On Linux with an AMD i/dGPU, you can set the `virtual_display` module parameter for `amdgpu`[1] and do what you want without the need for an HDMI dummy or weird software. It's also hardware accelerated.

> virtual_display (charp)

> Set to enable virtual display feature. This feature provides a virtual display hardware on headless boards or in virtualized environments. It will be set like xxxx:xx:xx.x,x;xxxx:xx:xx.x,x. It’s the pci address of the device, plus the number of crtcs to expose. E.g., 0000:26:00.0,4 would enable 4 virtual crtcs on the pci device at 26:00.0. The default is NULL.

[1]https://www.kernel.org/doc/html/latest/gpu/amdgpu/module-par...