linux-kernel - Re: [RFC 1/2] dt-bindings: pwm-backlight: add brightness-levels-scale property

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Date:   Tue, 5 Sep 2017 06:45:23 -0700
From:   Guenter Roeck <groeck@...gle.com>
To:     Enric Balletbo i Serra <enric.balletbo@...labora.com>
Cc:     Lee Jones <lee.jones@...aro.org>,
        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>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Doug Anderson <dianders@...gle.com>,
        Brian Norris <briannorris@...gle.com>,
        linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 1/2] dt-bindings: pwm-backlight: add brightness-levels-scale property

On Mon, Sep 4, 2017 at 8:35 AM, Enric Balletbo i Serra
<enric.balletbo@...labora.com> wrote:
> Brightness is not perceived linearly; rather, it typically follows some
> kind of parabolic curve. We can support this by skipping values in the
> brightness-levels array in a pseudo-quadratic curve. Typically we used
> less than 256 levels, which yields no more than 1KiB of memory in our
> device tree. But, we've noticed that on some devices the backlight
> performs much smoother at lower ranges if we have more than 256-levels of
> granularity. On kevin device, for example, if we support all 64K, that
> will waste us at least 256KiB in our device tree.
>
> Let's avoid to waste memory and have a huge table of numbers in our device
> tree of numbers by adding a brightness-levels-scale property to let the
> driver compute the brightness levels based on one algorithm and their
> property parameters.
>

Given Daniel's comments, would it possibly make sense to provide
something like "brightness-level-range = <0, 65536>" instead and let
the driver handle details ?

Thanks,
Guenter

> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
> ---
>  .../bindings/leds/backlight/pwm-backlight.txt       | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> index 764db86..11c5583 100644
> --- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> +++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> @@ -17,6 +17,16 @@ 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])
> +  - brightness-levels-scale: this can be used instead of 'brightness-levels',
> +                             to represent the perceived luminance. So rather
> +                             than specifying 'brightness-levels = <0
> +                             1 2 ... 65535>', one can simply say
> +                             'brightness-levels-scale = <255 65535>', where the
> +                             first number is the max number of levels and the
> +                             second number is the max PWM value that represent a
> +                             100% duty cycle (brightest). The result is a
> +                             correction table for PWM values to create linear
> +                             brightness based on the CIE1931 algorithm.
>
>  [0]: Documentation/devicetree/bindings/pwm/pwm.txt
>  [1]: Documentation/devicetree/bindings/gpio/gpio.txt
> @@ -33,3 +43,14 @@ Example:
>                 power-supply = <&vdd_bl_reg>;
>                 enable-gpios = <&gpio 58 0>;
>         };
> +
> +       backlight {
> +               compatible = "pwm-backlight";
> +               pwms = <&pwm 0 5000000>;
> +
> +               brightness-levels-scale = <255 65535>;
> +               default-brightness-level = <128>;
> +
> +               power-supply = <&vdd_bl_reg>;
> +               enable-gpios = <&gpio 58 0>;
> +       };
> --
> 2.9.3
>