linux-kernel - Re: PCI Express MMCONFIG and BIOS Bug messages..

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Message-Id: <200704291127.21263.jbarnes@virtuousgeek.org>
Date:	Sun, 29 Apr 2007 11:27:20 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Robert Hancock <hancockr@...w.ca>
Cc:	Andi Kleen <ak@...e.de>, Chuck Ebbert <cebbert@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org
Subject: Re: PCI Express MMCONFIG and BIOS Bug messages..

On Sunday, April 29, 2007, Robert Hancock wrote:
> Problem is that even if we read the MMCONFIG table location from the
> hardware registers, that doesn't mean we can trust the result. It could
> be that the BIOS hasn't lied about where it put the table, it just stuck
> it someplace completely unsuitable like on top of RAM or other
> registers. It seems that with some of those 965 chipsets the latter is
> what the BIOS is actually doing, and so when we think we're writing to
> the table we're really writing to random chipset registers and hosing
> things. (Jesse Barnes ran into this while trying to add chipset support
> for the 965).

Right, I've updated the BIOS since, but at least that version was totally 
buggy wrt MMconfig support.  I haven't yet looked at the new one to see if 
it properly reserves MCFG space in ACPI _CRS yet or properly programs it.

> Likely what we need to do is:
>
> -If chipset is known, take table address from registers, otherwise check
> the MCFG table
> -Take the resulting area (Ideally not just the first minimum part as we
> check now, but the full area based on the expected length) and make sure
> that the entire area is covered by a reservation in ACPI motherboard
> resources.
> -If that passes, then we still need to sanity check the result by making
> sure it hasn't been mapped over top of something else important. How to
> do this depends on exactly how they've set up the ACPI reservations on
> these broken boxes.. Does someone have a full dmesg from one on a recent
> kernel that shows all the pnpacpi resource reservation output?
> -If these checks fail, we don't use the table, and the chipset is known,
> we should likely try to disable decoding of the region so that it won't
> get in the way of anything else.

Yeah, that sounds like a good algorithm.

I'm not sure how to handle the fact that we don't have access to the _CRS 
until late in boot though...  Len?

Jesse
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