backlight: add backlight type

There may be multiple ways of controlling the backlight on a given
machine.  Allow drivers to expose the type of interface they are
providing, making it possible for userspace to make appropriate policy
decisions.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Airlie <airlied@linux.ie>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/stable/sysfs-class-backlight

@@ -34,3 +34,23 @@ Contact:	Richard Purdie <rpurdie@rpsys.net>
 Description:
 		Maximum brightness for <backlight>.
 Users:		HAL
+
+What:		/sys/class/backlight/<backlight>/type
+Date:		September 2010
+KernelVersion:	2.6.37
+Contact:	Matthew Garrett <mjg@redhat.com>
+Description:
+		The type of interface controlled by <backlight>.
+		"firmware": The driver uses a standard firmware interface
+		"platform": The driver uses a platform-specific interface
+		"raw": The driver controls hardware registers directly
+
+		In the general case, when multiple backlight
+		interfaces are available for a single device, firmware
+		control should be preferred to platform control should
+		be preferred to raw control. Using a firmware
+		interface reduces the probability of confusion with
+		the hardware and the OS independently updating the
+		backlight state. Platform interfaces are mostly a
+		holdover from pre-standardisation of firmware
+		interfaces.