linux-kernel - Re: [PATCH v2 2/4] backlight: Expose brightness curve type through sysfs

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Date:   Wed, 26 Jun 2019 16:56:11 +0200
From:   Pavel Machek <pavel@....cz>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Daniel Thompson <daniel.thompson@...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 2/4] backlight: Expose brightness curve type through
 sysfs

Hi!

> Export the type of the brightness curve via the new sysfs attribute
> 'scale'. The value of the attribute may be a simple string like
> 'linear' or 'non-linear', or a composite string similar to
> 'compatible' strings of the device tree. A composite string consists
> of different elements separated by commas, starting with the
> most-detailed description and ending with the least-detailed one. An
> example for a composite string is "cie-1931,perceptual,non-linear"
> This brightness curve was generated with the CIE 1931 algorithm, it
> is perceptually linear, but not actually linear in terms of the
> emitted light. If userspace doesn't know about 'cie-1931' or
> 'perceptual' it should at least be able to interpret the 'non-linear'
> part.

I'm not sure the comma-separated thing is a good idea. If it is, it should 
go to the Documentation, not to changelog.

> +What:		/sys/class/backlight/<backlight>/scale
> +Date:		June 2019
> +KernelVersion:	5.4
> +Contact:	Daniel Thompson <daniel.thompson@...aro.org>
> +Description:
> +		Description of the scale of the brightness curve. The
> +		description consists of one or more elements separated by
> +		commas, from the most detailed to the least detailed
> +		description.
> +
> +		Possible values are:
> +
> +		unknown
> +		  The scale of the brightness curve is unknown.
> +
> +		linear
> +		  The brightness changes linearly in terms of the emitted
> +		  light, changes are perceived as non-linear by the human eye.
> +
> +		non-linear
> +		  The brightness changes non-linearly in terms of the emitted
> +		  light, changes might be perceived as linear by the human eye.

non-linear is not too useful as described.

> +		perceptual,non-linear
> +		  The brightness changes non-linearly in terms of the emitted
> +		  light, changes should be perceived as linear by the human eye.
> +
> +		cie-1931,perceptual,non-linear
> +		  The brightness curve was calculated with the CIE 1931
> +		  algorithm. Brightness changes non-linearly in terms of the
> +		  emitted light, changes should be perceived as linear by the
> +		  human eye.

Is it useful to know difference between perceptual, and cie-1931?

Would it be useful to export absolute values in some well-known units?

If I'm in dark room, I may want 100mW/m^2 of backlight... And it would
be nice if I could set same backlight intensity on all my devices easily.

								Pavel