Re: [PATCH 1/5] PCI: let 'pcibios_root_bridge_prepare()' access to 'bridge->windows'

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Guenter from other thread:
https://lore.kernel.org/r/20211129015909.GA921717@roeck-us.net]

On Wed, Dec 01, 2021 at 09:56:22PM +0100, Sergio Paracuellos wrote:
> On Wed, Dec 1, 2021 at 9:24 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Fri, Nov 19, 2021 at 05:20:17PM -0600, Bjorn Helgaas wrote:
> > > [+cc Thierry]
> > >
> > > In subject,
> > >
> > >   PCI: Let pcibios_root_bridge_prepare() access bridge->windows
> > >
> > > On Mon, Nov 15, 2021 at 08:08:05AM +0100, Sergio Paracuellos wrote:
> > > > When function 'pci_register_host_bridge()' is called, 'bridge->windows' are
> > > > already available. However this windows are being moved temporarily from
> > > > there. To let 'pcibios_root_bridge_prepare()' to have access to this windows
> > > > move this windows movement after call this function. This is interesting for
> > > > MIPS ralink mt7621 platform to be able to properly set I/O coherence units
> > > > with this information and avoid custom MIPs code in generic PCIe controller
> > > > drivers.
> > > >
> > > > Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
> > > > ---
> > > >  drivers/pci/probe.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > > > index 087d3658f75c..372a70efccc6 100644
> > > > --- a/drivers/pci/probe.c
> > > > +++ b/drivers/pci/probe.c
> > > > @@ -898,8 +898,6 @@ static int pci_register_host_bridge(struct pci_host_bridge *bridge)
> > > >
> > > >     bridge->bus = bus;
> > > >
> > > > -   /* Temporarily move resources off the list */
> > > > -   list_splice_init(&bridge->windows, &resources);
> > >
> > > Arnd added this with 37d6a0a6f470 ("PCI: Add
> > > pci_register_host_bridge() interface") [1].
> > >
> > > I can't remember why this was done, but we did go to some trouble to
> > > move things around, so there must have been a good reason.
> > >
> > > Arnd or Thierry, do you remember?
> >
> > Nobody seems to remember, so I think we should go ahead and make this
> > change after the usual due diligence (audit the code between the old
> > site and the new site to look for any uses of bridge->windows).
> 
> It seems any user of the pcibios_root_bridge_prepare() does nothing
> with 'bridge->windows'. But I don't get the point of passing around a
> complete bridge pointer if windows are temporarily removed from there.
> That is an incomplete bridge and after parsing  windows from dts are
> supposed to be there... What do you mean with 'audit the code between
> the old and new site'?

I mean "look at all the code that is run between the old site and the
new site to make sure that none of that code depends on
bridge->windows being temporarily emptied."

> > I think this would be material for v5.17.
> 
> Do you prefer me to parse dts again inside
> pcibios_root_bridge_prepare() for ralink mt7621?. Not real sense since
> 'windows' should be already there, but it would be a way to get this
> patchset added for v5.16. Something like (not tested yet but it should
> work):

This is definitely too big for v5.16, regardless of which way you go.
For v5.16, the only thing that's practical is to avoid building as a
module.  It'd be *nice* if it could be built as a module, but it is
not a requirement.

Bjorn