Re: MMIO and gcc re-ordering issue

[Trent Piepho]

On Tue, 3 Jun 2008, Matthew Wilcox wrote:
> On Tue, Jun 03, 2008 at 11:47:00AM -0700, Trent Piepho wrote:
> > On Tue, 3 Jun 2008, Linus Torvalds wrote:
> > > On Tue, 3 Jun 2008, Nick Piggin wrote:
> > > > Linus: on x86, memory operations to wc and wc+ memory are not ordered
> > > > with one another, or operations to other memory types (ie. load/load
> > > > and store/store reordering is allowed). Also, as you know, store/load
> > > > reordering is explicitly allowed as well, which covers all memory
> > > > types. So perhaps it is not quite true to say readl/writel is strongly
> > > > ordered by default even on x86. You would have to put in some
> > > > mfence instructions in them to make it so.
> >
> > So on x86, these could be re-ordered?
> >
> > writel(START_OPERATION, CONTROL_REGISTER);
> > status = readl(STATUS_REGISTER);
>
> You wouldn't ask for write-combining memory mapping for control or
> status registers.

But Nick said, "store/load reordering is explicitly allowed as well, which
covers *all* memory types."
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