Unable to grab keyboard with VT on a headless node in stable/12

From: Cejka Rudolf <cejkar_at_fit.vutbr.cz>
Date: Sat, 23 Oct 2021 15:03:35 UTC
  I have a headless node without monitor and without keyboard. There
is just network and USB card reader in keyboard mode.

In stable/11, there was no problem to disconnect USB card reader
from kbdmux using kbdcontrol -A ukbd0 < /dev/console and open
/dev/ukbd0 in a program.

However, under stable/12 it works just with connected monitor.

Without monitor, there is an error:

# kbdcontrol < /dev/console
-bash: /dev/console: Device not configured

Furthermore with default VT, it is also impossible to use /dev/kbdmux0:

# kbdcontrol < /dev/kbdmux0
-bash: /dev/kbdmux0: Device busy

I have found a workaround using SC, where it is possible to atleast
open /dev/kbdmux0 and use kbdcontrol -A ukbd0 < /dev/kbdmux0, but why
keyboard operation is now dependend on connected monitor? And why VT
does not allow to open /dev/kbdmux0?

Thank you.

Rudolf Cejka <cejkar at fit.vut.cz> http://www.fit.vut.cz/~cejkar
Brno University of Technology, Faculty of Information Technology
Bozetechova 2, 612 66  Brno, Czech Republic