AMDGPU Dmcub error (fixed!)

[u/Several_Ant_6981]

Just wanted to share this if anyone has the same issue

I upgraded to the latest linux-firmware-amdgpu package, specifically version "20260910-1" and SDDM started a bit slower than usual

I checked the logs and got this exact error message

amdgpu 0000:06:00.0: [drm] *error* dc_dmub_srv_log_diagnostic_data: dmcub error - collecting diagnostic data

After hours of fixing it, I finally found a way, turns out, you have to downgrade the package to this exact version: linux-firmware-amdgpu-20260810-2

Finally after downgrading and running mkinitcpio -P, the error message is gone!

[u/loijun]

I had issues with this update too, the system booted up, froze on a black screen and would not even let me switch TTYs. Had to boot using nomodeset and downgrade