On Fri, Sep 21, 2012 at 23:13:39, Stephen Warren wrote: > On 09/21/2012 12:03 AM, Philip, Avinash wrote: > > Hi Stephen, > > > > On Fri, Sep 21, 2012 at 10:46:45, Stephen Warren wrote: > >> On 09/20/2012 10:51 PM, Philip, Avinash wrote: > >>> Some backlights 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. > >>> > >>> This patch adds support for an optional low-threshold-brightness > >>> property. > >> > >>> diff --git a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt > >> > >>> 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. > >> > >> For my education, why not just specify values above this value in the > >> brightness-levels array; how do those two interact? > > > > Please find details from > > https://lkml.org/lkml/2012/7/18/284 > > Hmm. That still doesn't really explain what this property does. > > I'm going to guess that if this property is present, and values in the > brightness-levels property get scaled between the > low-threshold-brightness and 255 instead of being used directly. This is correct. > But then, in the email you linked to, what does "But brightness-levels won't > be uniformly divided" mean? For some panels, backlight would absent on low end of brightness due to low percentage in duty_cycle. Consider following example where backlight absent for brightness levels from 0 - 51. pwms = <&pwm 0 50000>; brightness-levels = <0 51 53 56 62 75 101 152 255>; default-brightness-level = <6>; So in the example, brightness-levels are set to have values for backlight present. Here levels are not uniformly divided. But by providing, brightness_threshold_level = <50>; we can have a uniform division in brightness-levels as below brightness-levels = <0 2 4 8 16 32 64 128 255>; > Why would doing the calculation at run-time > be any better than simply putting the correct values into > brightness-levels in the first place? I think you mean to provide the backlight-levels as in example brightness-levels = <0 51 53 56 62 75 101 152 255>; In that case, I still feel adding brightness_threshold_level would be a better option. > > Either way, the DT binding should explain exactly what this value is > used for, and how it affects the interpretation of values in > brightness-levels. Is DT binding documentation a good place to explain this feature? Initially Thierry suggested document option. So I left out. Thanks Avinash > -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
