maintainer-feedback requested: [Bug 245923] graphics/drm-fbsd12.0-kmod: 12.1-stable crash on loading i915kms.ko

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Bug 245923: graphics/drm-fbsd12.0-kmod: 12.1-stable crash on loading i915kms.ko
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=245923



--- Description ---
After upgrading 12.1-STABLE from r359345 to r360192, with graphics/drm-kmod and
graphics/drm-fbsd12.0-kmod rebuilt and reinstalled together with the new
kernel,
the system crashes when loading i915kms.ko. When loading the module via
kld_list in /etc/rc.conf at boot, screen goes blank and system stops responding
until a power cycle. If booting without loading the module and loading it with
kldload manually, the screen goes blank and reboots after some time. In the
latter case a crashdump is found at the next boot.
Hardware is an Acer C720 Chromebook, using a Celeron 2955U. The graphics
controller shows in pciconf -lv as "Haswell-ULT Integrated Graphics
Controller".
Kernel config is based on GENERIC, but with options WITNESS and
WITNESS_SKIPSPIN added.

I don't know much about debugging kernel dumps, but if someone tells me how to
do it I'm ready to try getting data out of the existing dump.