linux-kernel - Re: [PATCH 2/2] ACPI / video / i915: Remove ACPI backlight if firmware expects Windows 8

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Date:	Wed, 11 Sep 2013 10:30:17 +0000
From:	Matthew Garrett <matthew.garrett@...ula.com>
To:	Jani Nikula <jani.nikula@...ux.intel.com>
CC:	Aaron Lu <aaron.lu@...el.com>, "Rafael J. Wysocki" <rjw@...k.pl>,
	"Daniel Vetter" <daniel@...ll.ch>,
	ACPI Devel Mailing List <linux-acpi@...r.kernel.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	"intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
	Yves-Alexis Perez <corsac@...ian.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Seth Forshee <seth.forshee@...onical.com>,
	"Lee, Chun-Yi" <joeyli.kernel@...il.com>,
	Igor Gnatenko <i.gnatenko.brain@...il.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Lee Chun-Yi <jlee@...ell.com>, Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 2/2] ACPI / video / i915: Remove ACPI backlight if
 firmware expects Windows 8

On Wed, 2013-09-11 at 13:29 +0300, Jani Nikula wrote:
> On Wed, 11 Sep 2013, Matthew Garrett <matthew.garrett@...ula.com> wrote:
> > On Wed, 2013-09-11 at 11:45 +0300, Jani Nikula wrote:
> >
> >> Before plunging forward, have you observed any difference between the
> >> boot modes? We have reports [1] that the backlight behaviour is
> >> different with UEFI vs. UEFI+CSM or legacy boot. So I'm wondering if the
> >> acpi_gbl_osi_data >= ACPI_OSI_WIN_8 check in patch 2/2 is the whole
> >> story.
> >> 
> >> Further, if we tell the BIOS we're Windows 8 to use the tested BIOS code
> >> paths, what guarantees do we have of UEFI+CSM or legacy boots working?
> >
> > We have no evidence of Windows behaving differently based on the exposed
> > firmware type.
> 
> By "behaving differently", do you mean internally adapting to the boot
> mode, or exhibiting different behaviour to the user?

As far as backlight control goes, both.

> We have evidence of the firmware behaving differently (VBT, backlight)
> based on the boot mode, all else being equal. We don't adapt to that,
> and we fail. I don't know if we should adapt, or do things differently
> altogether. I don't even know if Windows 8 works on all boot modes on
> the machines in question.

Sure, but Windows knows nothing about VBT or opregion-backed backlight
control. That's up to the Intel driver.

-- 
Matthew Garrett <matthew.garrett@...ula.com>