I guess it would depend on the structure of the PHY - whether the PHY
is structured similar to a two port switch internally, having a MAC
facing the host and another MAC facing the media side. (I believe this
is exactly how the MACSEC versions of the 88x3310 are structured.)

If you don't have that kind of structure, then I would guess that doing
duplex adaption could be problematical.

If you don't have that sort of structure, i think rate adaptation
would have problems in general. Pause is not very fine grained. You
need to somehow buffer packets because what comes from the MAC is
likely to be bursty. And when that buffer overflows, you want to be
selective about what you throw away. You want ARP, OSPF and other
signalling packets to have priority, and user data gets
tossed. Otherwise your network collapses.

	Andrew