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

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Message-ID: <20190617200314.GT137143@google.com>
Date:   Mon, 17 Jun 2019 13:03:14 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Pavel Machek <pavel@....cz>
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 Pavel,

On Mon, Jun 17, 2019 at 03:01:51PM +0200, Pavel Machek wrote:
> 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.

I see the merit in the sense that it allows to provide more precision
for if userspace wants/needs it, without requiring userspace to know all
possible (future) options. If userspace wants to keep things simple it
can just check for check for "s == 'non-linear'" and
"s.ends_with(',non-linear')"

In any case, I posted a first version of the patch:

https://lore.kernel.org/patchwork/patch/1088760/

Maybe best to center the discussion there?

> > > 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?

I'm open to either, I mentioned it earlier and Daniel seemed to prefer
the 'unknown' value so I went with it in the first version (it's also
slightly less code).

Cheers

Matthias