Created attachment 121493 [details]
It looks al little bit as this bug here:
But this time with a R9 380, Gigabyte, 4Gb
Created attachment 121494 [details]
Why NOTOURBUG? Where was the bug?
Ok notabug would be better.. the problem was in mkinitcpio.conf, the radeon module was loaded there and conflicted with the amdgpu module... After i removed this entry everything worked.
Wait a second. Why the heck should radeon conflict with amdgpu?
As long as nobody messed with the PCI-IDs the kernel should know very well what driver to load for a Tonga.