Re: [PATCH] PCI: Release unused bridge resources during resize

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jun 07, 2024 at 05:01:56PM -0600, Alex Williamson wrote:
> On Fri, 7 Jun 2024 17:33:20 -0500
> Bjorn Helgaas <helgaas@kernel.org> wrote:
> 
> > On Tue, May 07, 2024 at 03:31:23PM -0600, Alex Williamson wrote:
> > > Resizing BARs can be blocked when a device in the bridge hierarchy
> > > itself consumes resources from the resized range.  This scenario is
> > > common with Intel Arc DG2 GPUs where the following is a typical
> > > topology:
> > > 
> > >  +-[0000:5d]-+-00.0-[5e-61]----00.0-[5f-61]--+-01.0-[60]----00.0  Intel Corporation DG2 [Arc A380]
> > >                                              \-04.0-[61]----00.0  Intel Corporation DG2 Audio Controller
> ...

> > > pci 0000:60:00.0: BAR 2 [mem 0xbfe0000000-0xbfefffffff 64bit pref]: releasing
> > > pcieport 0000:5f:01.0: bridge window [mem 0xbfe0000000-0xbfefffffff 64bit pref]: releasing
> > > pci 0000:5e:00.0: bridge window [mem 0xbfe0000000-0xbfefffffff 64bit pref]: releasing
> > > pci 0000:5e:00.0: BAR 0 [mem 0xbff0000000-0xbff07fffff 64bit pref]: releasing
> > > pcieport 0000:5d:00.0: bridge window [mem 0xbfe0000000-0xbff07fffff 64bit pref]: releasing
> > > pcieport 0000:5d:00.0: bridge window [mem 0xb000000000-0xb2ffffffff 64bit pref]: assigned
> > > pci 0000:5e:00.0: bridge window [mem 0xb000000000-0xb1ffffffff 64bit pref]: assigned
> > > pci 0000:5e:00.0: BAR 0 [mem 0xb200000000-0xb2007fffff 64bit pref]: assigned

It didn't occur to me before, but don't we have a potential issue here
because we relocated 5e:00.0 BAR 0 from 0xbff0000000 to 0xb200000000
without notifying any code that might be using it?

I'm worried about vendor-specific perf/EDAC/etc drivers that can't
claim the the bridge using pci_register_driver() because the pcieport
driver is in the way.  I think some of those drivers go behind the
back of the driver model to locate their device and ioremap the BAR
direction.

Bjorn