PDC_TCR
bus_space_write_4(iot, ioh, offset + PDC_TCR, 0);
PUTREG(sc, SPI_PDC_BASE + PDC_TCR, 0);
at91usart_writereg(sc, US_PDC + PDC_TCR, 0);
at91usart_writereg(sc, US_PDC + PDC_TCR, 0); \
US_PDC + PDC_TCR) && !sc->sc_tx_busy) {