Linux Hardware

DP-alt-mode, datarates, cabling are all vastly more complicated than you’d think. (I’ve spent easily 500 hours of my life, buying dozens of products and specific cables, running debugging hardware and software … getting Pro Display XDR’s (6k displays) working at longer distances, behind Thunderbolt docks, on multiple OSes, etc — it’s it’s never intuitive.)

Some absurd things that may actually help:

1. flip the dock-to-display cable around (not even shitting you)
2. if the dock is a DP-alt consumer itself, plug an unterminated HDMI cable into it (and into nothing else),
3. change the order in which you plug various things into eachother post-boot (ideally, nothing plugged into anything other than that C-D-M chain of three items; then try C-D-first/D-M-second, and try D-M-first/C-D-second.)

Failing various fixes with existing equipment, the cheapest next steps will be cabling — despite the high promises of USB-C, all cables are absolutely not created equal when it comes to high-pixelclock-DP-alt-mode:

1. Shortest possible cables. Get shorter USB-C cables from reputable vendors. Make sure they’re maximum-data-rate certified.
2. This is counterintuitive as fuck, but do not use “better” Thunderbolt cables to substitute USB-C cables for USB-C-capable-displays. (They can work plenty often; but they can also be problematic with some EDID/OS combinations.)
3. If your display has other inputs, you may have more luck with a USB-C-to-DisplayPort-proper cable, than a traditional USB-C-to-USB-C cable.
4. Worst case, look into a cable with an active repeater. I’ve had luck with CableMatters’ Cable Matters Active USB C Cable... https://www.amazon.com/dp/B07PPVMYRV?ref=ppx_pop_mob_ap_share

tl;dr good luck this might be gon suuuuuck. )=