leds: add output inversion option to backlight trigger

Extend the LED backlight tirgger driver with an option that allows for
inverting the trigger output polarity.

With the invertion option provided, I (ab)use the backlight trigger for
driving a LED that indicates LCD display blank condtition on my Amstrad
Delta videophone.  Since the machine has no dedicated power LED, it was
not possible to distinguish if the display was blanked, or the machine was
turned off, without touching it.

The invert sysfs control is patterned after a similiar function of the GPIO
trigger driver.

[akpm@linux-foundation.org: make output match input, tighten input checking]
[akpm@linux-foundation.org: make output match input, tighten input checking]
Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Cc: Paul Mundt <lethal@linux-sh.org>
Acked-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-class-led b/Documentation/ABI/testing/sysfs-class-led
index 9e4541d71cb6..edff6630c805 100644
--- a/Documentation/ABI/testing/sysfs-class-led
+++ b/Documentation/ABI/testing/sysfs-class-led
@@ -26,3 +26,12 @@ Description:
                 scheduler is chosen. Trigger specific parameters can appear in
                 /sys/class/leds/<led> once a given trigger is selected.
 
+What:           /sys/class/leds/<led>/inverted
+Date:           January 2011
+KernelVersion:  2.6.38
+Contact:        Richard Purdie <rpurdie@rpsys.net>
+Description:
+                Invert the LED on/off state. This parameter is specific to
+                gpio and backlight triggers. In case of the backlight trigger,
+                it is usefull when driving a LED which is intended to indicate
+                a device in a standby like state.