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

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Message-ID: <20190611223019.GH137143@google.com>
Date:   Tue, 11 Jun 2019 15:30:19 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Brian Norris <briannorris@...gle.com>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Pavel Machek <pavel@....cz>,
        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.

On Tue, Jun 11, 2019 at 09:55:30AM -0700, Brian Norris wrote:
> On Tue, Jun 11, 2019 at 3:49 AM Daniel Thompson
> <daniel.thompson@...aro.org> wrote:
> > This is a long standing flaw in the backlight interfaces. AFAIK generic
> > userspaces end up with a (flawed) heuristic.
> 
> Bingo! Would be nice if we could start to fix this long-standing flaw.

Agreed!

How could a fix look like, a sysfs attribute? Would a boolean value
like 'logarithmic_scale' or 'linear_scale' be enough or could more
granularity be needed?

The new attribute could be optional (it only exists if explicitly
specified by the driver) or be set to a default based on a heuristic
if not specified and be 'fixed' on a case by case basis. The latter
might violate "don't break userspace" though, so I'm not sure it's a
good idea.