[PATCH] intel_display.c: make acpi_video last in backlight_interfaces

[Alon Levy]

On Sun, Aug 26, 2012 at 03:03:08PM +0100, Chris Wilson wrote:
> On Sun, 26 Aug 2012 15:45:42 +0200, Alon Levy <alevy at redhat.com> wrote:
> > On Sun, Aug 26, 2012 at 12:51:44PM +0100, Chris Wilson wrote:
> > > On Sun, 26 Aug 2012 21:46:01 +1000, Dave Airlie <airlied at gmail.com> wrote:
> > > > On Sun, Aug 26, 2012 at 9:43 PM, Alon Levy <alevy at redhat.com> wrote:
> > > > > ACPI is meant as a fallback, so it should be last.
> > > > 
> > > > eh no, acpi is never the fallback, its always the preferred way.
> > 
> > I guess I was confused by the comment:
> >  "acpi_video1", /* finally fallback to the generic acpi drivers */
> > 
> > > 
> > > Right, acpi works on far more machines than intel_backlight. If you want
> > > to override the automaticlly choose interface, how about an
> > >    Option "backlight" "intel_backlight"
> > > ?
> > 
> > Works for me, patch below. But I'm confused - wouldn't the
> > intel_backlight sys directory be non existant if the kernel found the
> > machine didn't support it? at least I thought that was the point - then
> > checking acpi last makes sense.
> 
> It's worse than you thought. We know the hardware always has PWM
> registers that may be used by the backlight controller. However, the
> manufacturer is at liberty to implement their own controller entirely
> and expose that interface through ACPI. So on quite a few machines
> intel_backlight does nothing at all, and on other machines the ACPI
> firmware is worse - but the kernel has no way of telling just which path
> is broken.

OK. I'm not sure what would be better for gnome / your-favorite desktop
manager. One option is to write an xorg.conf file with this patch.
Another is a property for the backlight interface. Even though it's
abuse of the property interface for a one time thing (not like it would
change during the lifetime of X). Right now it's a stopgap thing, but
not very user friendly. This patch came from my brother complaining that
with his mac he can lower the backlight all the way to 0, but not with
his linux laptop. I don't want to have to maintain an xorg.conf file.

Sorry for the blurb - bottom line, I'm not sure if an xorg.conf file
would be something management, i.e. libgnome-desktop / kde etc. would
want to deal with, setting a property is perhaps easier, or another
solution.

> 
> Thanks for the patch,
> -Chris
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre