Ah, I seem to have failed to point out the major issue here:
The standard configuration goes as follows: A PHY device is supposed to be configured by the MAC via the MII (media independent interface). (see
MII management)
This requires two signal lines that form a two-wire serial bus: MDC and MDIO.
So irrespective of that the MAC looks like, things like duplex mode, 10/100 speed selection and auto-negotiation options are configured via this interface.
Looking at it from the switch as the owner of the ethernet MAC, this interface should be accessible no matter what means of switch management are employed.
Now that bloody switch chip is
1) supposed to send configuration data to a PHY device as part of the MII interface via MDC/MDIO
2) at the same time configurable through SPI that shares the same I/Os?
I don't see how that would work because for 1) to happen the MDC/MDIO lines must operate as a TWI with the switch chip being the master device and for 2) the KSZ8873 has to shut up an receive commands as a slave SPI device.
Is the SPI only usable when the switch chip is in reset state (which it has to be for initial configuration but not necessarily for management afterwards)? If not, do I have to manage the third PHY myself?