linux-kernel - Re: [PATCH 1/2] dt-bindings: pwm-backlight: Add 'max-brightness' property

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Message-ID: <20190611102851.GA16611@ulmo>
Date:   Tue, 11 Jun 2019 12:28:51 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Lee Jones <lee.jones@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Pavel Machek <pavel@....cz>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        dri-devel@...ts.freedesktop.org, linux-leds@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pwm@...r.kernel.org, linux-fbdev@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        Brian Norris <briannorris@...omium.org>
Subject: Re: [PATCH 1/2] dt-bindings: pwm-backlight: Add 'max-brightness'
 property

On Mon, Jun 10, 2019 at 04:37:38PM -0700, Matthias Kaehlcke wrote:
> Add an optional 'max-brightness' property, which is used to specify
> the number of brightness levels (max-brightness + 1) when the node
> has no 'brightness-levels' table.
> 
> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> ---
>  .../devicetree/bindings/leds/backlight/pwm-backlight.txt       | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> index 64fa2fbd98c9..98f4ba626054 100644
> --- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> +++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> @@ -27,6 +27,9 @@ Optional properties:
>                              resolution pwm duty cycle can be used without
>                              having to list out every possible value in the
>                              brightness-level array.
> +  - max-brightness: Maximum brightness value. Used to specify the number of
> +                    brightness levels (max-brightness + 1) when the node
> +                    has no 'brightness-levels' table.

Back at the time when these bindings were defined we specifically didn't
add this because it was deemed impractical. That is, no real hardware is
actually capable of achieving useful results with a simplified
description like this.

Besides, we already have the num-interpolated-steps property which
should allow you to achieve the same thing:

	brightness-levels = <0 255>;
	default-brightness-level = <1>;
	num-interpolated-steps = <255>;

Though given the original discussion that we had around how backlight
hardware behaves, that doesn't seem like a good choice.

Thierry

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