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

John Partridge
2006-11-02T01:10:19Z

David Miller wrote:
> From: John Partridge <johnip@sgi.com>
> 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
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E-Mail: johnip@sgi.com