - unregister a backlight notifier
+ * @nb: notifier block to unregister
+ *
+ * @return 0 on success, otherwise a negative error code
+ *
+ * Register a notifier to get notified when backlight devices get registered
+ * or unregistered.
+ */
+int backlight_unregister_notifier(struct notifier_block *nb)
+{
+        return blocking_notifier_chain_unregister(&backlight_notifier, nb);
+}
+EXPORT_SYMBOL(backlight_unregister_notifier);
+
+/**
  * devm_backlight_device_register - resource managed backlight_device_register()
  * @dev: the device to register
  * @name: the name of the device
@@ -544,6 +582,8 @@ static int __init backlight_class_init(void)
         backlight_class->pm = &backlight_class_dev_pm_ops;
         INIT_LIST_HEAD(&backlight_dev_list);
         mutex_init(&backlight_dev_list_mutex);
+        BLOCKING_INIT_NOTIFIER_HEAD(&backlight_notifier);
+
         return 0;
 }
 
diff --git a/include/linux/backlight.h b/include/linux/backlight.h
index 72647429adf6..adb14a8616df 100644
--- a/include/linux/backlight.h
+++ b/include/linux/backlight.h
@@ -40,6 +40,11 @@ enum backlight_type {
         BACKLIGHT_TYPE_MAX,
 };
 
+enum backlight_notification {
+        BACKLIGHT_REGISTERED,
+        BACKLIGHT_UNREGISTERED,
+};
+
 struct backlight_device;
 struct fb_info;
 
@@ -133,6 +138,8 @@ extern void devm_backlight_device_unregister(struct device *dev,
 extern void backlight_force_update(struct backlight_device *bd,
                                    enum backlight_update_reason reason);
 extern bool backlight_device_registered(enum backlight_type type);
+extern int backlight_register_notifier(struct notifier_block *nb);
+extern int backlight_unregister_notifier(struct notifier_block *nb);
 
 #define to_backlight_device(obj) container_of(obj, struct backlight_device, dev)