SPI_FIFOLEN
avail = SPI_FIFOLEN - __SHIFTOUT(reg, SPI_TXFLR_TXFLR);
? MIN(sc->sc_rchunk->chunk_rresid, SPI_FIFOLEN / 2) : 0;
? MIN(sc->sc_wchunk->chunk_wresid, SPI_FIFOLEN / 2) : 0;
SPIREG_WRITE(sc, SPI_TXFTLR, SPI_FIFOLEN - len);