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>

1 files changed, 20 insertions, 0 deletions

diff --git a/Documentation/ABI/stable/sysfs-class-backlight b/Documentation/ABI/stable/sysfs-class-backlight
index 4d637e1c4ff7..70302f370e7e 100644
--- a/Documentation/ABI/stable/sysfs-class-backlight
+++ b/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.