Hello Philippe,Thank you for the logs. They narrow things down a lot, and they also show that something I told you last time was wrong.
The `.xsession-errors` from your `startx` session without the NVIDIA driver shows Cinnamon starting normally ("Cinnamon took 508 ms to start") on the framebuffer, and the session stays up for as long as you used it. So I was wrong when I wrote that Cinnamon does not get that far without a usable GL context: it works on the software GL it gets there, and the "Software Rendering" session would not have changed anything. Sorry for sending you in that direction.
What seems to fail is the step before, and `lightdm.log` points to it: [+0.00s] DEBUG: Monitoring logind for seats [+0.00s] DEBUG: Stopping Plymouth, no displays replace itlightdm never adds a seat, so it never starts X at all. By default it only starts the seats that systemd-logind reports as able to run graphics, and logind decides that from udev tags. In trixie's `/usr/lib/udev/rules.d/71-seat.rules` a DRM device (`/dev/dri/card0`) always qualifies, but a plain framebuffer like the EFI one you are left with only qualifies when the kernel is booted with `nomodeset`:
# Allow efifb / uvesafb to be a master if KMS is disabledSUBSYSTEM=="graphics", KERNEL=="fb[0-9]", IMPORT{cmdline}="nomodeset", TAG+="master-of-seat"
Your kernel command line only has `ro quiet`, so my guess is that seat0 is not considered graphical, lightdm waits for it forever and you are left on the console. `startx` does not ask logind about any of this, which would explain why it works.
I could not check this on hardware like yours, so if you are willing to test it, next time you are without the NVIDIA driver:
1. from the console, `loginctl show-seat seat0 -p CanGraphical` should print `CanGraphical=no`; 2. reboot, press `e` on the Debian entry in GRUB, add `nomodeset` at the end of the line starting with `linux`, and boot with Ctrl+X (this only affects that one boot, nothing is saved); 3. the same command should now print `CanGraphical=yes`, and the lightdm login screen should appear; 4. if it does, log in to the normal "Cinnamon" session from there and tell me whether you get a working desktop, as you do with `startx`.
If that is confirmed, the problem would not be in cinnamon: the missing piece would still be nouveau support for your card in the kernel, and the seat rule is a deliberate choice in systemd (a bare framebuffer only counts as a graphical seat when KMS is disabled on purpose). If instead lightdm still does not start, or the Cinnamon session fails when started from it, the logs from that boot (the same three files, plus the output of the `loginctl` command) would tell us where to look next.
Thanks again for your patience and for the careful testing, Fabio
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