Nouveau - May 2019 - [Bug 110631] New: EDID confusion with LG 4K TV as monitor running X rather than wayland

https://bugs.freedesktop.org/show_bug.cgi?id=110631

            Bug ID: 110631
           Summary: EDID confusion with LG 4K TV as monitor running X
                    rather than wayland
           Product: xorg
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/nouveau
          Assignee: nouveau at lists.freedesktop.org
          Reporter: horsley1953 at gmail.com
        QA Contact: xorg-team at lists.x.org

Starting with fedora 28 and going through fedora 30, the nouveau driver
produces a bad signal when trying to display X on my LG OLEDB6P 4K television.

Weirdly, with what is presumably the same driver, it displays a full UHD
resolution when running wayland rather than X. (I really don't understand
why
something as low level as the video timing would be different between wayland
and X, but if someone does know, they probably know where the bug is too :-).

This isn't restricted to fedora either. I've just been testing a
multi-boot USB
stick, and the drivers on the ubuntu 19.04 live CD and the System Rescue CD
6.0.3 produce the same "bad signal" errors on this monitor.

For details (like X log files) see:

https://bugzilla.redhat.com/show_bug.cgi?id=1575391

and 

https://bugzilla.redhat.com/show_bug.cgi?id=1648608

If I add nomodeset and forcevesa kernel parameters, I can boot and see (lower
resolution) video rather than a black screen.

Switching to the nvidia binary drivers makes the display work properly again.

Booting the USB stick on different systems with more conventional resolution
monitors, nouveau works fine.

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--- Comment #1 from Ilia Mirkin <imirkin at alum.mit.edu> ---
FWIW I have a LG C7P which I tested with a GM206 and GP107. Slightly different
than your setup, but comparable.

I used this setup for bringup of HDMI 2.0 support, available in kernel 4.20.
HDMI 2.0 is required for 4k at 60 over HDMI.

Looking at those logs, you appear to be using modeset instead of
xf86-video-nouveau (note how it says "modeset" instead of
"NOUVEAU"). The logs
make it look like modeset is trying to use a 3840x2160 594MHz mode. This
requires HDMI 2.0 to work (and should have been filtered out, but nonetheless
is the modeline presented).

I'd recommend (a) using xf86-video-nouveau [RH has made local changes to
this
driver to not load, but I believe this to be a very bad idea, you can override
with an xorg.conf iirc], and (b) updating to a 4.20+ kernel so that you get
HDMI 2.0 to work to get access to the higher refresh rates.

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Martin Peres <martin.peres at free.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |MOVED
             Status|NEW                         |RESOLVED

--- Comment #2 from Martin Peres <martin.peres at free.fr> ---
-- GitLab Migration Automatic Message --

This bug has been migrated to freedesktop.org's GitLab instance and has been
closed from further activity.

You can subscribe and participate further through the new bug through this link
to our GitLab instance:
https://gitlab.freedesktop.org/xorg/driver/xf86-video-nouveau/issues/484.

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