[RFC/RFT PATCH 03/18] PCI: Introduce pci_scan_root_bus_bridge()

[Bjorn Helgaas]

[+cc Ray, Scott, Jon]

On Tue, May 02, 2017 at 06:15:01PM +0100, Lorenzo Pieralisi wrote:
> On Fri, Apr 28, 2017 at 02:28:38PM +0200, Arnd Bergmann wrote:
> > On Wed, Apr 26, 2017 at 1:17 PM, Lorenzo Pieralisi
> > <lorenzo.pieralisi at arm.com> wrote:
> > > Current pci_scan_root_bus() interface is made up of two main
> > > code paths:
> > >
> > > - pci_create_root_bus()
> > > - pci_scan_child_bus()
> > >
> > > pci_create_root_bus() is a wrapper function that allows to create
> > > a struct pci_host_bridge structure, initialize it with the passed
> > > parameters and register it with the kernel.
> > >
> > > As the struct pci_host_bridge require additional struct members,
> > > pci_create_root_bus() parameters list has grown in time, making
> > > it unwieldy to add further parameters to it in case the struct
> > > pci_host_bridge gains more members fields to augment its functionality.
> > >
> > > Since PCI core code provides functions to allocate struct
> > > pci_host_bridge, instead of forcing the pci_create_root_bus() interface
> > > to add new parameters to cater for new struct pci_host_bridge
> > > functionality, it is more suitable to add an interface in PCI
> > > core code to scan a PCI bus straight from a struct pci_host_bridge
> > > created and customized by each specific PCI host controller driver.
> > >
> > > Add a pci_scan_root_bus_bridge() function to allow PCI host controller
> > > drivers to create and initialize struct pci_host_bridge and scan
> > > the resulting bus.
> > >
> > > Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
> > > Cc: Arnd Bergmann <arnd at arndb.de>
> > > Cc: Bjorn Helgaas <bhelgaas at google.com>
> > 
> > Good idea, yes. To avoid growing the number of interfaces too
> > much, should we change the existing users of pci_register_host_bridge
> > in host drivers over to this entry point, and make the other one
> > local to probe.c then?
> 
> Yes, the problem is that there are drivers (ie pcie-iproc.c) that
> require the struct pci_bus (created by pci_register_host_bridge())
> to fiddle with it to check link status and THEN scan the bus (so
> the pci_register_host_bridge() call can't be embedded in the scan
> interface - the driver requires struct pci_bus for pci_ops to work
> before scanning the bus itself).

I think code like iproc_pcie_check_link() that requires a struct
pci_bus before we even scan the bus is lame.  I think the driver
should be able to bring up the link before telling the PCI core about
the bridge.  Aardvark uses a typical pattern:

  advk_pcie_probe
    advk_pcie_setup_hw
      advk_pcie_wait_for_link
    pci_scan_root_bus

I would rather see iproc restructured along that line than add a
callback.

That would require replacing the pci_bus_read_config uses in
iproc_pcie_check_link() with something different, maybe iproc-internal 
accessors.  Slightly messy, but I think doable.

> I will see how I can accommodate this change because you definitely
> have a point.
> 
> > > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > > index 7e4ffc4..c7a7f72 100644
> > > --- a/drivers/pci/probe.c
> > > +++ b/drivers/pci/probe.c
> > > @@ -2412,6 +2412,44 @@ void pci_bus_release_busn_res(struct pci_bus *b)
> > >                         res, ret ? "can not be" : "is");
> > >  }
> > >
> > > +int pci_scan_root_bus_bridge(struct pci_host_bridge *bridge)
> > > +{
> > > +       struct resource_entry *window;
> > > +       bool found = false;
> > > +       struct pci_bus *b;
> > > +       int max, bus, ret;
> > > +
> > > +       if (!bridge)
> > > +               return -EINVAL;
> > > +
> > > +       resource_list_for_each_entry(window, &bridge->windows)
> > > +               if (window->res->flags & IORESOURCE_BUS) {
> > > +                       found = true;
> > > +                       break;
> > > +               }
> > > +
> > > +       ret = pci_register_host_bridge(bridge);
> > > +       if (ret < 0)
> > > +               return ret;
> > > +
> > > +       b = bridge->bus;
> > > +       bus = bridge->busnr;
> > > +
> > > +       if (!found) {
> > > +               dev_info(&b->dev,
> > > +                "No busn resource found for root bus, will use [bus %02x-ff]\n",
> > > +                       bus);
> > > +               pci_bus_insert_busn_res(b, bus, 255);
> > > +       }
> > > +
> > > +       max = pci_scan_child_bus(b);
> > > +
> > > +       if (!found)
> > > +               pci_bus_update_busn_res_end(b, max);
> > > +
> > > +       return 0;
> > > +}
> > > +
> > 
> > We probably want an EXPORT_SYMBOL() here as well.
> 
> Yep, sure.
> 
> Thanks for having a look !
> 
> Lorenzo
> 
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