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

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Message-ID: <20061031195811.GF26964@parisc-linux.org>
Date:	Tue, 31 Oct 2006 12:58:11 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Roland Dreier <rdreier@...co.com>
Cc:	"Michael S. Tsirkin" <mst@...lanox.co.il>,
	linux-kernel@...r.kernel.org, linux-ia64@...r.kernel.org,
	jeff@...zik.org, openib-general@...nib.org,
	linux-pci@...ey.karlin.mff.cuni.cz,
	David Miller <davem@...emloft.net>
Subject: Re: Ordering between PCI config space writes and MMIO reads?

On Tue, Oct 31, 2006 at 11:53:02AM -0800, Roland Dreier wrote:
>  > 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)

I don't see anything in the PCI spec which forbids it, but I would
expect that hardware designers don't actually do that in practice.
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