[PATCH] 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 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);
+
 	/* 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);
+
 	/*
 	 * 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