No accessible video output device

Dear forum,

I recently acquired a rock5B board and I would like to use it for a HDMI in, image processing and HDMI out purpose.
I have been able to get HDMI input and save to files so far.

Now I would like to connect HDMI in to HDMI out by using gstreamer. However, I can’t find any video out devices under /dev/ - there’s only video0 (in) and video0_enc and video0_dec. If I just try to have ffplay stream an captured file to the screen, it gives a segmentation fault.

The UI is displayed to the screen fine. Looking at DMESG I see:
[ 8.698360] dwhdmi-rockchip fde80000.hdmi: use tmds mode
[ 8.698574] rockchip-hdptx-phy-hdmi fed60000.hdmiphy: hdptx_ropll_cmn_config bus_width:24d998 rate:2415000
[ 8.698849] rockchip-hdptx-phy-hdmi fed60000.hdmiphy: hdptx phy pll locked!
[ 8.698885] dwhdmi-rockchip fde80000.hdmi: final tmdsclk = 241500000
[ 8.698928] dwhdmi-rockchip fde80000.hdmi: don’t use dsc mode
[ 8.698932] dwhdmi-rockchip fde80000.hdmi: dw hdmi qp use tmds mode
[ 8.698938] rockchip-hdptx-phy-hdmi fed60000.hdmiphy: bus_width:0x24d998,bit_rate:2415000
[ 8.699131] rockchip-hdptx-phy-hdmi fed60000.hdmiphy: hdptx phy lane locked!
[ 8.752988] dwhdmi-rockchip fde80000.hdmi: Rate 241500000 missing; compute N dynamically

It seems like the hdmi driver is having trouble with the tx lane?
Has anyone had a similar problem, and how can I troubleshoot it?

Kind regards