udl module, essential? comes from where?

[Jim2220]

Another clue:
If I boot my desktop PC without the Xenarc then do
lsmod | grep udl
nothing (i.e. no udl module) is there.
sudo modprobe udl
lsmod | grep udl
still nothing is there.

Repeating this experiment with the Xenarc plugged in at boot, I can load udl manually with modprobe.
Unfortunately, having udl pre-loaded has no beneficial effects on my program being able to initialize and output to the Xenarc one second after my program starts. It still goes offline.

Linux Mint hotplug / unplug sounds correspond exactly to the appearance / disappearance of Displaylink CD on the "Disks" utility screen.