Serial mouse (trackball) not working with GNOME and systemd

[mounty1]

As per title.

• The sermouse module is listed in /etc/modules-load.d/sermouse.conf and is loaded at boot time.
  

• # ls -ltr /proc/tty/driver/serial:

  -r--r--r-- 1 root root 0 May 19 08:42 /proc/tty/driver/serial

  
  

• # dmesg | egrep -iw '(serial|mouse)':

  [    0.668737] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled [    0.721399] mousedev: PS/2 mouse device common for all mice

  
  

but moving the trackball does not move the pointer on screen.
A search for gentoo serial mouse turns up only very old (pre-systemd) or irrelevant hits. What's the next step?
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Michael Mounteney