Hi Neil

Yes this would be a good idea if we were able to scan the internal
and external PHYs at the same time, but with our limited knowledge
the values we write in the register seems to switch a mux for the
whole RMII and MDIO signals to either interface.

Ah, O.K. So you need something like:

drivers/net/phy/mdio-mux-mmioreg.c

This seems to be limited to a single byte for the mux register, so you
might need to make some extensions.

      Andrew