On 09/24/2012 10:29 PM, Philip, Avinash wrote: > 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. So why not just change the values so they /are/ what you want? After all, it's just data and you can put whatever values you want there. What is preventing you from doing this? Perhaps e.g.: brightness-levels = <0 101 106 112 124 150 202 304 511>; (just multiplying everything by N, for arbitrary N=2, to get extra precision) ... plus whatever adjustments are required to make the data "uniformly divided", which I can't do in the example here since I'd need to know whatever non-linear equation characterizes the backlight's PWM % duty cycle to perceived brightness mapping. The only thing that could be preventing this is mathematical precision. While all the PWM DT examples I've seen have brightness-levels range from 0..255, I don't think there is any such actual limit; you could range from say 0..1000000 if you wanted, right? >> 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. The binding documents are supposed to be a standalone description of what the data in DT does; given general no-Linux-specific domain knowledge, the binding document should be detailed enough for someone to understand how to fill in the DT. So, yes, I think the binding document would be a great place to put such documentation. -- 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
