linux-kernel - Re: [PATCH 0/1] drm/i915: Allow specifying a minimum brightness level for sysfs control.

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Message-ID: <5152DE75.5010701@web.de>
Date:	Wed, 27 Mar 2013 12:56:37 +0100
From:	Danny Baumann <dannybaumann@....de>
To:	Matthew Garrett <mjg59@...f.ucam.org>
CC:	David Airlie <airlied@...ux.ie>, intel-gfx@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] drm/i915: Allow specifying a minimum brightness level
 for sysfs control.

Hi,

>> Well, the ACPI spec says this (section B.5.2):
>>
>> "
>> The OEM may define the number 0 as "Zero brightness" that can mean
>> to turn off the lighting (e.g. LCD panel backlight) in the device.
>> This may be useful in the case of an output device that can still be
>> viewed using only ambient light, for example, a transflective LCD.
>> "
>>
>> My interpretation of this is that the value 0 is supposed to still
>> be visible. I'm pretty sure I saw a statement that 0 is supposed to
>> mean "barely visible" somewhere, but can't find it at the moment.
>> I'll search for the source of it.
>
> I think that's a stretch - "This may be useful" isn't normative
> language, "The OEM may define" is. But even if we do assert it for the
> ACPI backlight, it's not true for other interfaces - zero backlight
> intensity is supposed to be screen off on Apple hardware, for instance.

OK, I see. And there is user space depending on that behaviour? And 
again - how is user space supposed to know about the behavioral 
differences? Is it something like 'if type is raw, don't expect anything'?
The reason for my question is that I want to determine what a) the 
correct place to fix this and b) the correct fix is. As Xrandr abstracts 
away the used backlight interface, I see no way for user space using 
Xrandr (e.g. KDE) to meaningfully handle this.

Thanks,

Danny
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