drm/i915: XPS13 backlight regression in v3.14

[Jani Nikula]

On Thu, 03 Apr 2014, James Hogan <james at albanarts.com> wrote:
> Hi,
>
> I've noticed that v3.14 breaks the backlight on Dell XPS13. Reverting the 
> following commit fixes the issue for me (i.e. the GUI brightness controls work 
> again):
>
> bc0bb9fd1c78 drm/i915: remove QUIRK_NO_PCH_PWM_ENABLE
>
> It appears that in v3.14 (with the above patch):
> * intel_backlight/brightness=4882 (max & default) causes 
> acpi_video0/brightness to have no effect.
> * intel_backlight/brightness=0 causes acpi_video0/brightness to behave 
> corrrectly (the firmware (I assume) fades the brightness gradually when 
> acpi_video0/brightness is set)
> * intel_backlight/brightness=something else in between often causes 
> acpi_video0 and intel_backlight to apparently "fight" over the backlight, 
> resulting in brightness flickering up and down every second or so.
>
> Reverting the above patch on v3.14:
> * intel_backlight/brightness has no effect
> * acpi_video0/brightness works normally
>
> It doesn't seem quite as simple as them trying to use the same hardware since 
> when intel_backlight/brightness=4882, acpi_video0/brightness has no effect at 
> all, so I don't really get what's going on.
>
> Can anybody shed some light or suggest a proper fix for this recurring issue?

Tracked at https://bugs.freedesktop.org/show_bug.cgi?id=76276

BR,
Jani.


>
> Thanks
> James

-- 
Jani Nikula, Intel Open Source Technology Center