Failure to change backlight for on integrated amd gpu laptop

Using nixos-unstable, after a recent upgrade graphical artifacts started appearing in wayland. This was solved by upgrading both firmware and changing kernel to use 6.13.

However after those changes, I cannot change the backlight anymore using brightnessctl or directly writing to amdbl1/brightness. Stuff like waybar or brightnessctl correctly detects the supposed brightness level and it changes with XF86 brightness keys, just that the actual brightness is always at max.

I tried changing kernel params to “acpi_backlight=none” “video”, “vendor”, “native” combined with “amdgpu.backlight=0” or not but none of them worked.

GPU is AMD Radeon 680M [Integrated].
Current kernel is 6.13 but downgrading to 6.11 did not help either

I solved this by downgrading to kernel 6.6.
Since I have been using the default lts kernel before this problem happened, I guess this means something broke during the 6.6 to 6.12 lts upgrade (nothing to do with the firmware upgrade), which was partially solved in 6.13 but not fully. I guess I’ll just have to hope that it’ll all be fixed again by 2027.