Messages in this thread | | | Subject | Re: [PATCH] Define wc_wmb, a write barrier for PCI write combining | From | Roland Dreier <> | Date | Tue, 28 Feb 2006 09:02:47 -0800 |
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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.
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