When doing busdma sync ops for BUSDMA_COHERENT memory, there is no need
for cache maintenance operations, but ensure that all prior writes have
reached memory when doing a PREWRITE sync.
Do not do a cache invalidate on a PREREAD sync that is also a PREWRITE sync.
Do the cache invalidate sequence from the outermost to innermost, required
for correct operation.
Correct the sequence of busdma sync ops involved with PRE/POSTREAD syncs.
When doing a PREREAD sync of an mbuf-type dma buffer, do a writeback of
the first cacheline if the buffer start address is not on a cacheline
boundary.
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