Comment # 9 on bug 1116611 from Michal Srb

In the Xorg.0.log in comment 0 the xf86-video-amdgpu is not even used. The
Aruba GPU is primary, which causes the xf86-video-radeon driver to load for it.
Once it is loaded, it seems to take over the second card as well instead of
xf86-video-amdgpu. It tries to initialize the surface manager, which uses ioctl
for the *radeon kernel* driver, but the device actually uses the *amdgpu
kernel* driver, so it fails.

Now strictly speaking it is a bug in X server's matching of user-side to
kernel-side drivers. But you do have quite unusual configuration - two AMD
GPUs, where both would normally use radeon, but you forced one of them to use
amdgpu. I think your best bet is to fix/workaround it using some configuration.

The point of my question was whether you actually intend to use *both* GPUs or
just one of them. If for example you want to use just the Radeon HD 8600M, you
could disable the Aruba one.

You could:
1) Disable the APU in bios. (If that is possible. I don't know.)
2) Blacklist the radeon driver.
3) Configure X to use only the one card:

  Section "ServerFlags"
    Option "AutoAddGPU" "False"
  EndSection

  Section "Device"
    Identifier "Default Device"
    Driver "amdgpu"
    BusId "PCI:0:0:0"
  EndSection


In case you really want to use both, then maybe some X configuration would be
enough to match the cards to the proper drivers.