[Bug 296372] linuxkpi does not set some flags nvidia-drm-kmod looks for activating vt_drmfb

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=296372

            Bug ID: 296372
           Summary: linuxkpi does not set some flags nvidia-drm-kmod looks
                    for activating vt_drmfb
           Product: Base System
           Version: 15.1-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: keivan@motavalli.me

brief context: Kde plasma >= 6.8 will be Wayland only, which requires using DRM
on FreeBSD, that includes Nvidia users.
Fortunately the proprietary nvidia-driver for FreeBSD can be used by an open
source platform layer, nvidia-drm{-version}-kmod, based on the opensource
nvidia linux platform layer, to expose drm functionality via drm-kmod.

Once patched to avoid a kernel panic (see bug #296195 - patch about to go both
into ports and in a future upstream nvidia release), it makes plasma6 able to
run using wayland, including suspend/resume, with an Nvidia RTX 3060 as the
sole system graphics adapter.

When running a wayland client over nvidia-drm-kmod, switching consoles with
crtl-alt-F{number} does not work and results in losing access to both the text
consoles and the wayland client until it is externally terminated, eg, via ssh. 

The current VT integration with DRM drivers is via vt_drmfb, whose
initialization is hooked into drm_fb_helper functions of drm-kmod, for what I
understand (https://github.com/freebsd/drm-kmod/pull/243), and both i915/amdgpu
call into drm_fb_helper to inizialize drm fb functionality instead of handling
it directly, even if this may not always have been that way.

When loading the nvidia-drm.ko and drm.ko modules, vt_drmfb does not get
initialized (I added some printfs to vt_drmfb_init and drm_fb_helper_init to
check and these functions are never called); VT continues to use vt_efifb.

I do not understand well the inner workings of the nvidia driver and believe
some calls are into the proprietary blob, so no assurance that it can work
trough linux drm_fb_helper like amdgpu and i915, but config code 
(in
/usr/ports/graphics/nvidia-drm-612-kmod/work/NVIDIA-FreeBSD-x86_64-595.84/src/nvidia-drm/nvidia-drm-conftest.h)
seems to check for CONFIG_DRM_TTM_HELPER via <linux/kconfig.h> to enable
framebuffer functionality.

From my understanding, CONFIG_DRM_TTM_HELPER is not set in the linuxkpi so
nvidia-drm-kmod never enables related functionalities.
Also from my understanding, ‎drm_fbdev_ttm_setup() from
drm-kmod/drivers/gpu/drm/drm_fbdev_ttm.c, if called into from the nvidia driver
in nvidia-drm-drv.c:2076 (for release 595.84), dependent on a C preprocessor
check around CONFIG_DRM_TTM_HELPER derived variables, would then invoke
drm_fb_helper functions in drm-kmod and thus inizialize the vt_drmfb backend,
as per drm-kmod implementation of drm_fbdev_ttm_setup().

This attribution of the bug is speculative, because I have no experience of
systems programming nor prior understanding of the drm layer. I can test
changes on my system, hoping that this is really just some config variables
away from working, without requiring adaptation from the part of nvidia or
changes into VT/vt_drmfb

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