w5100_read16
if (w5100_read16(priv, rtr) != RTR_DEFAULT)
offset = w5100_read16(priv, W5100_S0_TX_WR(priv));
u16 rx_buf_len = w5100_read16(priv, W5100_S0_RX_RSR(priv));
offset = w5100_read16(priv, W5100_S0_RX_RD(priv));