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

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Message-Id: <201001271345.54454.bjorn.helgaas@hp.com>
Date:	Wed, 27 Jan 2010 13:45:48 -0700
From:	Bjorn Helgaas <bjorn.helgaas@...com>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
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 Wednesday 27 January 2010 09:53:37 am Jesse Barnes wrote:
> 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)?

Without intel_bus.c, we essentially assume config 1 all the time.
If we keep intel_bus.c and this patch for .33, things should work
for configs 1 and 4.  Adding support for config 4 is good.

The bad part is that for config 4, intel_bus.c covers up any defects
in the _CRS or the Linux code that interprets it.  The reason Yinghai
added intel_bus.c in the first place was to work around a defect in
this area[1].  Keeping it will make it harder to fix the underlying
issue that keeps us from turning on _CRS for that box.

Bjorn

[1] http://lkml.org/lkml/2009/10/6/371
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