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

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Date:   Mon, 27 Nov 2017 12:21:45 +0100
From:   Enric Balletbo Serra <eballetbo@...il.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Pavel Machek <pavel@....cz>,
        Doug Anderson <dianders@...gle.com>,
        Brian Norris <briannorris@...gle.com>,
        Guenter Roeck <groeck@...gle.com>,
        Lee Jones <lee.jones@...aro.org>,
        Alexandru Stan <amstan@...gle.com>, linux-leds@...r.kernel.org,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 1/2] backlight: pwm_bl: linear interpolation between
 values of brightness-levels

Hi Rob,

2017-11-20 19:58 GMT+01:00 Rob Herring <robh@...nel.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@...labora.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