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

Bjorn Helgaas
2010-01-26T17:32:44Z

On Tuesday 26 January 2010 05:48:59 am Rafael J. Wysocki wrote:
> On Tuesday 26 January 2010, Jeff Garrett wrote:
> > On Sun, Jan 24, 2010 at 11:04:38PM +0100, Rafael J. Wysocki wrote:
> > > The following bug entry is on the current list of known regressions
> > > from 2.6.32. Please verify if it still should be listed and let me know
> > > (either way).
> > >
> > > Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=15124
> > > Subject : PCI host bridge windows ignored (works with pci=use_crs)
> > > Submitter : Jeff Garrett <jeff@jgarrett.org>
> > > Date : 2010-01-13 5:37 (12 days old)
> > > References : http://marc.info/?l=linux-kernel&m=126336296600307&w=4
> > > Handled-By : Yinghai Lu <yinghai@kernel.org>
> > > Bjorn Helgaas <bjorn.helgaas@hp.com>
> >
> > This regression should still be listed. No patch to test yet.
> ...
> IIRC, we already know how to fix this ...

As far as I know, we do NOT know how to fix this.

This regression occurred when we added intel_bus.c because it's not
yet smart enough to determine the correct host bridge apertures.
Here's what it thinks the bridge aperture is and the Radeon BAR:

IOH bus: 00 index 1 mmio: [e0000000, fdffffff]
pci 0000:04:00.0: reg 10: [mem 0xd0000000-0xdfffffff 64bit pref]

The IOH aperture is obviously not big enough to cover the Radeon BAR.
But the host bridge _CRS tells us this:

pci_root PNP0A08:00: host bridge window [mem 0xc0000000-0xdfffffff]
pci_root PNP0A08:00: host bridge window [mem 0xf0000000-0xfed8ffff]

which IS big enough, and we know the bridge is in fact forwarding the
[mem 0xd0000000-0xdfffffff 64bit pref] region, because the Radeon works
when Jeff boots with "pci=use_crs".

I'm quite concerned about this for .33 because I don't think Jeff's
configuration (Dell desktop with Intel x58 and large graphics device)
is unusual.

The benefit of intel_bus.c is on machines with multiple IOHs, where we
need to figure out which address ranges go to which IOHs so we can
program downstream devices correctly. But even there, _CRS should give
us the information we need, so "pci=use_crs" should make these machines
work.

I think we should remove intel_bus.c before .33. It's breaking boxes
and we don't know how to fix it. Even if we do find out how to fix it,
I think we should move toward using _CRS instead, because that's what
Windows uses and it's an easy way for the firmware to tell us about
platform quirks.

Bjorn