w5100_write16
w5100_write16(priv, W5100_S0_TX_WR(priv), offset + skb->len);
w5100_write16(priv, W5100_S0_RX_RD(priv), offset + rx_buf_len);
w5100_write16(priv, W5100_S0_RX_RD(priv), offset + 2 + rx_len);