OSIOP_MAX_XFER
err = bus_dmamap_create(sc->sc_dmat, OSIOP_MAX_XFER, OSIOP_NSG,
OSIOP_MAX_XFER, 0, BUS_DMA_NOWAIT, &acb->datadma);
if (bp->b_bcount > OSIOP_MAX_XFER)
bp->b_bcount = OSIOP_MAX_XFER;