Backlight values are a mixture of 16 bit and 8 bit and break saving and restoring levels

Brief summary of the problem:

With a change in the amdgpu kernel module, actual_brightness in /sys/class/backlight/amdgpu_bl0 returns a 5-digit value to allow for more precision. However, both brightness and max_brightness return 8 bit values. With a recent commit to systemd, restoring backlight levels on boot is broken: The value in actual_brightness will be stored, and restored to brightness on boot. As brightness only is 8 bit, and a 16 bit value is restored, numbers will be clamped and in turn always return maximum brightness.

Hardware description:

• CPU: AMD Ryzen 7 Pro 4750U
• GPU: AMD Radeon RX Vega 7
• System Memory: 16GB
• Display(s): IPS 1920x1080
• Type of Diplay Connection: eDP
• Laptop: Thinkpad X13

System infomration:

• Distro name and Version: Arch Linux
• Kernel version: 5.8.0-rc7

How to reproduce the issue:

• Read 16 bit value from /sys/class/backlight/amdgpu_bl0/actual_brightness
• Write value into /sys/class/backlight/amdgpu_bl0/brightness
• "Invalid argument" is returned because value is > 255.

Issues on other sites

• Lennart Poettering said this is a kernel issue, max_brightness and brightness apparently are expected to be in the same range as actual_brightness
• There is an issue on the Kernel.org bugzilla opened in 2019 dealing with this, however I suspect not many people have witnessed this issue since the change to systemd only happened a few weeks ago.

Any help would be appreciated :) Please let me know if you need more information.