backlight: introduce backlight_device_registered

Introduce a new API for modules to query if a specific type of backlight
device has been registered. This is useful for some backlight device
provider module (e.g. ACPI video) to know if a native control
interface(e.g. the interface created by i915) is available and then do
things accordingly (e.g. avoid registering its own on Win8 systems).

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

2 files changed, 35 insertions, 0 deletions

diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
index 94a403a9717a..5d05555fe841 100644
--- a/drivers/video/backlight/backlight.c
+++ b/drivers/video/backlight/backlight.c
@@ -21,6 +21,9 @@
 #include <asm/backlight.h>
 #endif
 
+static struct list_head backlight_dev_list;
+static struct mutex backlight_dev_list_mutex;
+
 static const char *const backlight_types[] = {
         [BACKLIGHT_RAW] = "raw",
         [BACKLIGHT_PLATFORM] = "platform",
@@ -349,10 +352,32 @@ struct backlight_device *backlight_device_register(const char *name,
         mutex_unlock(&pmac_backlight_mutex);
 #endif
 
+        mutex_lock(&backlight_dev_list_mutex);
+        list_add(&new_bd->entry, &backlight_dev_list);
+        mutex_unlock(&backlight_dev_list_mutex);
+
         return new_bd;
 }
 EXPORT_SYMBOL(backlight_device_register);
 
+bool backlight_device_registered(enum backlight_type type)
+{
+        bool found = false;
+        struct backlight_device *bd;
+
+        mutex_lock(&backlight_dev_list_mutex);
+        list_for_each_entry(bd, &backlight_dev_list, entry) {
+                if (bd->props.type == type) {
+                        found = true;
+                        break;
+                }
+        }
+        mutex_unlock(&backlight_dev_list_mutex);
+
+        return found;
+}
+EXPORT_SYMBOL(backlight_device_registered);
+
 /**
  * backlight_device_unregister - unregisters a backlight device object.
  * @bd: the backlight device object to be unregistered and freed.
@@ -364,6 +389,10 @@ void backlight_device_unregister(struct backlight_device *bd)
         if (!bd)
                 return;
 
+        mutex_lock(&backlight_dev_list_mutex);
+        list_del(&bd->entry);
+        mutex_unlock(&backlight_dev_list_mutex);
+
 #ifdef CONFIG_PMAC_BACKLIGHT
         mutex_lock(&pmac_backlight_mutex);
         if (pmac_backlight == bd)
@@ -499,6 +528,8 @@ static int __init backlight_class_init(void)
 
         backlight_class->dev_groups = bl_device_groups;
         backlight_class->pm = &backlight_class_dev_pm_ops;
+        INIT_LIST_HEAD(&backlight_dev_list);
+        mutex_init(&backlight_dev_list_mutex);
         return 0;
 }
 
diff --git a/include/linux/backlight.h b/include/linux/backlight.h
index 53b77949c79d..5f9cd963213d 100644
--- a/include/linux/backlight.h
+++ b/include/linux/backlight.h
@@ -100,6 +100,9 @@ struct backlight_device {
         /* The framebuffer notifier block */
         struct notifier_block fb_notif;
 
+        /* list entry of all registered backlight devices */
+        struct list_head entry;
+
         struct device dev;
 };
 
@@ -123,6 +126,7 @@ extern void devm_backlight_device_unregister(struct device *dev,
                                         struct backlight_device *bd);
 extern void backlight_force_update(struct backlight_device *bd,
                                    enum backlight_update_reason reason);
+extern bool backlight_device_registered(enum backlight_type type);
 
 #define to_backlight_device(obj) container_of(obj, struct backlight_device, dev)