linux-kernel - Re: [PATCH 2/2] backlight: pwm_bl: Get number of brightness levels for CIE 1931 from the device tree

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Message-ID: <20190611101843.GD7526@amd>
Date:   Tue, 11 Jun 2019 12:18:43 +0200
From:   Pavel Machek <pavel@....cz>
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>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Thierry Reding <thierry.reding@...il.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 2/2] backlight: pwm_bl: Get number of brightness levels
 for CIE 1931 from the device tree

On Mon 2019-06-10 16:37:39, Matthias Kaehlcke wrote:
> Commit 88ba95bedb79 ("backlight: pwm_bl: Compute brightness of LED
> linearly to human eye") uses pwm_period / hweight32(pwm_period) as
> as heuristic to determine the number of brightness levels when the DT
> doesn't provide a brightness level table. This heuristic is broken
> and can result in excessively large brightness tables.
> 
> Instead of using the heuristic try to retrieve the number of
> brightness levels from the device tree (property 'max-brightness'
> + 1). If the value is not specified use a default of 256 levels.
> 
> Fixes: 88ba95bedb79 ("backlight: pwm_bl: Compute brightness of LED linearly to human eye")

I don't think this one is suitable for stable. I'm pretty sure the
heuristics works well for many boards, and you just replaced it with
another heuristics ("256").

> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>

Acked-by: Pavel Machek <pavel@....cz>

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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