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GPU -> DP to USB-c cable -> USB-c KVM switch -> monitor connection is not working (alien.top)

submitted 1 year ago by soupdiver23@alien.top to c/homelab@selfhosted.forum

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I have a laptop connected to a thunderbolt hub and from there a usb-c cable goes into a usb-c kvm switch

The other connection is from a GPU with a DP to usb-c cable. The gpu is used in a VM if that matters.

The kvm switch is then connected via DP to my monitor.

However the monitor is not recognized in my vm (runs windows) with that cable.

Connecting HDMI from the gpu to the monitor works.

Using the DP to usb-c cable from my laptop direct to the monitor also works.

What could cause the issue? Is there some tool I could use to see if the machine recognises the cable and maybe reports an error or so?

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[–] merkuron@alien.top 0 points 1 year ago (2 children)

DP to USB-C adapter probably uses DP-alt mode and not true USB-C data transfer.

[–] soupdiver23@alien.top 1 points 1 year ago (1 children)

DP to USB-C adapter probably uses DP-alt mode and not true USB-C data transfer.

Is there a way to actually check that?

[–] merkuron@alien.top 1 points 1 year ago

USB by its very nature requires a host device. The connection method you described (PC—>DP—>DP/USBC—>Display) I can only assume works because the display’s USB-C port switches to DP-Alt mode, functioning solely as a DP input port. In this case, there is no host device.

USB-C hubs/docking stations, which provide many ports (HDMI, Ethernet, etc), require that any display signal be transmitted as data on the USB bus. In this case, DP-Alt mode cannot be used, and the PC is the host device. It goes without saying that display images are very bandwidth-intensive, and when using a such a hub, you want to maximize the upstream bandwidth. 5Gbps hubs are OK, 10Gbps are better, 40Gbps (USB4) is optimal.

YMMV on a setup that goes something like:

PC—>USBC—>KVM—>USBC—>Display

PC—>USBC—>KVM—>USBC hub—>DP/HDMI—>Display

I don’t know if a KVM knows how to handle this kind of situation. What does your USB switch advertise as capabilities?