[PATCH v4] PCI: introduce two interfaces to walk PCI buses

[Jiang Liu <liuj97@gmail.com>]

The pci_find_next_bus() is not hotplug safe, so introduce PCI hotplug
safe interfaces to walk PCI buses. To avoid some deadlock scenarios,
two interfaces are introduced.

The first one is pci_for_each_bus(), which walks all PCI buses holding
read lock on the pci_bus_sem.

The second one is pci_for_each_started_bus(), which walks all started
PCI buses without holding any global locks. Started PCI buses are those
which have been added to the device tree by calling device_add().
---
Hi Bjorn,
	How about this PCI bus iterator design? It's a little ugly that
we need to two interfaces to work around some deadlock scenarios.
And I plan to split the task into two parts:
	1) a hotplug safe PCI bus iteraror to replace pci_find_next_bus
	2) handle hotplug notifications to update bus related states.

My patchset at http://www.spinics.net/lists/linux-pci/msg17515.html has
patially solved issue 2 above for x86/ACPI, and will add more supports
for other platforms.
	Thanks!
	Gerry
---
 drivers/pci/bus.c   |   42 ++++++++++++++++++++++++++++++++++++++++++
 include/linux/pci.h |    1 +
 2 files changed, 43 insertions(+)

diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
index e16a8f0f..21b0ade 100644
--- a/drivers/pci/bus.c
+++ b/drivers/pci/bus.c
@@ -327,6 +327,48 @@ void pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
 }
 EXPORT_SYMBOL_GPL(pci_walk_bus);
 
+static int pci_bus_iter(struct pci_bus *bus,
+			int (* cb)(struct pci_bus *, void *), void *data)
+{
+	int rc;
+
+	rc = cb(bus, data);
+	if (rc == 0)
+		list_for_each_entry(bus, &bus->children, node) {
+			rc = pci_bus_iter(bus, cb, data);
+			if (rc)
+				break;
+		}
+
+	return rc;
+}
+
+/** pci_for_each_bus - walk all PCI buses and call the provided callback.
+ *  @cb   callback to be called for each bus found
+ *  @data arbitrary pointer to be passed to callback.
+ *
+ *  Walk all PCI buses and call the provided callback with pci_bus_sem held.
+ *
+ *  We check the return of @cb each time. If it returns anything
+ *  other than 0, we break out.
+ */
+int pci_for_each_bus(int (* cb)(struct pci_bus *, void *), void *data)
+{
+	int rc = 0;
+	struct pci_bus *bus;
+
+	down_read(&pci_bus_sem);
+	list_for_each_entry(bus, &pci_root_buses, node) {
+		rc = pci_bus_iter(bus, cb, data);
+		if (rc)
+			break;
+	}
+	up_read(&pci_bus_sem);
+
+	return rc;
+}
+EXPORT_SYMBOL(pci_for_each_bus);
+
 struct pci_bus *pci_bus_get(struct pci_bus *bus)
 {
 	if (bus)
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 3c5017d..1423a24 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -1060,6 +1060,7 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max,
 
 void pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
 		  void *userdata);
+int pci_for_each_bus(int (* cb)(struct pci_bus *, void *), void *data);
 int pci_cfg_space_size_ext(struct pci_dev *dev);
 int pci_cfg_space_size(struct pci_dev *dev);
 unsigned char pci_bus_max_busnr(struct pci_bus *bus);
-- 
1.7.9.5