Bug#944838: xorg: after recent upgrade on two different machines, X no longer responds to kbd or mouse input

[Peter Habcak]

 > On one machine (the AMDGPU one) `libinput debug-events --verbose` 
would complain about certain devices not being initialised by udev, 
including the keyboard device. Upgrading udev from version 243-5 to 
243-7 and reloading udev by running `udevadm control --reload-rules && 
udevadm trigger` and then restarting lightdm worked.

Yes, this fixed the issue for me as well. Upgrading udev was not enough, 
but after reloading udev everything runs smoothly and the fix seems to 
be permanent (no issues after reboot).
It fixed also another issue I was about to mention - "suspend when 
laptop lid is closed" stopped working with udev 243-5 as well.

 > Oh and another thing for people wanting to play around with this, 
when you run `systemctl disable lightdm` and then fix the problem and 
then run `systemctl enable lightdm` the latter does not work for some 
stupid reason, even though the former command worked. You have to run 
`dpkg-reconfigure lightdm` to make it work again.

Same here, dpkg-reconfigure lightdm helped

#systemctl enable lightdm.service
Synchronizing state of lightdm.service with SysV service script with 
/lib/systemd/systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install enable lightdm
The unit files have no installation config (WantedBy=, RequiredBy=, Also=,
Alias= settings in the [Install] section, and DefaultInstance= for template
units). This means they are not meant to be enabled using systemctl.

Possible reasons for having this kind of units are:
• A unit may be statically enabled by being symlinked from another unit's
   .wants/ or .requires/ directory.
• A unit's purpose may be to act as a helper for some other unit which has
   a requirement dependency on it.
• A unit may be started when needed via activation (socket, path, timer,
   D-Bus, udev, scripted systemctl call, ...).
• In case of template units, the unit is meant to be enabled with some
   instance name specified.