put_eth_data(port, offset, data, for_ioctl) is called only by reply_thr_put() with data, put_eth_data()'s third parameter, set to null. If there are no ethernet packets waiting to be delivered to the ip port, put_eth_data() simply clears the IEF_READ_IP flag. If data is null and there are ethernet packets waiting to be delivered to the ip port, put_eth_data() calls do_eth_read() to process the packets.