get_eth_data(fd, offset, count, for_ioctl) is (indirectly) called by a number of functions within the ethernet code, including eth_write(). get_eth_data() performs one of several tasks, depending on the state of the ip port and the value of count, get_eth_data()'s third parameter.
If the state of the ip port is IES_MAIN (its state during normal operations) and count is nonzero, get_eth_data() returns the packet from the de_frame field of the ip port. In this way, eth_write() gets the packet from the ip code to send off to eth_send().
If count is zero, get_eth_data() does something different. After eth_write() calls eth_send() (and the ethernet frame is therefore delivered), eth_write() calls the ethernet's reply_thr_get() with count equal to zero. If the ethernet file descriptor was opened up by the ip code, reply_thr_get() is simply a wrapper for eth_get_data(). In this scenario, get_eth_data() sets the ip port's de_frame field to null (since eth_send() just passed this packet to the ethernet driver) and calls ipeth_restart_send() if there are any ethernet packets that the ip code is waiting to send.
If the ip port's state is IES_PROTO (its configuration state), get_user_data() handles an initialization-related task. If count, get_eth_data()'s third parameter, is not zero (0), get_eth_data() sets various fields of an nwio_ethopt struct appropriate for the ip protocol and then returns a pointer to the struct.
When ipeth_main() calls eth_ioctl() the first time, eth_ioctl() in turn (indirectly) calls eth_get_data() to get the nwio_ethopt struct constructed by eth_get_data().
If count is zero and the ip port's state is IES_PROTO, get_eth_data() calls ipeth_main() if additional initialization is necessary.