Re: Ordering between PCI config space writes and MMIO reads?

Michael S. Tsirkin
2006-10-31T20:28:00Z

Quoting r. Roland Dreier <rdreier@cisco.com>:
> Subject: Re: Ordering between PCI config space writes and MMIO reads?
>
> > Here's what I don't understand: according to PCI rules, pci config read
> > can bypass pci config write (both are non-posted).
> > So why does doing it help flush the writes as the comment claims?
>
> No, I don't believe a read of a config register can pass a write of
> the same register. (Someone correct me if I'm wrong)

It can if PCI-X/PCI-Ex spec is anything to go by.

For example, see table 2-23, transaction ordering rules, in the PCI-express
spec: it is marked as "Y/N: there are no requirements. The second transaction
may optionally pass the first transaction or be blocked by it."

In typical systems the OS should take care not to start a new
non-posted transaction before the previous one completed.
In particular, all intel and ppc systems I've seen simply block the CPU
unti the split completion arrives.

I find it hard to believe that Altix des not supply a way to check
that completion for config write transaction has arrived.

--
MST