[PATCH RFC 11/11] PCI: hotplug: movable bus numbers: compact the gaps in numbering

[Sergey Miroshnichenko]

If bus numbers are distributed sparsely and there are lot of devices in the
tree, hotplugging a bridge into the end of the tree may fail even if it has
less slots then the total number of unused bus numbers.

Thus, the feature of bus renaming relies on the continuity of bus numbers,
so if a bridge was unplugged, the gap in bus numbers must be compacted.

Let's densify the bus numbering at the beginning of a next PCI rescan.

Signed-off-by: Sergey Miroshnichenko <s.miroshnichenko at yadro.com>
---
 drivers/pci/probe.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index fe9bf012ef33..0c91b9d453dd 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1319,6 +1319,30 @@ static bool pci_new_bus_needed(struct pci_bus *bus, const struct pci_dev *self)
 	return true;
 }
 
+static void pci_compact_bus_numbers(const int domain, const struct resource *valid_range)
+{
+	int busnr_p1 = valid_range->start;
+
+	while (busnr_p1 < valid_range->end) {
+		int busnr_p2 = busnr_p1 + 1;
+		struct pci_bus *bus_p2;
+		int delta;
+
+		while (busnr_p2 <= valid_range->end &&
+		       !(bus_p2 = pci_find_bus(domain, busnr_p2)))
+			++busnr_p2;
+
+		if (!bus_p2 || busnr_p2 > valid_range->end)
+			break;
+
+		delta = busnr_p1 - busnr_p2 + 1;
+		if (delta)
+			pci_move_buses(domain, busnr_p2, delta, valid_range);
+
+		++busnr_p1;
+	}
+}
+
 static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
 					      unsigned int available_buses);
 /**
@@ -3691,6 +3715,9 @@ unsigned int pci_rescan_bus(struct pci_bus *bus)
 		pci_bus_update_immovable_range(root);
 		pci_bus_release_root_bridge_resources(root);
 
+		pci_compact_bus_numbers(pci_domain_nr(bus),
+					&root->busn_res);
+
 		max = pci_scan_child_bus(root);
 
 		pci_reassign_root_bus_resources(root);
-- 
2.23.0