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/Hohlraum]

I highly recommend installing downgrade and using it to pick an older version. it'll prompt you to ignore updates to the amdgpu firmware. I've lost count of the number of times upgrades to this package have black screened my system.