Tried again, somehow there was no subject sent with my last mail.
So, well, I believe I got a solution. However, not everyone might be happy with it, but maybe it will work.
I am running the kernel 6.7.12+bpo-amd64 on my system. This is a backport kernel, but it might also work with other kernels, too.
You also need to install the build environment, the easiest way is to use module-assistant.
If you got this set and installed linux-headers and so on, then do the following:
First, enter the line for sid into your /etc/apt/sources.list
deb http://deb.debian.org/debian/ sid main contrib non-free non-free-firmware
then do
apt update
apt install nvidia-legacy-340xx-driver firmware-misc-nonfree
This should install all necessary files and build the kernel module. Here it did work. However, I
had to use 390xx instead of 340xx, but both built here fine.
After it, reboot and try again if it is working.
Sometimes, nvidia-detect says, use 340xx-legacy, but I had some cases, where I in real had to
use 390xx. So, if 340xx is not working, try 390xx.
Note: Do NOT upgrade any other files! I suggest, after installing both packages as above,
remove the sid entry from sources.list and do again an
apt update. Thus you are not going into the danger, to install any more packages from sid.
If you need bumblebee or primusrun, because you have two GPUs (one in the CPU and one
extern), then use the packages from stable. This will work!
My notebook is a Lenovo T520 with Intel CPU (and internal Intel GPU) and also NVidia GPU as
external GPU (soldered on mainboard). I have to use optimus, to get my external GPU.
I got my information from this site.
https://wiki.debian.org/NvidiaGraphicsDrivers[1]
Hope, this helps.
Ah, and last but not least: Big thanks to the lads and guys, who made 340xx and 390xx
buildable again, great work! Big big thank you!!!
Here on my system, 390xx is working like a charm.
Good luck!
Hans
--------
[1] https://wiki.debian.org/NvidiaGraphicsDrivers