Fedora: MST issues with FW13 AMD and Dell U2724DE

I’m having weird issues with my new display setup (USB-C → Dell U2724DE → Dell U2724D) and the FW 13 AMD on Fedora 40: All works well when connected to the front right port (The screen acknowledges the USB3 connection), but when I connect to one of the USB4 ports, I get unpredictable results for the displays. In all cases, the USB devices are working fine and I think in all cases the displays appear working in gnome settings.

Most times, the displays stay black and I get a “no thunderbolt signal detected” message. This usually comes with dmesg showing

amdgpu 0000:c1:00.0: [drm] *ERROR* Sending link address failed with -5

Sometimes the second display (U2724D, daisy chained through DP) switches on while the first display in the chain doesn’t show anything. The error message that I’ve seen repeatedly with this issue is

amdgpu 0000:c1:00.0: [drm] *ERROR* Step 2 of creating MST payload for 0000000078e80139 failed: -5

Sometimes - rarely - both displays work after waiting for a minute or two.

Can anyone make sense of this behaviour and point me to a solution?

I have no solutions. But I seen very similar behavior on my 12th gen with my MST setup (1 MST Hub in dock (Synaptics VMM53xx, that seems already problematic and Intel Linux drivers do not want to use DSC), Dell U3223QE behind (other, unknown MST hub, not Synaptics), having its own MST hub from which another monitor is connected.

Works perfectly fine in Windows, with DSC where expected. Very unstable under Fedora. Ever since I got my FW.

If you do not need to use that setup on non TB/USB4 hosts, you could work around MST being problematic, by using a USB4 host port and the TB-out to use 2 separate DP connections without needing to rely on MST at all. Though I have seen other reports of there being problems with 2 DP connections through TB/USB4 for AMD under Linux.

The long term solution would be to trace the issue back to a probable driver / module and report an issue against it so it can be fixed.