OK, apparently it's possible to force AMDGPU onto Southern Island / Sea
Island:

https://wiki.archlinux.org/index.php/AMDGPU#Enable_Southern_Islands_(SI)_and_Sea_Islands_(CIK)_support

We'd need to things:

- enable the following in linux-libre

--8<---------------cut here---------------start------------->8---
# CONFIG_DRM_AMDGPU_SI is not set
# CONFIG_DRM_AMDGPU_CIK is not set
--8<---------------cut here---------------end--------------->8---

- Make sure amdgpu is loaded before radeon.  Can we do that in the
  operating-system declaration?

-- 
Pierre Neidhardt
https://ambrevar.xyz/