HALF_BUF_SIZE
int offs = sc->sc_dmaoffs ^ HALF_BUF_SIZE;
bus_dmamap_sync(sc->sc_dmat, sc->sc_dmamap, offs, HALF_BUF_SIZE,
if ((len = chunk->chunk_rresid) > HALF_BUF_SIZE)
len = HALF_BUF_SIZE;
len = MIN(chunk->chunk_wresid, HALF_BUF_SIZE);
sc->sc_dmaoffs ^= HALF_BUF_SIZE;