On Tuesday, February 28, 2006 9:02 am, Roland Dreier wrote:
> Jes> Not quite correct as far as I understand it. mmiowb() is
> Jes> supposed to guarantee that writes to MMIO space have
> Jes> completed before continuing. That of course covers the
> Jes> multi-CPU case, but it should also cover the write-combining
> Jes> case.
>
> I don't believe this is correct. mmiowb() does not guarantee that
> writes have completed -- they may still be pending in a buffer in a
> bridge somewhere. The _only_ effect of mmiowb() is to make sure that
> writes which have been ordered between CPUs using some other mechanism
> (i.e. a lock) are properly ordered by the rest of the system. This
> only has an effect systems like very large ia64 systems, where (as I
> understand it), writes can pass each other on the way to the PCI bus.
> In fact, mmiowb() is a NOP on essentially every architecture.
I think it could be implemented meaningfully on ppc64, mips64, and
perhaps some parisc systems, but I don't think their respective
maintainers have gotten around to that yet.
Anyway, it looks like the write combine ordering Bryan is talking about
really is a distinct semantic. Not sure if it's possible (or desirable)
to overload an existing barrier op to include the semantics he wants.
Jesse
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