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

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Message-ID: <20061024223631.GT25210@parisc-linux.org>
Date:	Tue, 24 Oct 2006 16:36:32 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Roland Dreier <rdreier@...co.com>
Cc:	Jeff Garzik <jeff@...zik.org>, linux-pci@...ey.karlin.mff.cuni.cz,
	linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
	openib-general@...nib.org, John Partridge <johnip@....com>
Subject: Re: Ordering between PCI config space writes and MMIO reads?

On Tue, Oct 24, 2006 at 02:51:30PM -0700, Roland Dreier wrote:
>  > I think the right way to fix this is to ensure mmio write ordering in
>  > the pci_write_config_*() implementations.  Like this.
> 
> I'm happy to fix this in the PCI core and not force drivers to worry
> about this.
> 
> John, can you confirm that this patch fixes the issue for you?

Hang on.  I wasn't thinking clearly.  mmiowb() only ensures the write
has got as far as the shub.  There's no way to fix this in the pci core
-- any PCI-PCI bridge can reorder the two.

This is only really a problem for setup (when we program the BARs), so
it seems silly to enforce an ordering at any other time.  Reluctantly, I
must disagree with Jeff -- drivers need to fix this.
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