BCM2835_SPI_DLEN
bcm2835_wr(bs, BCM2835_SPI_DLEN, 0);
bcm2835_wr(bs, BCM2835_SPI_DLEN, bs->rx_prologue);
bcm2835_wr(bs, BCM2835_SPI_DLEN, tx_remaining);
bcm2835_wr(bs, BCM2835_SPI_DLEN, bs->tx_len);