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

Chris Wilson chris at chris-wilson.co.uk
Sun Aug 26 07:03:08 PDT 2012

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.

Thanks for the patch,

Chris Wilson, Intel Open Source Technology Centre

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