Can't control brightness - iMac 2013

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Fedora 33 Workstation works great except for controlling the brightness on my 2013 iMac (iMac14,1). This computer uses the Intel Iris Pro Graphics 5200 as its GPU.

After entering a GNOME session (both Xorg and Wayland), I can move the slider but the brightness doesn’t seem to change. I tried appending several acpi_backlight boot parameters with no luck, but then I noticed that when I suspended the machine and afterwards woke it up the slider would actually control the brightness properly.

Checking the Problem Reporting application shows that xorg-x11-drv-intel crashes after waking the computer from suspension because of “WARNING: CPU: 3 PID: 2350 at drivers/gpu/drm/i915/display/intel_display_power.c:4850 hsw_enable_pc8+0x61e/0x640 [i915] [i915]”.

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I’ve got an iMac 12.1 with an AMD Radeon GPU. At least under xorg I could somewhat control the brightness with xrandr --output eDP --brightness 0.6. This is of course no longer working under Wayland. Would be interesting to know if somebody else might be able to give a tip.