Which kernel module is cloud-amd64 missing for kmscon?

[u/OkWrongdoer7003]

I am currently testing kmscon on forky. It seems to run and switch fine, both between instances of itself and to/from fbcon, when forky runs on its generic/default linux-image-amd64 kernel(s).

On the other hand, for my lightweight VMs, I prefer running linux-image-cloud-amd64. Here, roughly 3 out of 5 times, switching to kmscon causes it to freeze up. Sometimes switching back to a default fbcon/agetty console from a frozen kmscon will freeze all ttys (whether they run fbcon or kmscon). The same applies after reboot if I try disabling [getty@.service](mailto:getty@.service) and enabling [kmsconvt@.service](mailto:kmsconvt@.service). If I install and boot into a build of linux-image-amd64 (instead of -cloud-amd64) on that very same guest VM, the problem seems to disappear completely. Boot back into -cloud-amd64, and it returns.

When I provoke the issue, kmscon itself outputs the following line in kmsconvt@$tty.service and syslog:

Error: video_fbdev: cannot get finfo (9): Bad file descriptor

...followed by repeated lines of:

Warning: terminal: cannot swap display [fbdev] -22

dmesg does not indicate any complaints from the kernel side. I know kmscon is a userland program, but thought I'd check anyway since switching from/to default fbcon is part of it.

I have unsuccessfully tried playing around with options like --no-drm and --no-mouse.

I suspect there is at least one required kernel module which - depending on how Debian's cloud kernels are built - either can be loaded with kmod/modprobe or is missing entirely. The project's own documentation does not seem to specify which kernel modules it needs, and although I can diff linux-header files for cloud vs. generic kernels as much as I want, that by itself will not get me any further without knowing what to look for.

Does anybody know where I can look to figure out which kernel module(s) that I am missing?

This will probably turn out to be out-of-scope for the cloud kernels, but since I like kmscon and would enjoy also using it in my lightweight cloud VMs, I thought it might be worth asking anyway, and possibly submit a wishlist bug if it seems more or less reasonable.

Thanks.

[u/dkopgerpgdolfg]

Here, roughly 3 out of 5 times

If something works the 2/5 times on the same install/machine, the issue is likely not a missing kernel module

[u/OkWrongdoer7003 → u/dkopgerpgdolfg]

Then why does rebooting that very same install/machine into linux-image-amd64 (a kernel with many more modules) make the issue disappear, and booting back into linux-image-cloud-amd64 (the leaner kernel with fewer modules) makes it return? Both ways consistently.