On Wed, 26 Sep 2012 20:17:07 +0530
"Philip, Avinash" [off-list ref] wrote:

quoted

Some back lights perform poorly when driven by a PWM with a short
duty-cycle. For such devices, the low threshold can be used to specify a
lower bound for the duty-cycle and should be chosen to exclude the
problematic range.

Add device tree probing support for lth_brightness putting
low-threshold-brightness as optional property.

...

--- a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
+++ b/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt

@@ -14,6 +14,15 @@ Required properties:
 Optional properties:
   - pwm-names: a list of names for the PWM devices specified in the
                "pwms" property (see PWM binding[0])
+  - low-threshold-brightness: brightness threshold low level. Low threshold
+    brightness set to value so that backlight present on low end of
+    brightness.
+    Some panels, backlight would absent if duty percentage of PWM wave is less
+    than certain level (say 20%). By setting low-threshold-brightness to a
+    value above (percentage of brightness-levels max) 50 (20% of 255, if 255
+    is max). On setting low-threshold-brightness, range of brightness-levels
+    is calculated in a region of low-threshold-brightness to brightness-levels
+    max.

hoo boy, that's hard to follow.  How does this look?

--- a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt~pwm_backlight-add-device-tree-support-for-low-threshold-brightness-fix
+++ a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt

@@ -14,15 +14,15 @@ Required properties:
 Optional properties:
   - pwm-names: a list of names for the PWM devices specified in the
                "pwms" property (see PWM binding[0])
-  - low-threshold-brightness: brightness threshold low level. Low threshold
-    brightness set to value so that backlight present on low end of
-    brightness.
-    Some panels, backlight would absent if duty percentage of PWM wave is less
-    than certain level (say 20%). By setting low-threshold-brightness to a
-    value above (percentage of brightness-levels max) 50 (20% of 255, if 255
-    is max). On setting low-threshold-brightness, range of brightness-levels
-    is calculated in a region of low-threshold-brightness to brightness-levels
-    max.
+  - low-threshold-brightness: brightness threshold low level. Sets the lowest
+    brightness value.
+    On some panels the backlight misbehaves if the duty cycle percentage of the
+    PWM wave is less than a certain level (say 20%).  In this example the user
+    can set low-threshold-brightness to a value above 50 (ie, 20% of 255), thus
+    preventing the PWM duty cycle from going too low.
+    On setting low-threshold-brightness the range of brightness levels is
+    calculated in the range low-threshold-brightness to the maximum value in
+    brightness-levels, described above.
 
 [0]: Documentation/devicetree/bindings/pwm/pwm.txt
 

Also, I'm wondering if we really needed a new property - couldn't one
do this simply by setting brightness-levels to 50..255?