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SubjectRe: [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@collabora.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@collabora.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
>

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