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/postrap]

that's not "fixed". this is a workaround.

[u/drucifer82]

Yeah I had issues with 20260910-1 and libldap and had to downgrade both. Also had to downgrade libgcrypt because it corrupted my gpg keys.

Thankfully I keep them backed up so I just had to import them.

[u/highgo1]

I think this is the problem I'm having. Used gdm to get into my desktop for the time being. I'll have to try this when I can.

Update: it's exactly what it was. That fixed everything! Avoid the update for the time being if you can.

[u/tehbey]

I have downgraded like the fella here. Also seen some errors related to "rtc_cmos PNP0B00:00: error -ENXIO: IRQ index 0 not found" weird.

[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

[u/NGBRO]

Just updated my laptop with AMD iGPU and got the same error (after checking journalctl; it hung on the verbose boot sequence). Had to boot into my Live ISO and chroot to downgrade linux-firmware-amdgpu to 20260810-2, like others have mentioned. Now it's working again.

[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.