HDMI monitor stopped working after windows laptop plugged in

Question

I'm running Xubuntu 20.04.3 LTS with Nvidia GTX 1050 Ti graphics card on my desktop. Everything was fine until I plugged my windows laptop into the same monitors that my Linux system uses. My desktop uses HDMI and displayport for monitors 1 and 2 respectively and my laptop used displayport and HDMI for monitors 1 and 2 respectively. After I disconnected my laptop and reconnected my desktop my HDMI monitor was undetected by Xubuntu (the displayport monitor was still ok).

I installed proprietary Nvidia drivers 470 (replacing the Nouveau drivers), which fixed the problem. But today I plugged my Windows laptop into the monitors and the same thing has happened, even though I was careful to make sure my desktop was switched off when I connected and disconnected the laptop.

Most solutions I have seen suggest reinstalling the graphics drivers or replacing the cable but it seems unlikely that either of these can be the root of the problem. I'm confused about how Xubuntu can have been affected when I connected and disconnected the laptop when the desktop machine was shut down. Can anyone help? I'm not an expert so please go easy.

Edit:

In /var/log/Xorg.0.log I see multiple entries like this:

[     5.180] (II) config/udev: Adding input device HDA NVidia HDMI/DP,pcm=8 (/dev/input/event9)
[     5.180] (II) No input driver specified, ignoring this device. 
[     5.180] (II) This device may have been added with another device file.

Earlier in the log I see:

[     4.741] (II) NOUVEAU driver Date:   Mon Jan 28 23:25:58 2019 -0500
[     4.741] (II) NOUVEAU driver for NVIDIA chipset families :
[     4.741]    RIVA TNT            (NV04)
[     4.741]    RIVA TNT2           (NV05)
[     4.741]    GeForce 256         (NV10)
[     4.741]    GeForce 2           (NV11, NV15)
[     4.741]    GeForce 4MX         (NV17, NV18)
[     4.741]    GeForce 3           (NV20)
[     4.741]    GeForce 4Ti         (NV25, NV28)
[     4.741]    GeForce FX          (NV3x)
[     4.741]    GeForce 6           (NV4x)
[     4.741]    GeForce 7           (G7x)
[     4.741]    GeForce 8           (G8x)
[     4.741]    GeForce 9           (G9x)
[     4.741]    GeForce GTX 2xx/3xx (GT2xx)
[     4.741]    GeForce GTX 4xx/5xx (GFxxx)
[     4.741]    GeForce GTX 6xx/7xx (GKxxx)
[     4.741]    GeForce GTX 9xx     (GMxxx)
[     4.741]    GeForce GTX 10xx    (GPxxx)

It looks like the Nouveau driver is being loaded before the Nvidia driver.

Answer 1

I solved this by blacklisting the Nouveau driver, following the instructions on this page: https://linuxconfig.org/how-to-disable-blacklist-nouveau-nvidia-driver-on-ubuntu-20-04-focal-fossa-linux

$ sudo bash -c "echo blacklist nouveau > /etc/modprobe.d/blacklist-nvidia-nouveau.conf"
$ sudo bash -c "echo options nouveau modeset=0 >> /etc/modprobe.d/blacklist-nvidia-nouveau.conf"

$ cat /etc/modprobe.d/blacklist-nvidia-nouveau.conf
blacklist nouveau
options nouveau modeset=0

$ sudo reboot