MMIO and gcc re-ordering issue

[Matthew Wilcox]

On Tue, Jun 03, 2008 at 12:57:56PM -0700, Trent Piepho wrote:
> 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."

Then Nick is confused.  PCI only defines one way to flush posted writes
to a device -- doing a read from it.  There's no way that reads can
be allowed to pass writes (unless you've asked for it, like with write
combining).

-- 
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