How works MDIO

If you are familiar with I2C, MDIO is similar. The PHY address or ID is the address of the PHY device on the MDIO bus. There can be more than PHY device on the MDIO bus. Messages are sent with PHY addresses to select the proper device. Register addresses are specify addresses within each PHY. PHYs usually have a standard set of registers that a generic driver can talk to. PHYs usually have non-standard extensions to the register map for special functions. The MDIO bus is usually not directly connected to a MDIO controller in the processor. The MDIO bus is usually connected to the MAC which is then connected to the processor. Code will go through the MAC to access the MDIO bus.