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Symptom

Upgrading from a WQHD ViewSonic display to a giant 4K UHD ViewSonic display yields a No Signal error (via HDMI) after my workstation successfully makes it through GRUB and Plymouth screens. The Debian-branded Plymouth splash screen seems to display at 4K, but then it never displays the LightDM greeter; the new monitor loses the signal at that point.

I can switch over to TTY1 where that TTY seems to be running at 4K, based on tiny font size and its sharpness. Furthermore a forest-style ps reveals that the greeter is hanging off a TTY7 X process:

root       810     1  0 59886  9164   2 16:09 ?        00:00:00 /usr/sbin/lightdm
root       817   810  0 138738 82472  2 16:09 tty7     00:00:00  \_ /usr/lib/xorg/Xorg :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
root       843   810  0 41710  7412   2 16:09 ?        00:00:00  \_ lightdm --session-child 16 19
lightdm    858   843  0 196049 133484 2 16:09 ?        00:00:01  |   \_ /usr/sbin/lightdm-gtk-greeter
root       897   810  0  4687  5112   0 16:09 ?        00:00:00  \_ lightdm --session-child 12 19

Configuration

The sole GPU:

01:00.0 VGA compatible controller: NVIDIA Corporation GK107 [GeForce GT 640] (rev a1) (prog-if 00 [VGA controller])
    Subsystem: eVga.com. Corp. GK107 [GeForce GT 640]
    Flags: bus master, fast devsel, latency 0, IRQ 27
    Memory at f6000000 (32-bit, non-prefetchable) [size=16M]
    Memory at e0000000 (64-bit, prefetchable) [size=256M]
    Memory at f0000000 (64-bit, prefetchable) [size=32M]
    I/O ports at e000 [size=128]
    Expansion ROM at 000c0000 [disabled] [size=128K]
    Capabilities: [60] Power Management version 3
    Capabilities: [68] MSI: Enable+ Count=1/1 Maskable- 64bit+
    Capabilities: [78] Express Endpoint, MSI 00
    Capabilities: [b4] Vendor Specific Information: Len=14 <?>
    Capabilities: [100] Virtual Channel
    Capabilities: [128] Power Budgeting <?>
    Capabilities: [600] Vendor Specific Information: ID=0001 Rev=1 Len=024 <?>
    Capabilities: [900] #19
    Kernel driver in use: nouveau
    Kernel modules: nouveau

And

  • Debian 10
  • Kernel Debian 4.19.132-1 (2020-07-24) x86_64 GNU/Linux
  • lightdm 1.26.0-4
  • xfce4 4.12.5
  • xserver-xorg 1:7.7+19

Troubleshooting

Verified that the new monitor can be driven via HDMI at 4K by connecting it to a notebook running Pop_OS + Gnome.

Back on my main workstation I collected various LightDM, X11, and XFCE config and log files after booting separately with the WQHD and 4K monitors, looking for a smoking gun.

X11 seems to try to drive the monitor at 4K,

:~/troubleshooting/xfce_at_4k_uhd$ egrep -C2  "3840.+2160" *
Xorg.0.log-[    26.169] (II) modeset(0):    0000000000000000000000000000001a
Xorg.0.log-[    26.169] (--) modeset(0): HDMI max TMDS frequency 600000KHz
Xorg.0.log:[    26.169] (II) modeset(0): Not using default mode "3840x2160" (bad mode clock/interlace/doublescan)
Xorg.0.log-[    26.169] (II) modeset(0): Not using default mode "2560x1440" (bad mode clock/interlace/doublescan)
Xorg.0.log:[    26.169] (II) modeset(0): Not using default mode "3840x2160" (bad mode clock/interlace/doublescan)
Xorg.0.log:[    26.169] (II) modeset(0): Not using default mode "3840x2160" (bad mode clock/interlace/doublescan)
Xorg.0.log-[    26.169] (II) modeset(0): Printing probed modes for output HDMI-1
Xorg.0.log:[    26.169] (II) modeset(0): Modeline "3840x2160"x60.0  533.00  3840 3888 3920 4000  2160 2163 2168 2222 +hsync -vsync (133.2 kHz d)
Xorg.0.log:[    26.169] (II) modeset(0): Modeline "3840x2160"x24.0  297.00  3840 5116 5204 5500  2160 2168 2178 2250 +hsync +vsync (54.0 kHz e)
Xorg.0.log:[    26.169] (II) modeset(0): Modeline "3840x2160"x24.0  296.70  3840 5116 5204 5500  2160 2168 2178 2250 +hsync +vsync (53.9 kHz e)
Xorg.0.log:[    26.169] (II) modeset(0): Modeline "3840x2160"x30.0  262.75  3840 3888 3920 4000  2160 2163 2168 2191 +hsync -vsync (65.7 kHz e)
Xorg.0.log-[    26.169] (II) modeset(0): Modeline "3200x1800"x60.0  492.00  3200 3456 3800 4400  1800 1803 1808 1865 -hsync +vsync (111.8 kHz d)
Xorg.0.log-[    26.169] (II) modeset(0): Modeline "3200x1800"x59.9  373.00  3200 3248 3280 3360  1800 1803 1808 1852 +hsync -vsync (111.0 kHz d)
--
Xorg.0.log-[    26.170] (II) modeset(0): Output HDMI-1 connected
Xorg.0.log-[    26.170] (II) modeset(0): Using exact sizes for initial modes
Xorg.0.log:[    26.170] (II) modeset(0): Output HDMI-1 using initial mode 3840x2160 +0+0
Xorg.0.log-[    26.170] (==) modeset(0): Using gamma correction (1.0, 1.0, 1.0)
Xorg.0.log-[    26.170] (==) modeset(0): DPI set to (96, 96)

But it's unclear to me where things go wrong. Can anybody suggest what to look for, or what else to post here as clues?

Thanks

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I finally got to the bottom of most of this. For whatever reason this card, stock nouveau driver, etc. can't truly drive the monitor at 4K. XFCE and LightDM/GTK Greeter do work at 3K (2880x1620) by dialing back the resolution in the XFCE Display settings widget and configuring a resolution override via xrandr in /etc/lightdm/lightdm.conf:

display-setup-script=/usr/bin/xrandr --output HDMI-1 --primary --mode 2880x1620

More details of my troubleshooting are on another forum.

1
  • And the diagnosis came somewhat after the fixes above, overnight. My GPU HDMI output is almost certainly HDMI 1.4, which can drive 4K, but at 30Hz maximum. I assume that all the X bits are defaulting to the unsupported 60Hz refresh rate. – Brian Ray Nov 23 '20 at 16:43

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