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UHD TV says mode not recognized for UHD or FHD at 60 Hz on 20.04

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Got a 2020 Samsung UHD TV hanging off internal Iris graphics HDMI native res 3840x2160 at 60 Hz either latest Win 10 or vanilla Ubuntu 20.04.

Recently the TV started saying "mode not recognized" when I boot into Ubuntu.

When I switch to a little multi-sync moni on the fly, res is at FHD at 60, which the TV should very well support. (When I boot 20.04 on a >10y old PC the 49" TV goes through all kinds of weird resolutions which must be VGA 640x480,.. scaled to 49" at some point!)

When I start in recovery mode and resume normal boot everything is fine on the TV and it just said it is at UHD 30p. (And, yes, ****, refresh rate setting GUI is gone on 20.04. ****!)

AFAIK the PC differentiates between monitors, everyone having some UID. There must be some weird setting be stored with the TV when Ubuntu starts normally.

Where can I find this?

When I change settings on the small moni it doesn't help.

Just deleted ~/.config/monitors.xml. Didn't help.

Or what is the culprit? TIA G.

gue22 avatar
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Now this is where things get rrreally interesting.
gue22 avatar
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Luckily I had set up RealVNC on the Mac to Vino on 20.04 fully patched. When I connect I can all of a sudden see the details of a recurring crash. gnome-shell (3.36.9) crashed with SIGSEGV in meta_monitor_get_outputs() DisplayManager gdm3 xrandr says output is to DP-1 while the TV actually hangs off HDMI-1 Will physically disconnect the external NVIDIA 2070 (powered off) on Thunderbolt 3 Any takers now if I'm still stuck then?
gue22 avatar
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First off: Maybe I shoulda edited the q and not commented, but too late now. So: Unplugging the Thunderbolt 3 / USB-C cable didn't help. External GPU was off anyway. xrandr --output HDMI-1 didn't change anything. Interestingly refresh rate is available in Settings in normal (non-recovery) mode. Changing it I got a ~/.config/monitors.xml, Changing the connector entry from DP-1 to HDMI-1 didn't do the trick either. Sadly I got no DP-port-monitor to check. This must be a bug in Gnome, because GRUB 2.04 displays its very few boot lines nicely on the huge TV. Thanks for any help.
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