Hardware: MacbookPro8,2 (Late 2011) Kernel: 4.9.13 from NixOS Xorg: 1.19.2 i915 provides /sys/class/backlight/intel_backlight. This interface behaves correctly except that it does not affect screen brightness at all, as this is handled by gmux. The existence of the intel_backlight sysfs entry causes Xorg to use that rather than gmux_backlight and stops xbacklight from working correctly.
What does Xorg.0.log say? Option "Backlight" "name" can be used to override the automatic selection.
The backlight sysfs class ABI is that platform backlight should be preferred over raw backlight. gmux_backlight is platform type, intel_backlight is raw type. From ABI perspective there is nothing wrong in kernel here.
acc@sphalerite.org, as Chris was pointing out, can you attach Xorg.0.log? thanks
based on the lack of activity and response in the bug the bug will be closed, if the issue is detected in a newer configuration please open a new bug
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