One comment:

Since the `op' argument to bus_dmamap_sync is most likely going to be
a compile-time constant at each call site, from the sake of
microefficiency, it might be better to have four distinct
bus_dmamap_sync ops ..

	bus_dmamap_sync_preread()
	bus_dmamap_sync_postread()
	bus_dmamap_sync_prewrite()
	bus_dmamap_sync_postwrite()

					- Bill