I was doing the digging too. While stock img would result in "sunxi-gmac gmac0 eth0: eth0: Type(6) PHY ID 001cc915 at 7 IRQ poll (gmac0-0:07)", Pine H64 img would give "sunxi-gmac gmac0 eth0: No PHY found!". So I'd guess it's either tx/rx delays or pin mapping.