Date:	Fri, 26 Jul 2013 08:43:10 +0100
From:	Steven Newbury <steve@...wbury.org.uk>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	James Hogan <james@...anarts.com>,
	Kamal Mostafa <kamal@...onical.com>,
	Aaron Lu <aaron.lu@...el.com>, Kalle Valo <kvalo@...rom.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Matthew Garrett <matthew.garrett@...ula.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Jörg Otte <jrg.otte@...il.com>,
	Martin Steigerwald <Martin@...htvoll.de>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	intel-gfx <intel-gfx@...ts.freedesktop.org>,
	Jani Nikula <jani.nikula@...ux.intel.com>
Subject: Re: Linux 3.11-rc2 (acpi backlight, revert)

On Thu, 2013-07-25 at 15:00 +0200, Rafael J. Wysocki wrote:
> On Monday, July 22, 2013 09:54:21 PM Rafael J. Wysocki wrote:
> > On Monday, July 22, 2013 11:11:54 AM Linus Torvalds wrote:
> > > On Mon, Jul 22, 2013 at 6:02 AM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > > >
> > > > Linus, do you want me to send a pull request reverting 8c5bd7a and efaa14c?
> > > 
> > > Yes, but let's wait a while. Not because I think we'll fix the problem
> > > (hey, miracles might happen), but because I think it would be useful
> > > to couple the reverts with information about the particular machines
> > > that broke (and the people who reported it). So that when we
> > > inevitably try again, we can perhaps get some testing effort with
> > > those machines/people.
> > > 
> > > It doesn't seem to be a show-stopped for a large number of people, so
> > > there's no huge hurry. I'd suggest doing the revert just in time for
> > > rc3, but waiting until then to gather info about people who see
> > > breakage.
> > > 
> > > Sound like a plan?
> > 
> > Yes, it does.
> 
> OK, time to revert I guess.
> 
> James, Kamal, Steven, Jörg, Martin, Kalle, please check if the apppended patch
> fixes the backlight for you.
> 
> Aaron, please double check if acpi_video_backlight_quirks() will still work as
> needed.
> 
> Thanks,
> Rafael
> 
> 
> ---
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Subject: Revert "ACPI / video / i915: No ACPI backlight if firmware expects Windows 8"
> 
> We attempted to address a regression introduced by commit a57f7f9
> (ACPICA: Add Windows8/Server2012 string for _OSI method.) after which
> ACPI video backlight support doesn't work on a number of systems,
> because the relevant AML methods in the ACPI tables in their BIOSes
> become useless after the BIOS has been told that the OS is compatible
> with Windows 8.  That problem is tracked by the bug entry at:
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=51231
> 
> Commit 8c5bd7a (ACPI / video / i915: No ACPI backlight if firmware
> expects Windows 8) introduced for this purpose essentially prevented
> the ACPI backlight support from being used if the BIOS had been told
> that the OS was compatible with Windows 8 and the i915 driver was
> loaded, in which case the backlight would always be handled by i915.
> Unfortunately, however, that turned out to cause problems with
> backlight to appear on multiple systems with symptoms indicating that
> i915 was unable to control the backlight on those systems as
> expected.
> 
> For this reason, revert commit 8c5bd7a, but leave the function
> acpi_video_backlight_quirks() introduced by it, because another
> commit on top of it uses that function.
> 
Works fine for me.

Tested-by: Steven Newbury <steve@...wbury.org.uk>

By the way, I'm willing to test any i915 backlight patches if it helps.

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