linux-kernel - Re: [PATCH v2 4/4] backlight: pwm_bl: Set scale type for brightness curves specified in the DT

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Message-ID: <20190628075516.dwev7fxz7kmox2i2@holly.lan>
Date:   Fri, 28 Jun 2019 08:55:16 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Pavel Machek <pavel@....cz>
Cc:     Matthias Kaehlcke <mka@...omium.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-pwm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Douglas Anderson <dianders@...omium.org>,
        Brian Norris <briannorris@...omium.org>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>
Subject: Re: [PATCH v2 4/4] backlight: pwm_bl: Set scale type for brightness
 curves specified in the DT

On Wed, Jun 26, 2019 at 04:56:18PM +0200, Pavel Machek wrote:
> On Mon 2019-06-24 13:31:13, Matthias Kaehlcke wrote:
> > Check if a brightness curve specified in the device tree is linear or
> > not and set the corresponding property accordingly. This makes the
> > scale type available to userspace via the 'scale' sysfs attribute.
> > 
> > To determine if a curve is linear it is compared to a interpolated linear
> > curve between min and max brightness. The curve is considered linear if
> > no value deviates more than +/-5% of ${brightness_range} from their
> > interpolated value.
> 
> I don't think this works. Some hardware does takes brightness in perceptual units,
> converting it in the LED controller.

This check is exclusive to PWM backlights so I'd like to double check
that you are thinking specifically of hardware that takes it's signal
from the PWM and works in perceptual units?

I don't recall any examples being offered when we reviewed the
auto-generated CIE tables (although since that can be overriden by DT it
was not of the same gravity and this example).


Daniel.