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Mon Jun 27 16:47:34 EDT 2011

doesn't need to provide that level of ordering as its supposed to be
an inter-CPU barrier - which probably means we need to invent a new
barrier to deal with DMA memory ordering.  However, given the
difficulty of getting the existing barriers placed correctly, I don't
think inventing new barriers is a very good idea.

What we can do is view devices which perform DMA as being strongly
ordered with respect to their memory accesses - iow, they have an
implicit memory barrier before and after their accesses to memory.
This would make the CPUs use of mb() have a conceptual pairing with
the DMA agents.

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