I'm running Xubuntu 20.04 on an HP Elitebook 845 G7 which has an AMD Ryzen 7 PRO 4750U with Radeon Graphics. I've been trying to use two daisy-chained DisplayPort monitors (Dell P2421DC) connected via the laptop's USB-C port. The first monitor is detected and works correctly, however the second monitor (daisy-chained from the first) is detected but cannot be enabled either via the Display settings or xrandr. Attempting to enable the second display gives me the following output in dmesg:

[drm] Failed to add display topology, DTM TA is not initialized.

I've enabled the HWE stack for 20.04 so I'm currently running kernel 5.8.0-49 which looks like it's correctly using the amdgpu module for the GPU:

$ inxi -G

Graphics:  Device-1: Advanced Micro Devices [AMD/ATI] Renoir driver: amdgpu v: kernel 

           Display: x11 server: X.Org 1.20.9 driver: amdgpu,ati unloaded: fbdev,modesetting,vesa resolution: 2560x1440~60Hz 

           OpenGL: renderer: AMD RENOIR (DRM 3.38.0 5.8.0-49-generic LLVM 11.0.0) v: 4.6 Mesa 20.2.6 

xrandr correctly detects the daisy-chained second monitor (DisplayPort-3) along with its resolutions:

$ xrandr --query

Screen 0: minimum 320 x 200, current 2560 x 1440, maximum 16384 x 16384

eDP connected (normal left inverted right x axis y axis)

   1920x1080     60.02 +  39.98  

   1680x1050     60.02  

   1280x1024     60.02  

   1440x900      60.02  

   1280x800      60.02  

   1280x720      60.02  

   1024x768      60.02  

   800x600       60.02  

   640x480       60.02  

HDMI-A-0 disconnected (normal left inverted right x axis y axis)

DisplayPort-0 disconnected (normal left inverted right x axis y axis)

DisplayPort-1 disconnected (normal left inverted right x axis y axis)

DisplayPort-2 connected primary 2560x1440+0+0 (normal left inverted right x axis y axis) 527mm x 296mm

   2560x1440     59.95*+

   1920x1200     59.88  

   2048x1080     60.00  

   1920x1080     60.00    50.00    59.94  

   1600x1200     60.00  

   1680x1050     59.95  

   1280x1024     75.02    60.02  

   1280x800      59.81  

   1152x864      75.00  

   1280x720      60.00    50.00    59.94  

   1024x768      75.03    60.00  

   800x600       75.00    60.32  

   720x576       50.00  

   720x480       60.00    59.94  

   640x480       75.00    60.00    59.94  

   720x400       70.08  

DisplayPort-3 connected (normal left inverted right x axis y axis)

   2560x1440     59.95 +

   1920x1200     59.88  

   2048x1080     60.00  

   1920x1080     60.00    50.00    59.94  

   1600x1200     60.00  

   1680x1050     59.95  

   1280x1024     75.02    60.02  

   1280x800      59.81  

   1152x864      75.00  

   1280x720      60.00    50.00    59.94  

   1024x768      75.03    60.00  

   800x600       75.00    60.32  

   720x576       50.00  

   720x480       60.00    59.94  

   640x480       75.00    60.00    59.94  

   720x400       70.08  

When I attempt to enable the monitor via xrandr or the Display configuration of XFCE it doesn't get enabled. xrandr gives the following output:

$ xrandr -d :0 --output DisplayPort-3 --auto

xrandr: Configure crtc 1 failed

and I get the dmesg output about DTM TA is not initialized

Any ideas about how to resolve this or further diagnose the issue would be most welcome.