[Intel-gfx] Linux 3.11-rc2 [backlight] [ASUS N56VZ]

Rafael J. Wysocki rjw at sisk.pl
Thu Jul 25 13:58:33 CEST 2013

On Thursday, July 25, 2013 11:09:27 AM Jani Nikula wrote:
> On Thu, 25 Jul 2013, "Rafael J. Wysocki" <rjw at sisk.pl> wrote:
> > On Wednesday, July 24, 2013 02:05:15 PM Linus Torvalds wrote:
> >> On Wed, Jul 24, 2013 at 2:02 PM, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> >> >
> >> > I think a i915 module option should be doable, otoh people seem to have a
> >> > viable workaround by setting a different acpi os version already.
> >> 
> >> At least the original claim was that if you set a non-windows8 acpi os
> >> version, you hit other bugs..
> >> 
> >> But yeah, if we just do a plain revert, that may be the only option
> >> for the people for whom the current (to-be-reverted) patches made
> >> things work.
> >
> > 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 at intel.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. :-)

> 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

> Buried deep in the calls from
> acpi_video_bus_add(), acpi_video_verify_backlight_support() is used
> before acpi_video_backlight_quirks() gets called. (Perhaps if i915 is
> reloaded, this goes right as the flags are already set.)

Please see above.

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

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