[Bug 271897] graphics/drm-510-kmod: loading amdgpu and i915kms kernel modules simultaneously causes the system to crash

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271897

--- Comment #11 from Patrick McMunn <doctorwhoguy@gmail.com> ---
I spent ~5 hours after work this evening troubleshooting at the prompting of
rkoberman's comments.

First of all, under 13.2-RELEASE, I found that the order in which the modules
appear in kld_list does make a difference. If kld_list="amdgpu i915kms", the
the kernel panics, dumps core, and reboots. If kld_list="i916kms amdgpu", the
the system reboots without dumping core.

I tried building drm-515-kmod under 13.2-RELEASE, but it faild to compile. So I
downloaded and installed
FreeBSD-14.0-CURRENT-amd64-20230608-653738e895ba-263444. Under 14.0-CURRENT,
both modules can appear in kld_list, and the system will continue to boot
successfully without panicking. So that's good. But it is still exhibiting
undesirable as well as baffling behavior.

Under 14.0-CURRENT, if i915ms either appears first or alone, the system will
boot, but SDDM will error with "Failed to read display number from pipe" and
"Could not start Display server on vt 9". But if kld_list="amdgpu i915kms",
then SDDM will start successfully, and I can log into Plasma 5. But, only my
secondary display connected to the integrated Intel graphics will have a
graphical display. The boot messages appear on my primary display connected to
my Radeon RX 240 until the drm kernel modules load. Then the text on the
primary display freezes and continues on my secondary display. So once the drm
kernel modules load, only my secondary display is functional.

At least with 13.2-RELEASE, I could load amdgpu alone and my primary display
alone would be functional. But under 14.0-CURRENT, amdgpu alone results in a
black screen on my primary display as soon as the drm kernel module loads. All
I can do at that point is press CTRL-ALT-DEL to reboot. All tests on
14.0-CURRENT were with drm-515-kmod-5.15.25_3. I have not yet tried
drm-510-kmod on 14.0-CURRENT.

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