quoted

I think there are two other valuable features provided by serio:

- an existing set of drivers written to the API
- the implementation of the tty_ldisc  

True, though I'd expect little of the data flow part of it to be reused.

Then your design is broken.

- a child of the uart node
- a reg property containing the line number if the parent has multiple
uarts (I'd expect this to rarely be used).

That surprises me as for current x86 platforms it would be the norm,
except that we use ACPI.

- baudrate and other line configuration (though I would expect the
slave driver to know all this and set it w/o DT. Also, we already have
a way to set baudrate in the parent node at least.)
- other standard device properties for interrupt, gpios, regulators.

Also to consider is whether muxing of multiple slaves is needed. It's
not anything I've seen come up, but it's not hard to imagine. I think
that can be considered later and shouldn't impact the initial binding
or infrastructure.

You can describe the child of the serial device as a mux and the children
of the mux as whatever so it comes out fine when you get to that point.

Alan