Re: [RFC v2 1/2] backlight: pwm_bl: linear interpolation between values of brightness-levels

Enric Balletbo Serra
2017-11-27T11:21:48Z

Hi Rob,

2017-11-20 19:58 GMT+01:00 Rob Herring <robh@kernel.org>:
> On Thu, Nov 16, 2017 at 03:11:50PM +0100, Enric Balletbo i Serra wrote:
>> Setting use-linear-interpolation in the dts will allow you to have linear
>> interpolation between values of brightness-levels.
>>
>> There are now 256 between each of the values of brightness-levels. If
>> something is requested halfway between 2 values, we'll use linear
>> interpolation.
>>
>> This way a high resolution pwm duty cycle can be used without having to
>> list out every possible value in the dts.
>
> I thought we already had a way to do that.
>
>> This system also allows for
>> gamma corrected values (eg: "brightness-levels = <0 2 4 8 16 32>;").
>>
>> Patch based on the Alexandru M Stan work done for ChromeOS kernels.
>>
>> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
>> ---
>> .../bindings/leds/backlight/pwm-backlight.txt | 2 +
>> drivers/video/backlight/pwm_bl.c | 55 +++++++++++++++++-----
>> include/linux/pwm_backlight.h | 2 +
>> 3 files changed, 47 insertions(+), 12 deletions(-)
>>
>> diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
>> index 764db86..7c48f20 100644
>> --- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
>> +++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
>> @@ -17,6 +17,8 @@ Optional properties:
>> "pwms" property (see PWM binding[0])
>> - enable-gpios: contains a single GPIO specifier for the GPIO which enables
>> and disables the backlight (see GPIO binding[1])
>> + - use-linear-interpolation: set this propriety to enable linear interpolation
>> + between each of the values of brightness-levels.
>
> Isn't this really just whether you allow values not listed because what
> other kind of interpolation would you do?

Yes, the idea behind this is just allow values not listed.

> Seems like you should just be
> able to query the resolution of the PWM and decide if it can take more
> values than listed.
>

Without using a new DT propriety and let decide the driver when use
intepolation and when not? Shouldn't this break current behavior then?
You can have a high resolution PWM but I'm not sure if you want allow
always more values than listed.

Regards,
Enric

> Rob