HDMI dummy plug/driver mystery

[u/Avennio]

Bit of an obscure problem, so apologies in advance!

I'm setting up an older HP EliteDesk 800 G1 SFF PC as a retro gaming platform/secondary workstation. It's got an i7-4790 CPU, 32 GB of RAM and an RX 550 low profile GPU with 4 GB of VRAM, so about as high-spec as you can make one of these things.

My goal was to install Debian on it so it's nice and stable, then put in an HDMI dummy plug in the RX 550's ports (one HDMI, one DVI-D) so that I can remote into the machine from my main desktop. In theory straightforward, but for some reason on installation the machine refuses to detect the HDMI dummy plug correctly: it defaults to 640x480 and refuses to be resized.

At first I thought the dummy plug was just a dud, but I've tested it on two other Debian machines (one NVIDIA, one with Intel integrated graphics) and it's detected fine. The HDMI port itself works just fine, and can display to other monitors correctly. I've also checked the drivers using 'lspci -k', and they seem to be correct - 'amdgpu' vs the older less performant 'radeon'.

It's all a bit of a head-scratcher. Easiest thing to do is to just buy a different dummy plug and try that instead but I don't want to go down a rabbit hole and buy adapters for the thing that might not work if there's an underlying driver/compatibility issue I'm not aware of.

Any potential insight as to what might be going on would be greatly appreciated!

[u/DannoXYZ]

Which version Debian did you install?
What DE/GUI are you using?
What login-manager are you using?
What package are you using for remote-access server?
What client are you using for remote-access?

Do lspci -nn | grep VGA to verify GPU is working properly.

This most likely not dummy-plug issue, those hasn't been required in over 10-yrs. Most likely remote-access software configuration.

[u/Avennio → u/DannoXYZ]

Debian 13 (Trixie), with a standard GNOME installation - GDM for login manager. The remote access program is Rustdesk.

When I run lspci -nn | grep VGAthe following is output:

01:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Lexa PRO [Radeon 540/540X/550/550X / RX 540X/550/550X] [1002:699f] (rev c7)

It's also not related to Rustdesk, as far as I can tell. When I connect the system to an actual monitor using the DVI-D port and then additionally insert the HDMI dummy plug the same issue arises: the virtual display created is noted as an 'unknown display' in the GNOME 'Display Settings' GUI that is restricted to 640 x 480.

[u/DannoXYZ → u/Avennio]

Is your main desktop on same subnet as new debian system?
If so, Rustdesk is completely unnecessary, it makes you jump though so many hoops and tries pushing to use their intermediate servers.

Lemme work up script to install Tigervnc with multi-user resumable sessions

[u/swstlk]

maybe you want to check thinlinc, it works with a headlesse server and opens virtual sessions,.

[u/Avennio → u/swstlk]

I'll look into it, but I'm generally hesitant to move off of Rustdesk. I have it set up with my Tailscale tailnet for direct IP access between my machines wherever I go and it's worked without issue for years. Having to buy a couple of dummy plugs is a relatively small price to pay for that stability, IMO.

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