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

[John Partridge]

David Miller wrote:
> From: John Partridge <johnip@xxxxxxx>
> Date: Wed, 01 Nov 2006 10:27:19 -0600
>
>
> > Sorry, but I find this change a bit puzzling. The problem is
> > particular to the PPB on the HCA and not Altix. I can't see anywhere
> > that a PCI Config Write is required to block until completion, it is
> > the driver and the HCA ,not the Altix hardware that requires the
> > Config Write to have completed before we leave mthca_reset()
> > Changing pci_write_config_xxx() will change the behavior for ALL
> > drivers and the possibility of breaking something else. The fix was
> > very low risk in mthca_reset(), changing the PCI code to fix this is
> > much more onerous.
>
>
> The issue is that something as simple as:
>
> 	val = pci_read_config(REG);
> 	val |= bit;
> 	pci_write_config(REG, val);
> 	newval = pci_read_config(REG);
> 	BUG_ON(!(newval & bit));
>
> is not guarenteed by PCI (aparently).
>
> I see no valid reason why every PCI device driver should
> be troubled with this lunacy and the ordering should thus
> be ensured by the PCI layer.
>
> It just so happens to take care of the original driver
> issue too :-)

Yeah, Matthew has convinced me of that now.

Thanks


--
John Partridge

Silicon Graphics Inc
Tel:	651-683-3428
Vnet:	233-3428
E-Mail:	johnip@xxxxxxx
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