Prime Sync not working with amdgpu

Hi,

Prime Synchronization seems to be non-functional on my RX 590, and the tearing is driving me insane:

andrew@andrew-gentoo-pc ~ % xrandr --properties | grep PRIME
PRIME Synchronization: 1
PRIME Synchronization: 1
PRIME Synchronization: 0
PRIME Synchronization: 0
PRIME Synchronization: 1

The 0's are from the connected monitors, the 1's are disconnected outputs.

Using xrandr --output DP-1-5 --set "PRIME Synchronization" 1 does not change anything.

I also tried toggling the amdgpu.dc boot paramter to 0 and 1, without effect.

What does work, is to force usage of the modesetting driver:

Section "Device"
  Identifier "modesetting"
  Driver "modesetting"
EndSection 

This results in working Prime Sync and a tear-free display. However the modesetting driver gives me problems with one of the two monitors connected to the iGPU. I get massive screen corruption on that monitor, while the other one is nice and tear-free. (This might be a different unrelated issue though)

andrew@andrew-gentoo-pc ~ % glxinfo | grep "OpenGL renderer"
OpenGL renderer string: Radeon RX 590 Series (POLARIS10, DRM 3.36.0, 5.6.7-gentoo, LLVM 9.0.1)

This whole setup, same monitors, same configuration, same everything, was working just fine with working Prime Sync before I switched out my Nvidia GTX 1060 with the RX 590.

Xorg.0.log dmesg.txt

xorg-server-1.20.8 mesa-20.0.5 xf86-video-amdgpu-19.1.0

What can I do to further debug this?

To summarize: Prime Sync does not work with amdgpu, but does work with modesetting.