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

[Jes Sorensen <jes@sgi.com>]

Roland Dreier wrote:
>     Jes> Could you explain why the current mmiowb() API won't suffice
>     Jes> for this?  It seems that this is basically trying to achieve
>     Jes> the same thing.
> 
> I don't believe mmiowb() is at all the same thing.  mmiowb() is all
> about ordering writes between _different_ CPUs without incurring the
> cost of flushing posted writes by issuing a read on the bus.

Not quite correct as far as I understand it. mmiowb() is supposed to
guarantee that writes to MMIO space have completed before continuing.
That of course covers the multi-CPU case, but it should also cover the
write-combining case.

 > wc_wmb()
> would just act like a true wmb(), even when using write-combining
> regions on x86 -- in other words, there would be no cross-CPU synchronization.

I wary of adding yet another variation unless there is a clear
distinction between them that is easy to understandn for driver authors.

Cheers,
Jes