Is PHY always needed?
The following are the scenarios there such a connection can be established:


MAC-to-MAC full-duplex MII-based connection
MAC-to-MAC full-duplex GPSI(SNI)-based connection


The following are the scenarios there such a connection cannot be established:


MAC-to-MAC half-duplex MII-based connection
MAC-to-MAC half-duplex GPSI(SNI)-based connection
PHY-to-PHY any-duplex MII-based connection
PHY-to-PHY any-duplex GPSI(SNI)-based connection
MAC-to-MAC any-duplex RMII-based connection
PHY-to-PHY any-duplex RMII-based connection


Note: GPSI(SNI) is for 10 Mbps only, MII and RMII are for 10/100 Mbps.