[Bug 265920] drm radeonkms doesn't work for DVI on RS780, RS880

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=265920

            Bug ID: 265920
           Summary: drm radeonkms doesn't work for DVI on RS780, RS880
           Product: Base System
           Version: CURRENT
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: alomobeto277@magenta.de

Created attachment 235980
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=235980&action=edit
logfiles from FreeBSD 11.2 on RS880 and 14-CURRENT on RS780

I'm running FreeBSD 11.2 with radeonkms without problems, but I need to
upgrade. So I tried 14-CURRENT. I built the drm modules and firmware from
ports.
When I load radeonkms my monitor attached to DVI goes blank. I found a similar
bug here:  https://github.com/freebsd/drm-kmod/issues/117, so I played around a
bit. My RS880 board (GA-880GA-UD3H) has one monitor attached to the dual-link
DVI output using a 2560x1440 solution. I also have a RS780 board (M3A78-CM)
with 3 monitors for VGA, DVI and DisplayPort. I focused on the Asus board since
it allows more test cases.

When only DVI is connected, the screen goes blank after loading radeonkms. The
connected monitor cannot be found. If I start Xorg with xf86-video-ati from
ports, Xorg logs about using KMS to the console and the kernel hangs - no
output, no ping, nothing (no crash dump).

When DVI and DisplayPort is connected the DVI monitor goes blank after loading
radeonkms, but the Displayport Monitor works as expected. The DVI connection is
not detected, the Displayport is. Xorg only works on the Displayport monitor in
the correct resolution.

When DVI and VGA is connected, after loading radeonkms the DVI monitor shows
exactly what the VGA monitor shows, same resolution. However, no monitor
connection is detected. On this board VGA and DVI can't be used separately -
DVI and Displayport can. So, the VGA Monitor works as expected, the DVI Monitor
is a clone - if you unplug the VGA monitor the DVI monitor goes blank. Same
behavior when you start Xorg.

A single monitor attached to VGA or Displayport works as expected, monitor and
resolutions get detected.

I attach logfiles for the different scenarios. A few additional remarks:
loading the drm module before the radeonkms never made a difference. It also
doesn't matter if the DVI connection is single- or dual-link. In case the
radeonkms module found a connected monitor dmesg shows kernel locking problems,
so there's something not working correctly, yet.
Since using dual-link DVI is the only option to drive my monitor I'd really
like you to fix this regression ;-) BTW. earlier attempts with live CDs
(GhostBSD) had a similar effect - this bug may already be on previous releases.

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