External Nvidia GPUs won't work with recent proprietary drivers

[Germain Chazot]

Since Nvidia drivers version 396 (still present in 440), `gpu-manager` generates a configuration that does not allow External GPUs.

The reason is a new XConfig option introduced by Nvidia: "AllowExternalGpus" which defaults to False.

https://download.nvidia.com/XFree86/Linux-x86_64/396.18/README/xconfigoptions.html#AllowExternalGpus

The rationale behind their default value seems to be that eGPUs are not necessarily used for display but can be used to add general purpose GPU computing capacity to computers that don't need it on a permanent basis. It seems to me that eGPU usage for display should also be covered on Ubuntu.

As there is no clear way to configure the output of gpu-manager in the documentation, it makes it fiddly to add this setting.

Workaround available:
For now I've done it by duplicating `/usr/share/X11/xorg.conf.d/11-nvidia-prime.conf` generated by gpu-manager into `/etc/X11/xorg.conf.d/12-nvidia-prime-fix.conf` and adding the setting in there, in order to override the configuration generated by gpu-manager. However this is not future-proof as this means that future updates to gpu-manager will not be taken into account in my configuration. It's also quite an obscure solution for most users.

[Germain Chazot (gchazot)]

Little update about this.

The problem is still present with nvidia-driver 460, on Ubuntu 20.04.

Also I've set up another laptop with nvidia-driver 495 on Ubuntu 21.10 and the problem is still there.