[Bug 295567] graphics/drm-612-kmod panics HP 840 G5 laptop

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=295567

            Bug ID: 295567
           Summary: graphics/drm-612-kmod panics HP 840 G5 laptop
           Product: Ports & Packages
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Individual Port(s)
          Assignee: x11@FreeBSD.org
          Reporter: cy@FreeBSD.org
             Flags: maintainer-feedback?(x11@FreeBSD.org)
          Assignee: x11@FreeBSD.org

This is a strange bug. I will describe it first then offer my analysis (guess).

drm-612-kmod will panic a very recent 16-CURRENT but only under strange
circumstances. When the laptop is powered on (from a powered off state), when X
loads (via xdm) the system will crash -- no panic string, just a long pause and
a reboot.

However if drm-66-kmod is installed when the laptop boots, drm-66-kmod is
uninstalled, drm-612-kmod is installed, the laptop is rebooted, X starts.
drm-612-kmod only works if it is installed after removal of drm-66-kmod and the
laptop is NOT powered off. Only rebooted.

My guess is that drm-66-kmod initializes the graphics hardware such that when
drm-612-kmod subsequently uses it (after drm-66-kmod has been uninstalled,
drm-612-kmod installed, and the laptop rebooted). But if drm-612-kmod was
installed before the laptop was powered off I think drm-612-kmod fails to
initialize the hardware correctly resulting the crash.

This only affects my HP 840 G5. The Framework 13 (with AMD graphics) has no
such problem.

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