I meant "Either some function similar to userspace's poll() is
needed, ...". Something like uart_dev_wait_for_rx()

You can't really do that - it might never return and then how do you want
to handle timeouts and cleanups

Alternatively the rx function could be a callback, that
is called when there is new data.

That's what the existing API gives you as an ldisc, it can't be immediate
in many cases however but must be buffered.

quoted

I'm assuming the only immediate consumers are in-kernel.  

Yes, but the driver should be notified about incoming data.

Alan