PDC_BLOCK_SIZE
at91usart_rx_rts_ctl(sc, (AT91PDC_FIFO_SPACE(&sc->sc_rx_fifo) > PDC_BLOCK_SIZE * 2));
&sc->sc_rx_fifo, PDC_BLOCK_SIZE)) {
US_PDC, &sc->sc_tx_fifo, PDC_BLOCK_SIZE)) {