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How do I get my extra monitors working again?

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James Barton

I'm running 21.04, with an nvidia Quadro P400 to drive three HP VH240a monitors. This worked fine for over a year ago when I was running xubuntu, and it seemed to work fine when I switched to ubuntu 21.04 for reasons that had nothing to do with screens displays. The only think I ever noticed after switching distro was that all three screens would sometimes go blank for a couple of seconds; this happened perhaps once per hour.

Anyway, I just rebooted the machine today, and suddenly only one monitor seems to be visible to the system. The "Screen Display" section of settings has no options for multiple screens, and the one screen that is working has been forced from its native resolution to 1024 * 768.

The "Additional Drivers" section of Software and Updates showed that the system was using X.org X server, rather than an nvidia package. I don't need any particular level of performance, so I don't much care which driver I use, as long as it works reliably.

I guess this could be a hardware problem, but coming right after a reboot I suspect a bug related to an update? Normally I suspend the system when I'm done using it, so it probably hadn't been rebooted for a a week or two. How do I investigate this? What additional information do I need?

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WinEunuuchs2Unix avatar
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WinEunuuchs2Unix
Try rebooting with the previous kernel version.
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in flag
James Barton
@WinEunuuchs2Unix That seems to be the right answer. The monitors became available to the OS again immediately. I just needed to remember how to use GRUB. There are some weird patterns displaying when the system boots, and the mouse is freezing as well after booting is complete, at least a few times. Not sure what the cause is, but I am tempted to go back to 20.04. If I don't want problems, I should probably stay on LTS.
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WinEunuuchs2Unix avatar
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WinEunuuchs2Unix
After rebooting, setup grub so it always boots with the last kernel version chosen from the menu, rather than the most recent kernel. I'm a big fan on staying on LTS rather than upgrading all the time. I posted an answer you can accept if appropriate. If not please post an answer of your own.
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Nate T avatar
it flag
Nate T
Kernel version will only change via update (unless you changed it manually.) Did you update apt? I see it was solved, but it still doesn't make sense to me how the kernel was updated. Maybe the changes didn't take effect until reboot?
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WinEunuuchs2Unix avatar Ubuntu
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WinEunuuchs2Unix

As per comments the problem was solved by booting to the kernel version previous to the upgrade:

This link also shows you how to make grub always boot to a specific kernel version each time.

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