using amdgpu headless (no monitor)

[Ian Rogers]

Hi,

Why can't virtual_display be used along with a physical display?

I see from here: https://bugzilla.kernel.org/show_bug.cgi?id=203339 that the intention of the virtual_display module option is to allow for virtual displays and purposely disable physical/real displays.
Is there a technical limitation that required this?

I ask because I want to be able to run a system that is sometimes headless and sometimes not.  And I'd like to be able to access a current X session (either logged in or at the login greeter) both remotely (say via VNC) and locally via the physical display (when it is plugged in).without having to reboot or lose that X session.

However I've noticed that (at least with a Ryzen 3 3200G with Radeon Vega 8) an X session does not login successfully when accessed remotely if there is no monitor connected.
I assume this is caused by something in the amdgpu driver but I haven't been able to figure out what.
So I was hoping that perhaps a solution would involve using virtual_display, but that won't work as it's currently designed because of it disabling the physical display.

But maybe I've gone down the wrong track and there is another solution.  I've tried using the xorg dummy driver in addition to amdgpu, but when doing so, I can never get a physical display working.
This is an Ubuntu 20.04 system with kernel 5.4.0.

I can provide lots more detail on what I've tried and my config where needed.

Thanks much