Bug#1008296: gdm3: No option to choose between gnome 42 wayland or x11, always goes x11

[Drew Parsons]

Package: gdm3
Followup-For: Bug #1008296

Poking the internet around wayland and nvidia, I found that gdm3 is
set up to deactivate wayland if it finds nvidia is available but not fully
configured.  This is why gdm does not show the Wayland option for the
Gnome session.  It has deliberately been disabled.

The logic for the disabling lies in /lib/udev/rules.d/61-gdm.rules
as already discussed in this bug. Sending it to /dev/null is not a
great workaround, I think.

Weiwen Hu identified a small patch to disable the disabling, so
61-gdm.rules is read, but doesn't disable wayland.
But that skips some of the nvidia checks.

Inspecting the checks themselves, /lib/udev/rules.d/61-gdm.rules is
looking for capabilities set in /proc/driver/nvidia/params, as well as
suspend/hibernate support.

Reporting the checks in order (the variables checked are the entries
in /proc/driver/nvidia/params),

1) NVIDIA_ENABLE_S0IX_POWER_MANAGEMENT
   This one seems not crucial.  If your card has it, then the other
   checks are not needed. But it's not needed itself, it just makes
   power management simpler if it is available.
   
2) suspend/hiberate support via scripts
     /usr/bin/nvidia-sleep.sh
     /usr/lib/systemd/system-sleep/nvidia
   These are provided by nvidia-suspend-common, so you'll already have
   them if you're using the debian nvidia packages.
   
3) NVIDIA_PRESERVE_VIDEO_MEMORY_ALLOCATIONS
   This was not activated on my system. I had to activate it in
   /etc/modprobe.d/nvidia-options.conf with
     options nvidia-current NVreg_PreserveVideoMemoryAllocations=1
   (i.e. uncommenting the line already in the file)
   
4) systemctl is-enabled nvidia-hibernate
   systemctl is-enabled nvidia-resume
   systemctl is-enabled nvidia-suspend

   These were already enabled on my system, no other action needed
   (except perhaps kernel option nvidia_drm_modeset=1)

5) gdm_hybrid_graphics_check_end
   gdm_virt_passthrough_check_end
   gdm_virt_multi_gpu_check_end
   
   Perhaps these tests are relevant to more complex installations
   (virtual GPUs). They test /run/udev/gdm-machine-has-* scripts.
   The debian package isn't providing them so these tests jump
   past gdm_disable_wayland (they don't cause the disbling)


So the main action I needed to take was to set the driver option
NVreg_PreserveVideoMemoryAllocations=1
to get NVIDIA_PRESERVE_VIDEO_MEMORY_ALLOCATIONS defined.
Looks like modesetting should also be activated.
These can both be done in
/etc/modprobe.d/nvidia-options.conf

options nvidia-current NVreg_PreserveVideoMemoryAllocations=1
options nvidia_drm modeset=1

For hibernation, Archlinux also suggests setting the tmp path to a
location with enough space to hold the GPU memory,
see https://wiki.archlinux.org/title/NVIDIA/Tips_and_tricks#Preserve_video_memory_after_suspend
e.g.
options nvidia-current NVreg_TemporaryFilePath=/var/tmp

There is some chatter on the internet about activating nvidia_drm option
fbdev=1, but we don't currently need it since it's not supported in
our current 545 driver version, and might be set by default in the
future versions anyway, see
https://forums.developer.nvidia.com/t/555-release-feedback-discussion/293652/14
https://forums.developer.nvidia.com/t/drm-fbdev-wayland-presentation-support-with-linux-kernel-6-11-and-above/307920/11

After setting NVreg_PreserveVideoMemoryAllocations=1 (and nvidia_drm
modeset=1), gdm3 gives me access to a wayland Gnome session,
with the nvidia driver accessible via PRIME OFFLOADING
(i.e. __NV_PRIME_RENDER_OFFLOAD=1)
https://wiki.archlinux.org/title/PRIME#PRIME_render_offload
https://gitlab.archlinux.org/archlinux/packaging/packages/nvidia-prime/-/blob/main/prime-run?ref_type=heads

Framerate with glxinfo
(with __GL_SYNC_TO_VBLANK=0,
see https://stackoverflow.com/questions/17196117/disable-vertical-sync-for-glxgears)
seems extremely poor (100 to 10 fps instead of 7000 fps0.
Hopefully it's fixed in the coming 550/560 drivers.
That said, the real world performance (tested in wine, Ctrl-r in World of Warcraft)
is fine for me (30-50 fps).

Thank you to ArchLinux for good documentation around the issue of
using Nvidia cards with linux.


-- System Information:
Architecture: amd64 (x86_64)
Kernel: Linux 6.12.17-amd64 (SMP w/8 CPU threads; PREEMPT)
ii  gnome-shell                                 48~beta-4
ii  gnome-session [x-session-manager]           47.0.1-1
ii  gnome-session-xsession [x-session-manager]  47.0.1-1
ii  xserver-xorg                                1:7.7+24

-- debconf information:
  gdm3/daemon_name: /usr/sbin/gdm3
* shared/default-x-display-manager: gdm3