[Bug 278930] x11/xorg no support of DisplayPort 1.2 specification, resulting a blank screen till reboot

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=278930

            Bug ID: 278930
           Summary: x11/xorg no support of DisplayPort 1.2 specification,
                    resulting a blank screen till reboot
           Product: Ports & Packages
           Version: Latest
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: Individual Port(s)
          Assignee: x11@FreeBSD.org
          Reporter: rosti.bsd@gmail.com
          Assignee: x11@FreeBSD.org
             Flags: maintainer-feedback?(x11@FreeBSD.org)

In case a monitor is connected to a computer by a displayport and this monitor
is in DisplayPort 1.2 mode, logging into any Desktop Environment (I tried LXQT
and XFCE) makes screen completely blank, even after trying to switch into
another virtual console (Ctrl+Alt+F1, F2... Fn). The system isn't stuck and
could be properly rebooted by pressing the power button (most likely through
SSH too). Before login into DE display managers like sddm and lightdm work
properly even when DisplayPort 1.2 is enabled.

Hardware in my case: the i7-4790 CPU with integrated graphics that is the only
graphic card; Dell U2414H monitor.

Windows 10 Pro works properly on this hardware and with that monitor connected
through mini displayport with and without the DisplayPort 1.2 mode is enabled
in the monitor itself.

System: FreeBSD 14.1-BETA2 x86_64.

The drm-kmod drivers set from ports is installed and i915kms added into the
kld_list in /etc/rc.conf

For more information read my emails in the freebsd-stable mailing list:
https://lists.freebsd.org/archives/freebsd-stable/2024-May/002132.html

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