1 files changed, 26 insertions, 0 deletions

diff --git a/include/linux/backlight.h b/include/linux/backlight.h
index 287c62d956f2..d1426b852bdf 100644
--- a/include/linux/backlight.h
+++ b/include/linux/backlight.h
@@ -9,8 +9,24 @@
 #define _LINUX_BACKLIGHT_H
 
 #include <linux/device.h>
+#include <linux/mutex.h>
 #include <linux/notifier.h>
 
+/* Notes on locking:
+ *
+ * backlight_device->sem is an internal backlight lock protecting the props
+ * field and no code outside the core should need to touch it.
+ *
+ * Access to update_status() is serialised by the update_lock mutex since
+ * most drivers seem to need this and historically get it wrong.
+ *
+ * Most drivers don't need locking on their get_brightness() method.
+ * If yours does, you need to implement it in the driver. You can use the
+ * update_lock mutex if appropriate.
+ *
+ * Any other use of the locks below is probably wrong.
+ */
+
 struct backlight_device;
 struct fb_info;
 
@@ -44,12 +60,22 @@ struct backlight_device {
         struct semaphore sem;
         /* If this is NULL, the backing module is unloaded */
         struct backlight_properties *props;
+        /* Serialise access to update_status method */
+        struct mutex update_lock;
         /* The framebuffer notifier block */
         struct notifier_block fb_notif;
         /* The class device structure */
         struct class_device class_dev;
 };
 
+static inline void backlight_update_status(struct backlight_device *bd)
+{
+        mutex_lock(&bd->update_lock);
+        if (bd->props && bd->props->update_status)
+                bd->props->update_status(bd);
+        mutex_unlock(&bd->update_lock);
+}
+
 extern struct backlight_device *backlight_device_register(const char *name,
         struct device *dev,void *devdata,struct backlight_properties *bp);
 extern void backlight_device_unregister(struct backlight_device *bd);