On Thu, Sep 27, 2012 at 02:33:09PM -0700, Andrew Morton wrote: > 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? Yes. This was discussed in the thread that followed the posting of this patch's v2. We've decided to drop it and go with brightness-levels only for device tree data. Eventually all existing users should convert to that as well so we can remove some of the cruft from the platform data up. Thierry
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