So far used AMD as my primary GPU on the first PCIe slot and Nvidia as secondary on the second slot, to be used for CUDA compute.


Now for other needs I need the Nvidia on the first slot and so need to switch the AMD on the second one.


Trying to boot on KDE Neon result in a core dump. Trying to shift the HDMI connection to the AMD card looking at GRUB menu result in black screen.


I suppose the only way is to switch again the GPU so to being able to boot, but what about from there. Need I to delete the AMD and Nvidia drivers and use only Nuveau drivers after switching back the Nvidia on the first slot?

The HD7950 can run on the default DDX, modesetting. Most NVidia GPUs can run on the modesetting DDX as well, which should include your 1070. Thus with neither xserver-xorg-video-amdgpu nor xserver-xorg.video-ati nor xserver-xorg-video-nouveau nor any proprietary NVidia driver installed, both GPUs should run on the modesetting DDX. Whether CUDA is supported on the modesetting DDX I have no idea.

With NVidia's proprietary drivers installed, traditionally KMS is blocked. Whether that remains the case in latest versions I don't know, but all DDXes competent to run AMD's GPUs require KMS be unblocked. Blocking of KMS for NVidia only should be sufficient via cmdline option nouveau.modeset=0. This might make slot placement of the cards unimportant. Whether the NVidia driver can function without using an xorg.conf that NVidia setup creates without conflicting with other GPUs I don't know either. Hopefully it wouldn't be a problem to exclude anything from xorg.conf that would prevent the 7950 from functioning.

I'd be using two PCs instead, one for each graphics card.

Maybe I better needed to post in the Beginners section, since I didn't get almost anything in your post...
Pardon me, What's a DDX, and what I'm supposed to be? Searching on Ubuntu seems it's some option in GPU drivers, but not sure how this apply since I'm not able to boot at all.

Your OP included indications of significant competence in these matters, particularly the plan to utilize CUDA, but you didn't make it clear whether you have managed to install Neon or not.

What exactly does "not at all mean"? At what point in the boot process does a coredump occur? Can you append a 3 to the kernel cmdline (Tab key if installation menu; E key if Grub menu) to produce a shell prompt from which you can login?

Video output is the result of many software layers that include multiple so-called "drivers". DDX is one category of "driver" on which both Xorg and Wayland depend. xserver-xorg-video-* are the names of packages that provide various DDXes. By removing them all, the modesetting DDX should be able to support both your GPUs at least to some useful degree.

Whether already installed or not, I suggest if the cmdline 3 wasn't helpful that you consider appending nouveau.modeset=0 to kernel cmdline to see if boot will progress far enough to enable useful troubleshooting steps to be taken.

1 members found this post helpful.

I have experimented a lot, but mostly by trial and error and reading tutorials, so in fact didn't have a deep understanding of what I'm doing and acting as beginner.
I'm using KDE Neon since some months and before that Kubuntu since beginning 2019 (first Linux installation ever), and my dual GPU setting is working fine, also using CUDA for ML-related training.
Currently writing on Neon since I tried to switch back the two GPU in the original status and all is working again.

So just wanting to return my setup to a neutral status with both GPU using "generic" drivers, might I use
and maybe
?
I think AMD card isn't using any proprietary driver so I suppose didn't need to uninstall anything on his side.

I don't have any significant experience unrelated to purging NVidia, and last I did have hands on was many years ago. I do know that simple NVidia package removal such as with apt(-get) may not be sufficient. NVidia provides instructions, accompanying installation, for removal which need to be strictly followed, as installation traditionally replaces at least one standard library, and without its replacement with the original, along with purging of NVidia's own binaries, nothing can work right.

You may wish to purge the reverse engineered xserver-xorg-video-nouveau and allow the DDX I use almost exclusively regardless of GPU, modesetting, to control both your GPUs. I would expect it to stand a good chance of getting past the black screen.