[PATCH 1/5] PCI: Make sure all bridges reserve at least one bus number

[Mika Westerberg <mika.westerberg@linux.intel.com>]

When distributing extra buses between hotplug bridges we need to make
sure each bridge reserve at least one bus number, even if there is
currently nothing connected to it. For instance ACPI hotplug may bring
in additional devices to non-hotplug bridges later on. To make sure we
don't run out of bus numbers for non-hotplug bridges reserve one bus
number for them upfront before distributing buses for hotplug bridges.

Fixes: 1c02ea810065 ("PCI: Distribute available buses to hotplug-capable bridges")
Reported-by: Mario Limonciello <mario.limonciello@dell.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: stable@vger.kernel.org
---
 drivers/pci/probe.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index ef5377438a1e..3e91d2191a7d 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2561,7 +2561,8 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
 	for_each_pci_bridge(dev, bus) {
 		cmax = max;
 		max = pci_scan_bridge_extend(bus, dev, max, 0, 0);
-		used_buses += cmax - max;
+		/* Reserve one bus for each bridge */
+		used_buses += cmax - max ?: 1;
 	}
 
 	/* Scan bridges that need to be reconfigured */
@@ -2584,12 +2585,14 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
 			 * bridges if any.
 			 */
 			buses = available_buses / hotplug_bridges;
-			buses = min(buses, available_buses - used_buses);
+			buses = min(buses, available_buses - (used_buses - 1));
 		}
 
 		cmax = max;
 		max = pci_scan_bridge_extend(bus, dev, cmax, buses, 1);
-		used_buses += max - cmax;
+		/* One bus is already accounted so don't add it again */
+		if (max - cmax > 1)
+			used_buses += (max - cmax) - 1;
 	}
 
 	/*
-- 
2.15.1