linux-kernel - Re: [Bug #15124] PCI host bridge windows ignored (works with pci=use

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Message-ID: <20100127085337.6ff06f6e@jbarnes-piketon>
Date:	Wed, 27 Jan 2010 08:53:37 -0800
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Bjorn Helgaas <bjorn.helgaas@...com>
Cc:	Yinghai Lu <yinghai@...nel.org>, Jeff Garrett <jeff@...rrett.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Linux PCI <linux-pci@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Myron Stowe <myron.stowe@...com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [Bug #15124] PCI host bridge windows ignored (works with
 pci=use_crs)

On Wed, 27 Jan 2010 09:45:15 -0700
Bjorn Helgaas <bjorn.helgaas@...com> wrote:

> On Tuesday 26 January 2010 03:57:31 pm Yinghai Lu wrote:
> > [PATCH] x86/pci: don't use ioh resource if only have one ioh
> > 
> > some system could use reosurce out of IOH resources when only one ioh is there.
> > 
> > could be BIOS have wrong IOH resources and not enable them.
> 
> The subtractive decode theory makes sense and would explain what's
> happening, but I don't like this patch.
> 
> If we assume that this really is a subtractive decode issue, this
> patch approaches it the wrong way.  We need to know whether a
> particular host bridge is configured for subtractive decode.  This
> patch tests whether we have more than one host bridge, which is quite
> a different question.
> 
> Imagine these system configurations:
> 
>   1) a single host bridge with subtractive decode
>   2) a single host bridge with only positive decode
>   3) multiple host bridges with subtractive decode enabled on one
>   4) multiple host bridges with only positive decode
> 
> This patch will break if we encounter configs 2 or 3.  In config 2,
> this patch assumes the bridge performs subtractive decode, so we
> think the bridge forwards more address space than it actually does.
> If we try to use that address space, the device will never see the
> accesses.  In config 3, this patch assumes there's no subtractive
> decode, so we would see Jeff's problem all over again.

Right, but OTOH:
  - multiple IOH has already been tested with the intel_bus.c code
  - we want to move to using _CRS data in these cases instead

So do you have any objection to applying this patch for 2.6.33 and then
moving away from intel_bus.c in .34 (assuming we can get _CRS working
well on the same machines where intel_bus.c was needed)?

-- 
Jesse Barnes, Intel Open Source Technology Center
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