Re: [PATCH] PCI: Make sure the bus bridge powered on when scanning bus

Bjorn Helgaas
2020-09-18T16:17:41Z

On Fri, Sep 18, 2020 at 05:31:54PM +0800, Yicong Yang wrote:
> On 2020/9/18 5:07, Bjorn Helgaas wrote:
> > On Wed, Jul 29, 2020 at 07:30:23PM +0800, Yicong Yang wrote:
> >> When the bus bridge is runtime suspended, we'll fail to rescan
> >> the devices through sysfs as we cannot access the configuration
> >> space correctly when the bridge is in D3hot.
> >> It can be reproduced like:
> >>
> >> $ echo 1 > /sys/bus/pci/devices/0000:80:00.0/0000:81:00.1/remove
> >> $ echo 1 > /sys/bus/pci/devices/0000:80:00.0/pci_bus/0000:81/rescan
> >>
> >> 0000:80:00.0 is root port and is runtime suspended and we cannot
> >> get 0000:81:00.1 after rescan.
> >>
> >> Make bridge powered on when scanning the child bus, by adding
> >> pm_runtime_get_sync()/pm_runtime_put() in pci_scan_child_bus_extend().
> >>
> >> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> >> ---
> >> drivers/pci/probe.c | 11 +++++++++++
> >> 1 file changed, 11 insertions(+)
> >>
> >> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> >> index 2f66988..5bb502b 100644
> >> --- a/drivers/pci/probe.c
> >> +++ b/drivers/pci/probe.c
> >> @@ -2795,6 +2795,14 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
> >>
> >> dev_dbg(&bus->dev, "scanning bus\n");
> >>
> >> + /*
> >> + * Make sure the bus bridge is powered on, otherwise we may not be
> >> + * able to scan the devices as we may fail to access the configuration
> >> + * space of subordinates.
> >> + */
> >> + if (bus->self)
> >> + pm_runtime_get_sync(&bus->self->dev);
> >
> > I think if we do this, we should be able to remove the call from
> > pci_scan_bridge() added by d963f6512e15 ("PCI: Power on bridges before
> > scanning new devices"), right?
> >
> > The reason we need it here is because there are two paths to
> > pci_scan_child_bus_extend() and only one of them calls
> > pm_runtime_get_sync():
> >
> > pci_scan_bridge_extend
> > pm_runtime_get_sync
> > pci_scan_child_bus_extend
> >
> > pci_scan_child_bus
> > pci_scan_child_bus_extend
> >
> > If we move the pm_runtime_get_sync() from pci_scan_bridge_extend() to
> > pci_scan_child_bus_extend(), both paths should be safe.
>
> A bit different, I think. The issue I met is a bit different from
> Mika, as we go through different sysfs files. Think about rescanning
> device under a root port,
>
> when echo 1 > /sysfs/bus/pci/devices/${RootPort}/rescan:
>
> rescan_store()
> pci_rescan_bus(pdev->bus) /* we will rescan the root bus */
> pci_rescan_child_bus()
> pci_scan_child_bus_extend() /* we cannot wake up the bus bridge here as is on the root bus */
> pci_scan_bridge_extend() /* we have to wake up the root port here */
>
> when echo 1 > /sysfs/bus/pci/devices/${RootPort}/pci_bus/${PciBus}/rescan:
>
> rescan_store()
> pci_rescan_bus(bus) /* we will rescan the bus of the root port */
> pci_rescan_child_bus()
> pci_scan_child_bus_extend() /* we can wake up the bus bridge - root port here */
>
> As different bus is rescanned, so it'll have problem without patch
> d963f6512e15.

Sorry, I didn't quite follow the above.

The problem here is about scanning a bridge's secondary bus when the
bridge may be runtime-suspended. The bridge may be in D0, D1, D2, or
D3hot. It is not in D3cold. pm_runtime_get_sync() brings a device
that may have been runtime-suspended back to D0.

All PCI devices respond to config accesses when they are in D0, D1,
D2, or D3hot [1], so we don't need pm_runtime_get_sync() to access a
bridge's config space.

But when a bridge is not in D0, it does not initiate transactions on
its secondary bus [2], so we do need pm_runtime_get_sync() before we
attempt config accesses for any children.

pci_scan_bridge_extend() does not directly do anything with the
secondary bus, which is why I'm not sure it needs
pm_runtime_get_sync().

The accesses to the secondary bus are in pci_scan_slot(), so the
pm_runtime_get_sync() you added immediately before calling
pci_scan_slot() makes sense to me. Although possibly it could go in
pci_scan_slot() itself, since there are several other callers.

[1] PCIe r5.0, sec 5.3.1.4.1
[2] PCIe r5.0, sec 5.3.1 implementation note

> >> /* Go find them, Rover! */
> >> for (devfn = 0; devfn < 256; devfn += 8) {
> >> nr_devs = pci_scan_slot(bus, devfn);
> >> @@ -2907,6 +2915,9 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
> >> }
> >> }
> >>
> >> + if (bus->self)
> >> + pm_runtime_put(&bus->self->dev);
> > I would probably do this:
> >
> > struct pci_dev *bridge = bus->self;
> >
> > if (bridge)
> > pm_runtime_get_sync(&bridge->dev);
> > ...
> > if (bridge)
> > pm_runtime_put(&bridge->dev);
>
> Sure.
>
> Regards,
> Yicong
>
>
> >
> >> /*
> >> * We've scanned the bus and so we know all about what's on
> >> * the other side of any bridges that may be on this bus plus
> >> --
> >> 2.8.1
> >>
> > .
> >
>