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?
Thanks James