linux-kernel - Re: [Intel-gfx] Linux 3.11-rc2 [backlight] [ASUS N56VZ]

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Date:	Thu, 25 Jul 2013 14:57:52 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jani Nikula <jani.nikula@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Aaron Lu <aaron.lu@...el.com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	* SAMÍ * <miaousami@...mail.com>,
	intel-gfx <intel-gfx@...ts.freedesktop.org>,
	LKML <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Igor Gnatenko <i.gnatenko.brain@...il.com>
Subject: Re: [Intel-gfx] Linux 3.11-rc2 [backlight] [ASUS N56VZ]

On Thursday, July 25, 2013 03:34:10 PM Jani Nikula wrote:
> On Thu, 25 Jul 2013, "Rafael J. Wysocki" <rjw@...k.pl> wrote:
> > On Thursday, July 25, 2013 11:09:27 AM Jani Nikula wrote:
> >> On Thu, 25 Jul 2013, "Rafael J. Wysocki" <rjw@...k.pl> wrote:
> >> > Well, I wonder what about the appended (untested) patch?
> >> 
> >> Rafael, before going there, I've been trying to wrap my (poor, rusty
> >> after vacation) head around
> >> 
> >> commit 8c5bd7adb2ce47e6aa39d17b2375f69b0c0aa255
> >> Author: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >> Date:   Thu Jul 18 02:08:06 2013 +0200
> >> 
> >>     ACPI / video / i915: No ACPI backlight if firmware expects Windows 8
> >> 
> >> and I can't see how it could work.
> >
> > Well, if it didn't work, people wouldn't see either improvement or breakage
> > from it, but they do see that, so it evidently works. :-)
> 
> I didn't claim it didn't work, just that *I* didn't see how it could. ;)
> 
> >> First, the ACPI_VIDEO_SKIP_BACKLIGHT flag seems to be checked before
> >> it's actually set anywhere.
> >
> > Are you sure about that?
> >
> > acpi_video_bus_add() is the .add() callback routine for acpi_video_bus which
> > in fact is an ACPI driver (the naming sucks, but I didn't invent it).  This
> > means that acpi_video_bus_add() can only be called *after* acpi_video_bus
> > has been registered with the ACPI subsystem (and the driver core).  That
> > is done by acpi_bus_register_driver() and, guess what?, this happens in
> > __acpi_video_register().  So clearly, acpi_video_bus_add() *cannot* run before
> > __acpi_video_register().
> 
> Right. I totally missed the call within the ternary operator. Thanks for
> the explanation, and apologies for the noise.
> 
> >> Second, with i915 that has opregion support, __acpi_video_register()
> >> should only ever get called once. Which means the acpi_walk_namespace()
> >> with video_unregister_backlight() should never get called in register.
> >> 
> >> Please enlighten me.
> >
> > Actually, that's correct, so we don't need the whole
> > video_unregister_backlight() thing, calling acpi_video_backlight_quirks() would
> > be sufficient.
> >
> > Ah, one more reason to do a full revert.  I'm thinking, though, that I'll leave
> > acpi_video_backlight_quirks() as is so that it can be used by
> > acpi_video_bus_(start)|(stop)_devices(), because that doesn't seem to cause
> > problems to happen.
> 
> I observe that for the regular non-quirk acpi_video_register() call,
> acpi_video_backlight_quirks() won't be called during register, but it
> will get called later. This might have subtle effects later on, don't
> you think?

Yes, it might, but after dropping ACPI_VIDEO_SKIP_BACKLIGHT it should be OK.

> As to the original problem, and your patch in this thread, what do you
> think about having another value in acpi_backlight kernel parameter for
> it? Having an i915 module parameter to tell acpi to use or not use
> quirks seems odd, since the i915 is not really taking over
> anything. It's just passing the info on to acpi.

I agree, I'm going to send a full revert in a while and we'll think what to
do about all that later.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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