linux-kernel - Re: [Patch v2] Make PCI extended config space (MMCONFIG) a driver opt-in

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Date:	Sat, 12 Jan 2008 19:56:01 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Matthew Wilcox <matthew@....cx>, Greg KH <gregkh@...e.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Greg KH <greg@...ah.com>, linux-kernel@...r.kernel.org,
	Jeff Garzik <jeff@...zik.org>,
	linux-pci@...ey.karlin.mff.cuni.cz,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Martin Mares <mj@....cz>
Subject: Re: [Patch v2] Make PCI extended config space (MMCONFIG) a driver
 opt-in

Linus Torvalds wrote:
> 
> On Fri, 11 Jan 2008, Matthew Wilcox wrote:
>> Did I miss a bug report?  The only problems I'm currently aware of are
>> the ones where using MMCONFIG during BAR probing causes a hard lockup on
>> some Intel machines, and the ones where we get bad config data on some
>> AMD machines due to the configuration retry status being mishandled.
> 
> Hmm. Were all those reports root-caused to just that BAR probing? If so, 
> we may be in better shape than I worried.

As far as I'm aware, the known MMCONFIG-related issues that I'm aware of 
are or have been:

-Some devices built into the AMD K8 integrated northbridge can't be 
reached by MMCONFIG - already handled

-Overlap of device BAR and MMCONFIG aperature during BAR sizing causing 
lockup - can be avoided by disabling device decode during BAR sizing.

-PCI Express CRS-related issues - already handled by disabling CRS by 
default

-Devices behind certain host bridges (some AMD HT to PCI-X bridges, 
others?) can't be reached by MMCONFIG - can be handled by Tony Camuso's 
patch or something similar (note that this is really a BIOS bug, it 
should not list those buses in the MCFG table if MMCONFIG cannot access 
them, and if it didn't I think we could already handle that)

-Some issue with some AMD CPUs needing MMCONFIG accesses to use a 
certain register I believe? already handled?

Of these, I think the PCI BAR/MMCONFIG overlap problem is responsible 
for by far the most cases of machines thought to have "broken MMCONFIG", 
when in fact they were nothing of the sort. I don't recall hearing of a 
single machine where MMCONFIG really just didn't work at all.

As I've mentioned before, all of these issues (well, I suppose not the 
BAR overlap one) need to be resolved whether we have Arjan's patch or 
not, otherwise if a driver does opt in and tries to use extended config 
space it will still break. And if they are resolved, the patch seems 
quite pointless.

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@...pamshaw.ca
Home Page: http://www.roberthancock.com/

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