External USB C Monitor not working

[Dendrocalamus64]

EDIT: Today dmesg is giving something:

Code:

[  116.626032] cdn-dp fec00000.dp: [drm:cdn_dp_pd_event_work [rockchipdrm]] Connected with sink. Re-train link
[  117.851634] usb 7-1: new full-speed USB device number 4 using xhci-hcd
[  118.034760] usb 7-1: New USB device found, idVendor=06c4, idProduct=c100, bcdDevice= 1.24
[  118.034783] usb 7-1: New USB device strings: Mfr=1, Product=2, SerialNumber=5
[  118.034796] usb 7-1: Product: USB-C To DisplayPort Adapter
[  118.034807] usb 7-1: Manufacturer: BizLink Technology Inc.
[  118.034818] usb 7-1: SerialNumber: 101

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It's Xfce, and dmesg doesn't give much to go on.

Code:

$ sudo dmesg --clear
$ dmesg --follow | grep cdn-dp
[656498.179883] cdn-dp fec00000.dp: [drm:cdn_dp_pd_event_work [rockchipdrm]] Not connected. Disabling cdn
[656498.650125] cdn-dp fec00000.dp: [drm:cdn_dp_pd_event_work [rockchipdrm]] Connected, not enabled. Enabling cdn

I tried another brand of USB-C -> DisplayPort cable and the behavior is the same.

It works the same using xrandr instead of the Xfce Display control panel.

Code:

$ xrandr --output DP-1 --mode 1920x1920 --rate 59.94   # DisplayPort Signal Error
$ xrandr --output DP-1 --mode 1920x1920 --rate 29.94   # Back on again

The X server defaults to the "preferred" refresh rate of 59.94 Hz. To make the monitor work on the sddm login screen, we can change that:
https://blog.victormendonca.com/2018/06/...e-screens/
(the reboot mentioned at the end isn't necessary; it's sufficient to log out & back in to Xfce.)

Code:

xrandr --output DP-1 --mode 1920x1920 --rate 29.94 --primary

All I had to add to /usr/share/sddm/scripts/Xsetup .

Todo: write a file for /usr/share/X11/xorg.conf.d/ so it will apply across all display managers.

The framebuffer video is another matter. Judging by the stretched M at startup & shutdown, it's defaulting to 1920x1080. Setting the resolution on the kernel command line in /boot/extlinux/extlinux.conf seems to affect it since framebuffer video on the monitor stops working, but so far I haven't found a working mode for 1920x1920. Tried IIRC

Code:

video=DP-1:1920x1920@29.94
video=DP-1:1920x1920@59.94
video=DP-1:1920x1920@60
video=DP-1:1920x1920