backlight: Fix external uses of backlight internal semaphore

backlight_device->sem has a very specific use as documented in the
header file. The external users of this are using it for a different
reason, to serialise access to the update_status() method.

backlight users were supposed to implement their own internal
serialisation of update_status() if needed but everyone is doing
things differently and incorrectly. Therefore add a global mutex to
take care of serialisation for everyone, once and for all.

Locking for get_brightness remains optional since most users don't
need it.

Also update the lcd class in a similar way.

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>

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);