linux-kernel - Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye.

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Date:   Mon, 17 Jun 2019 15:01:51 +0200
From:   Pavel Machek <pavel@....cz>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Brian Norris <briannorris@...gle.com>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Doug Anderson <dianders@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Guenter Roeck <groeck@...gle.com>,
        Lee Jones <lee.jones@...aro.org>,
        Alexandru Stan <amstan@...gle.com>, linux-leds@...r.kernel.org,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        kernel@...labora.com
Subject: Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED
 linearly to human eye.

Hi!

> > Certainly "linear" (this device will work more or less correctly if the
> > userspace applies perceptual curves). Not sure about logarithmic since
> > what is actually useful is something that is "perceptually linear"
> > (logarithmic is merely a way to approximate that).
> > 
> > I do wonder about a compatible string like most-detailed to
> > least-detailed description. This for a PWM with the auto-generated
> > tables we'd see something like:
> > 
> > cie-1991,perceptual,non-linear
> > 
> > For something that is non-linear but we are not sure what its tables are
> > we can offer just "non-linear".
> 
> Thanks for the feedback!
> 
> It seems clear that we want a string for the added flexibility. I can
> work on a patch with the compatible string like description you
> suggested and we can discuss in the review if we want to go with that
> or prefer something else.

Compatible-like string seems overly complicated.

> > Instead one valid value for the sysfs should be "unknown" and this be
> > the default for drivers we have not analysed (this also makes it easy to
> > introduce change here).
> 
> An "unknown" value sounds good, it allows userspace to just do what it
> did/would hace done before this attribute existed.

What about simply not presenting the attribute when we don't have the
information?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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