linux-kernel - Re: [REGRESSION/PATCH] acpi: blacklist win8 OSI for ASUS Zenbok Prime UX31A

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Message-ID: <20130731051421.GA15766@srcf.ucam.org>
Date:	Wed, 31 Jul 2013 06:14:22 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Felipe Contreras <felipe.contreras@...il.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, Aaron Lu <aaron.lwe@...il.com>,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	Len Brown <lenb@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [REGRESSION/PATCH] acpi: blacklist win8 OSI for ASUS Zenbok
 Prime UX31A

On Tue, Jul 30, 2013 at 07:11:06PM -0500, Felipe Contreras wrote:

> No, the ACPI driver is exposing a backlight interface, which has a
> defined stable API.

>From the ACPI spec:

'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.'

There's no mechanism for an OS to know whether or not a firmware 
implementation will actually turn the backlight off at 0, so there's no 
way the OS can define the lowest backlight state as anything other than 
"May or may not turn the screen off". If your userspace application 
depends on specific numbers having specific meanings, your userspace 
application is broken. Don't ascribe meanings to arbitrary values.

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