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

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Date:	Wed, 27 Mar 2013 09:06:04 -0400
From:	Alex Deucher <alexdeucher@...il.com>
To:	Danny Baumann <dannybaumann@....de>
Cc:	Matthew Garrett <mjg59@...f.ucam.org>,
	intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 0/1] drm/i915: Allow specifying a minimum brightness level
 for sysfs control.

On Wed, Mar 27, 2013 at 8:56 AM, Danny Baumann <dannybaumann@....de> wrote:
> 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.
>
>
> BTW, I found the source for that statement: [1], section
> System.Client.BrightnessControls.SmoothBrightness. While formally it's not
> part of the ACPI spec, I'm pretty sure most vendors (except Apple,
> obviously) will follow it as if it were, if not more strictly.
>
>>> 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.
>>
>>
>> In practice does it really matter?  As a user if you set the
>> brightness really low and you either can't see the screen or can
>> barely make it out does it matter if the screen is off or just really,
>> really dim?  The 0 brightness setting is probably not practically
>> usable regardless of what it does.
>
>
> That's right. I'm not intending to use the laptop with that low brightness,
> though, I'd just like to distinguish between my laptop being turned off /
> suspended or just its display being dimmed down by the desktop environment
> to conserve power. In order to do the latter, KDE sets brightness to 0
> ([2]), which worked fine for me as long as acpi_video was working on this
> laptop. This isn't the case at present, which is why I'm using
> intel_backlight at the moment. As you may have noticed, things aren't
> working as expected with it, which in turn is what brought me over here ;)
> I'm fine with sending a patch to KDE if that's the correct thing to do, but
> I'm not yet sure what the correct thing to do is.

FWIW, when I implemented native backlight support in the radeon
driver, I special cased level 0 as off since that was what a lot of
the other native backlight drivers did.  I'm open to changing it if
there is a plan for some kind of consistency, but it seems pretty
random at the moment.

Alex
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