[RESEND PATCH v5] PCI: Make sure the bus bridge powered on when scanning bus

[Yicong Yang <yangyicong@hisilicon.com>]

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 a Root Port and it is runtime-suspended, so
0000:81:00.1 is unreachable after a rescan.

Power up the bridge when scanning the child bus and allow it to
suspend again by adding pm_runtime_get_sync()/pm_runtime_put()
in pci_scan_child_bus_extend().

Cc: Rafael J. Wysocki <rafael@kernel.org>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 drivers/pci/probe.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 17a969942d37..b108e72b6586 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2859,11 +2859,20 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
 	unsigned int used_buses, normal_bridges = 0, hotplug_bridges = 0;
 	unsigned int start = bus->busn_res.start;
 	unsigned int devfn, fn, cmax, max = start;
+	struct pci_dev *bridge = bus->self;
 	struct pci_dev *dev;
 	int nr_devs;
 
 	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 (bridge)
+		pm_runtime_get_sync(&bridge->dev);
+
 	/* Go find them, Rover! */
 	for (devfn = 0; devfn < 256; devfn += 8) {
 		nr_devs = pci_scan_slot(bus, devfn);
@@ -2976,6 +2985,9 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
 		}
 	}
 
+	if (bridge)
+		pm_runtime_put(&bridge->dev);
+
 	/*
 	 * 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
-- 
2.24.0