how can i know the type of input in ethernet generated from pluggin?

writedata and ff_tx_data are part of two different interfaces. writedata is part of an Avalon Memory Mapped interface, used to configure the MAC registers, while ff_tx_data is the Avalon Stream interface used to send actual packets. The clocked used on the PHY side depends on the interface between the MAC and the PHY and needs to be precise. The clock used on the avalon side should just be at least fast enough to support the bandwidth.