Comment 7 for bug 1861925

I tried 19.10 and 20.04 (daily build) on an AMD platform (AMD Ryzen 5 2500U with Radeon Vega Graphics), and the backlight works without problems.

This platform has the same symptom as described in the bug description - actual_brightness is a fixed and higher value, and it does not change with brightness. The max_brightness is 255 (which is fixed in amdgpu driver) and brightness can be changed from 0 to 255.

I built a kernel, based on 5.6rc5, that has actual_brightness = brightness. It is available for testing @ https://people.canonical.com/~alexhung/LP1861925/poc_kernel/

The actual_brightness is read from hardware but it doesn't reflect actual brightness. Only AMD developers can answer why it doesn't work accordingly - perhaps different hardware has different address and this needs updating.