Created attachment 297819 [details]
HW: ThinkPad P14s gen 2a
CPU: AMD Ryzen 7 PRO 5850U with Radeon Graphics
Panel: BOE 2434 and CSO 5127
brightness can't be controlled through AUX.
It works with parameter "amdgpu.backlight=0"
Created attachment 297821 [details]
4k display edid
I would suggest to work with BOE and CSO to fix the DPCD values in panel firmware to not report aux control if it is not functional.
You can see the kernel checks the DPCD to determine whether to use AUX or PWM.
BTW this behavior has been also been reported in the past, so a heuristic may be introduced at some point in the future for the buggy panel firmware: https://gitlab.freedesktop.org/drm/amd/-/issues/1438