diff --git a/drivers/acpi/pci_root.c b/drivers/acpi/pci_root.c
index bce469c0b48a..27adbfdffb0c 100644
--- a/drivers/acpi/pci_root.c
+++ b/drivers/acpi/pci_root.c
@@ -644,12 +644,19 @@ static int acpi_pci_root_start(struct acpi_device *device)
 	struct acpi_pci_root *root = acpi_driver_data(device);
 	struct acpi_pci_driver *driver;
 
+	if (system_state != SYSTEM_BOOTING)
+		pci_assign_unassigned_bus_resources(root->bus);
+
 	mutex_lock(&acpi_pci_root_lock);
 	list_for_each_entry(driver, &acpi_pci_drivers, node)
 		if (driver->add)
 			driver->add(root);
 	mutex_unlock(&acpi_pci_root_lock);
 
+	/* need to after hot-added ioapic is registered */
+	if (system_state != SYSTEM_BOOTING)
+		pci_enable_bridges(root->bus);
+
 	pci_bus_add_devices(root->bus);
 
 	return 0;