There is no problem with ati/radeon vs amdgpu. Your card is supported by ati/radeon, and the optional/experimental amdgpu support for it is correctly disabled. X failed to start because llvm called abort(), meaning you are hitting a llvm or mesa bug. You can disable 3d in your xorg.conf or try older versions.
Right I hit a bug in LLVM or mesa.
So for clarity, lets forget ALL future discussion about amdgpu, and concentrate only on radeon.
Xorg-3D
without graphics-KERNEL works OK, using xorg-FBDEV (so is not accelerated).
If you don't load xf86-video-amdgpu or xf86-video-ati, but do load graphics-KERNEL, does the modesetting driver work?
1. After exit xorg and loading (in console) the graphics-KERNEL, then the console/prompter is frozen and I must do cold reset the laptop.
2.Fresh boot.After exit xorg and loading (in console) FIRST the firmware-radeon.tcz and THEN the graphics-KERNEL, now I stay console/prompter. When I start Xorg now, it shows briefly (1 second) the Xorg-window + mouse cursor, then it quickly quits back in console.
The less /var/log/Xorg.0.log shows that Xorg successfully loaded modeset-drv +llinput etc and then Xorg spits (EE) about libLLVM-13.so + radeonsi_dri.so.
3. Fresh boot.After exit xorg and loading (in console), first firmare-radeon, then Xf86-video-ati then graphics-kernel. And now I have the same briefly Xorg shown and then it quickly quits back in console. The less /var/log/Xorg.0.log shows that Xorg successfully loaded RADEON-drv +llinput etc, and then it spits (EE) about libLLVM-13.so + radeonsi_dri.so, right afler "RADEON (0): Modeline "1600x900" etc.