https://bugs.freedesktop.org/show_bug.cgi?id=108585
--- Comment #7 from Joel joel.stan@gmail.com --- (In reply to Benjamin Herrenschmidt from comment #6)
Dan... did you do some firmware changes here ? Could it have to do with the versions differences between petitboot and the final kernel ?
FWIW, Talos II machines use a fork of op-build that include the amdgpu driver in petitboot. (They also appear to be stuck on 4.16).
I was experimenting with the same.