For FAT the same buffer is used for read and writes, there
is a possibility a cache line is dirty. But the fs is
not dirty and will not write the sector to disk. This can
be seen https://github.com/PX4/NuttX/pull/175
When the system is busy that cache line can be evicted after the
RX DMA has completed and overwrite the data in memory. The solution
is to invalidate before the DMA to prevent an evection causing an
overwite, and after the DMA it to insure coherency.
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