linux-kernel - Re: [PATCH] thinkpad_acpi: don't yell on unsupported brightness interfaces

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Date:	Fri, 8 Jan 2016 12:55:13 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	Eric Curtin <ericcurtin17@...il.com>
Cc:	Darren Hart <dvhart@...radead.org>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	platform-driver-x86@...r.kernel.org,
	ibm-acpi-devel@...ts.sourceforge.net,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] thinkpad_acpi: don't yell on unsupported brightness interfaces

Hi

On Thu, Jan 7, 2016 at 2:03 AM, Eric Curtin <ericcurtin17@...il.com> wrote:
> Also another query, could be a mini-side project for me as I'm a
> beginner kernel dev when I have some spare time. Could we change the
> minimum brightness value not to be off? 0 does not have to mean
> backlight off from reading various things around the web:
>
> http://www.x.org/wiki/Events/XDC2014/XDC2014GoedeBacklight/backlight.pdf
>
> And to be honest having the ability to completely turn off the
> backlight is a useless function and is probably unintended as discussed
> in the github thread above also. You would need night vision googles to
> read the screen in this state! I'm pretty sure the Windows driver does
> not completely turn off the screen on minimum brightness either, no
> need to have different functionality on different platforms especially
> when this functionality isn't very useful.

Whether '0' means 'off' or 'lowest brightness level' has been
discussed several times. Sadly, we cannot change semantics without
breaking user-space, so you better work around it. There have been
several proposals to fix it, but I guess no-one cared so far.

Anyway, both features (turning backlight off *and* lowest backlight
level) _are_ used actively by user-space. Hence, both should be
supported.

Thanks
David