L1 KVM and USB-C DP-Alt madness: Help with troubleshooting

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Hi Folks! I’ve got a question about the L1 KVMs and use with dp-alt over usb-c on linux.

The target setup is the following:
Minisforum UM790Pro → usb-c dp-alt (or usb-c to display adaptor to DP) → L1 KVM → samsung odyssey G9 neo
Everything suggests this chain should be capable of the full 5120x1440 at 240Hz but I can not for the life of me get this to yield anything but a blank screen.

I’ve read many things, reviewed all the L1 KVM forum posts, and gone through every resource I could find and have just been hitting the most baffling wall. I’d like to describe the scenarios that work and describe what doesn’t and would be so grateful for guidance in how to troubleshoot.

The pieces of the setup are the following:

  • Quad host dual monitor DP 1.4 L1 KVM
  • Samsung Odyssey G9 Neo (firmware up-to date as of Oct. 19 2023 1015.0)
  • Minisforum UM790Pro (BIOS up-to date as of Oct. 19 2023 1.07)
  • OS: Fedora 38 Sericea (Silverblue sway spin, kernel 6.5.7)
  • Club3d 6ft DP1.4++ cable
  • one of the following:
    • Club3d usb-c DP1.4 bidirectional cable
    • Club3d usb-c to DP 1.4 adaptor
    • Anker 556 USB4 hub with DP output
    • monoprice bidirectional usb-c to DP 1.4 cable

Here are the different setups i’ve tried and their outcomes:

  1. UM790 → Anker hub → DP 1.4 → L1 KVM → G9 Neo
    • max output 5120x1440 @ 120Hz
  2. UM790 → club 3d usb-c to DP adaptor → DP 1.4 → L1 KVM → G9 Neo
    • black screen, but querying sway for available outputs correctly lists DP with up to 5120x1440@240Hz
  3. UM790 → club 3d bidirectional usb-c to DP → L1 KVM → G9 Neo
    • black screen, but querying sway for available outputs correctly lists DP with up to 5120x1440@240Hz
  4. UM790 → club 3d bidirectional usb-c to DP → G9 Neo
    • solid 5120x1440@240Hz connection

Now the whackier thing that I came across that is adding to my confusion is that if I start with an HDMI cable plugged in to the UM790 and put the G9 into PIP mode, the usb-c DP connection via the L1 KVM works perfectly well! To boot, if i then turn off the PIP mode, and use the already active usb-c DP connection, then the system works, albeit at 2560x1440@120Hz. If i then switch the output to be 5120x1440@any freq. the connection fails back to just a black screen.

It happened exactly once that following the PIP → to single output → correct display settings succeeded at giving me a stable connections at the max resolution and frequency, but it then failed once I changed the connection and I could never get it to replicate.

It almost seems as though the computer is failing to allocate the correct bandwidth to the cable but I’m not sure if thats how that works and at this point I my efforts feel more like divination than actual troubleshooting. I’d be immensely grateful for any input on possible ways to troubleshoot this further, or what I could look for, or try. While I’ve been using linux as my daily driver and am pretty comfortable with the commandline, I’m not sure what to look for in dmesg, journalctl, or other logging utilities for hints as to the issue.

I’ve tried journalctl -k and dmesg -k at different log levels but none of the messages seem to be related to the display (though im pretty unsure as to what i’m even looking for). I’ve also queried lsusb for the usb-c dp-alt cables and as far as I can tell, they report correctly but even that i’m not totally sure I understand how to interpret what I’m seeing.

If anyone has any thoughts on next steps I could try, whether configurations, or system commands to query for states or logs I would really appreciate any kind of input

Addendum:

  • UM790 → club 3d bidirectional usb-c to DP → L1 KVM → G9 Neo
    • This actually now works if I force the resolution to [email protected]. Curiously though, additional frequencies listed are
      60.001Hz
      72.010Hz
      96.020Hz
      120.001Hz
      239.761Hz
      But selecting anything else 60.001 and above) results in a black screen with the system struggling. I’ve tried with both adaptive sync on and off. Confusing…
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kvm usb in u.2 mode?

[email protected]
sounds good

120.001Hz
does not use display stream compression, so this is actually harder than

240.001Hz
which requires display stream compression. not every driver stack does display stream compression properly, wonder if that’s related here

the g9 is really buggy to start with in general… you’re better off with native dp than usbc which shares power/usb bandwidth if you possibly can. glad that 60hz works at least.

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Man, I’m learning this the hardway with the G9… What is it with this monitor?? And I really wish I could do native DP but sadly the minisforum folks decided HDMIs were the way to go.

At this point though this has become a learning exercise for me and understanding what drivers where possibly control what. I’m not familiar though with what u.2 mode for the usb would be. Could you give a tip on how i’d check? My knowledge and understanding is a weird hodgepodge of self taught so I frequently am missing pieces.

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It’s the switch on the back of the kvm, u 2 mode :slight_smile: unplug the kvm before changing.

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Oh I see, i see. This is actually one of the 4 computer dual screen DP KVMs. Not the actual usb-c one.

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Ohhh. Well that’s good.

Which minisforum machine do you have??
You might be closer to working then you think. Moshi USBC to dp and then 3 foot club3d dp to dp from the kvm might be exactly what you need. And full bandwidth!

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I gotta say, you have a saintly level of patience for calmly making that recommendation time and time again. I probably should’ve specified that I’m in canada, and was trying to avoid those moshi cables (they carry a hefty cost this side of the border). I suppose having exhausted all other options though, the time has come to bite the bullet! I’ll get it and report back.

The minisforum is the UM790. Which according to every spec sheet should be capable of what I’m asking of it, though admittedly the forums seem to suggest some users not finding that to be quite the case.

But just for my own understanding, I would’ve thought that if signal integrity was the primary concern, the quicker you convert to display to get the signal into a decent quality display cable, the better. so why does something like this club3d usb-c to female DP1.4 w/HBR3 support not seem to work at all? it seems to report correctly to the machine, and everything seems happy to use it, but then no image at all! It seems so strange…

Or even; would that mean that a purely usb-c switch hub (say n usb-c ports in and 1 usb-c port out) would almost be the best system? simply have it plug in to whatever hub of your choice and let the usb system preserve data integrity?

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Club3d USBC to dp cables can work too. Have you tried that one and it detects but black screen? That could be the dp cable on the other side of the kvm.

The club3d USBC to dp should work… tried a lower resolution? Just to try to sort if it truly is an integrity issue or some other issue? It sounded like you’d only tried HDMI.

Active HDMI adapters do work too generally but itsba little more complicated if the design of the minisforum goes edp>HDMI internally first… HDMI back to dp in that case can be… trouble.

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Okay so I redid the cabling, repositioned stuff and am using only 3ft display cables, except for the usb-c to male DP which seem to only be available as 6ft long cables.

So this is the situation:

Minisforum (MF) → usb-c to female DP → DP → KVM → DP → G9
is identified by the system and it lists the full available resolution and refresh rate as possible, but nothing but a black screen, on any resolution.

Minisforum (MF) → usb-c to male DP → KVM → DP → G9
This works and is stable at 60Hz, but interestingly, now it works at 240Hz for a bit, but after a while flickers on and off, and then eventually stays off.

I’ve tried the HDMI adaptors as well and they do work but are pretty limited in the refresh rate.

So from this I suppose the conclusion is signal integrity strikes again? I’ve ordered the moshi cable so I’m extremely curious about how that’ll work now.

For what its worth, the current best performing cable i’ve used is a 1m U-green usb-c to DP cable, but it is a very unstable connection and will only work briefly at 5120x1440 240Hz before flickering and going to a black screen.

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So at last the moshi cable came and I was able to try it out in the new setup, and with updated system (fedora 39 and graphics drives and all else at the latest) but still no luck on running with any sort of stability unfortunately. I guess then this means I’m out of luck on this one then eh?

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What’s the g9 firmware? If on 1011 try rolling back to 1008? Could be worth trying fibbr from kvm to g9 also.

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Firmware was actually already up to 1015. In the end however, the minipc was giving me issues with other unrelated elements and I’ve decided to return it. the KVM works fantastic with other direct DP devices so thats great! Thanks for your input!

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