Struggling to get USB-C DP to work

[gravedigger]

Hello

i am using Lenovo ThinkPad 40A9 dock for my daily driver Lenovo E595. I thought to test it with pinebook, but result was mixed, as i've described here.

Today i've returned to seek for rootcause, and what i came with, was this message

Jul 21 02:21:04 pinebook kernel: cdn-dp fec00000.dp: Direct firmware load for rockchip/dptx.bin failed with error -2

when dock was plugged.
I've started to connecting dots: firmware was present in /usr/lib/firmware, but yet somehow it was invisible for rockchipdrm module. After some tinkering, i've modified a file

[root@pinebook etc]# sdiff -s mkinitcpio.conf mkinitcpio.conf_20210730
FILES=(/usr/lib/firmware/rockchip/dptx.bin)       | FILES=()

regenerated my initrd and verified that file is actually included before rootfs gets mounted, so exactly when it's needed.
After reboot, somewhat success - no errors about missing firmware ,yet no image after connecting to dock, despite both plugs orientations

kernel says

[  26.816477] cdn-dp fec00000.dp: [drm:cdn_dp_pd_event_work [rockchipdrm]] Not connected. Disabling cdn

Is there anything else that i can do/ mess with/ try? i really am into docking Pinebook to my existing dock, as i have already nicely set up my desk and buying another "maybye compatible one" is missing my point.

my kernel: 5.13.4-1-MANJARO-ARM, vanilla from repo (yet i don't see a problem to recompile if needed, Gentoo fan here ;d)

[Arwen]

There are several things related to external displays on a Pinebook Pro;

• Must be USB C alternate mode Display Port
  
• If the laptop dock is USB C alternate mode HDMI, it will never work
  
• Some laptop docks use USB Display Link, which is basically a USB attached video card. These require drivers on the Pinebook Pro and maybe slower.
  

Checking your device it "appears to be using USB C alternate mode Display Port". Notice to quotes. It appears to be. That's part of the problem, manufactures not being crystal clear in their specifications.

So, try several other OSes and see if they work with your laptop dock. If so, then it's a software problem. Not "USB C alternate mode HDMI", which will never work with the Pinebook Pro.