Re: [PATCH] Define wc_wmb, a write barrier for PCI write combining

[Roland Dreier <rdreier@cisco.com>]

    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.

 - R.