Re: [Bug #15124] PCI host bridge windows ignored (works with pci=use_crs)

Jesse Barnes
2010-01-27T16:53:37Z

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