Are you talking about this
I dont know exactly - I know that Nvidia Nouveau have problem with kernel page size, custom kernel with 4kb page is needed on big-endian. I was playing with this when I had FX4500 inside my Quad.
Regarding the amdgpu I know only that it doesn't works on big-endian
, thus we must use radeon, i.e. we are limited to northern island and lower cards.
AmigaOS: Pegasos II, Sam440ep-flex, Micro A1-C, AmigaOneX1000
MorphOS: Pegasos II, Powerbook G4, iMac G5, Powermac G5 Quad