On Wed, 26 Sep 2012 20:17:07 +0530
"Philip, Avinash" <avinashphilip@xxxxxx> wrote:
> 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?
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