systemd-backlight: Using actual_brightness causes amdgpu always setting to maximum brightness

[YHNdnzj]

systemd version the issue has been seen with

245.7

Used distribution

Arch Linux

After upgrading systemd to 245.7, I noticed the brightness is set to maximum after every reboot.
Related issue: #16302

For amdgpu on my laptop, value from /sys/class/backlight/amdgpu_bl0/actual_brightness is way above max_brightness, causing backlight brightness to be set to maximum when restoring. For example, when value of brightness is 128, that of actual_brightness is 34481. If I change brightness in KDE Plasma, both values change.

[poettering]

Smells like a driver bug.

i.e. we assumed that brightness and actual_brightness are defined on the same range of values. If that's not the case this looks like an issue with the driver...

Can you report this to the kernel driver maintainers please?

[poettering]

Let's close this here, since this really looks like a kernel issue. Feel free to reopen if the kernel driver maintainers say otherwise.

[bocki]

This seems to be an intentional change on AMD's side, offering higher precision for current backlight values:
torvalds/linux@262485a

This unfortunately breaks reading values from actual_brightness and writing them back to brightness...

EDIT: Found an open bug report at kernel.org: https://bugzilla.kernel.org/show_bug.cgi?id=203905 and referenced this issue there.

[poettering]

This really looks like a driver fuckup. The two brightness values' range should really be in the max_brightness range, even though Documentation/ABI/stable/sysfs-class-backlight isn't very clear on that. Everything else would just be borked.