On 09/02/19 at 03:00, Gonzalo Pérez de Olaguer Córdoba wrote:
>
> 'the screen' is not clear to me. I suspect it means something else that
> is not a screen, because screens are not in charge of responding to
> keyboards or mouses.


  Of course they are.  Their response to kb/mouse events is the major
way you can tell the device is working, after leds going on/off.


> The kernel does it, Xorg also does it.


  And eudev does it, too.


> I'm not sure of whether eudev can be responsible of locking the
> keyboard management functions of the kernel,


  I did not write eudev is "responsible of locking the keyboard
management functions of the kernel", but for many years udev and eudev
have been the primary way Xorg detects and configures input devices. 
And in my case eudev is indeed the culprit, because no kb/mouse device
is present in /dev/input/ after boot (when eudev is not running) but
they do show up after I execute /etc/init.d/eudev start.  And after this
operation the DM starts fine and I can login as both the keyboard and
the mouse respond.