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

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Date:	Tue, 26 Mar 2013 17:21:04 +0000
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Danny Baumann <dannybaumann@....de>
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.

On Tue, Mar 26, 2013 at 06:10:30PM +0100, Danny Baumann wrote:
> Am 26.03.2013 18:02, schrieb Matthew Garrett:
> >I'm not quite clear what you mean here. The behaviour of "0" isn't well
> >defined for the ACPI backlight driver - it's perfectly reasonable for it
> >to turn the backlight off entirely. Anything assuming that "0" is still
> >visible is broken.
> 
> 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.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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