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

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Message-ID: <20061024230908.GB13022@mellanox.co.il>
Date:	Wed, 25 Oct 2006 01:09:08 +0200
From:	"Michael S. Tsirkin" <mst@...lanox.co.il>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Roland Dreier <rdreier@...co.com>, linux-ia64@...r.kernel.org,
	Jeff Garzik <jeff@...zik.org>, linux-kernel@...r.kernel.org,
	openib-general@...nib.org, linux-pci@...ey.karlin.mff.cuni.cz
Subject: Re: Ordering between PCI config space writes and MMIO reads?

Quoting r. Matthew Wilcox <matthew@....cx>:
> 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.

This can be true for any bridge.
Most arches, however, simply block until config write completes - this is why
driver doesn't issue any MMIO writes - and this is what we are looking for here
- a way to block the CPU until split completion for config write arrives.

By the way, e.g. the PCI Express spec says:
"Read Requests and I/O or Configuration Write Requests are permitted to be
blocked by or to pass other Read Requests and I/O or Configuration Write Requests."
so it is not clear that doing a config read will always flush all config writes
as you want.

-- 
MST
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