diff --git a/drivers/acpi/pci_root.c b/drivers/acpi/pci_root.c
index b07eda1e7b05..bf601d4df8cf 100644
--- a/drivers/acpi/pci_root.c
+++ b/drivers/acpi/pci_root.c
@@ -614,6 +614,16 @@ static int acpi_pci_root_add(struct acpi_device *device,
 	if (hotadd) {
 		pcibios_resource_survey_bus(root->bus);
 		pci_assign_unassigned_root_bus_resources(root->bus);
+		/*
+		 * This is only called for the hotadd case. For the boot-time
+		 * case, we need to wait until after PCI initialization in
+		 * order to deal with IOAPICs mapped in on a PCI BAR.
+		 *
+		 * This is currently x86-specific, because acpi_ioapic_add()
+		 * is an empty function without CONFIG_ACPI_HOTPLUG_IOAPIC.
+		 * And CONFIG_ACPI_HOTPLUG_IOAPIC depends on CONFIG_X86_IO_APIC
+		 * (see drivers/acpi/Kconfig).
+		 */
 		acpi_ioapic_add(root->device->handle);
 	}
 
diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
index c74059e10a6d..ec538d3d2bd5 100644
--- a/drivers/pci/setup-bus.c
+++ b/drivers/pci/setup-bus.c
@@ -25,6 +25,7 @@
 #include <linux/ioport.h>
 #include <linux/cache.h>
 #include <linux/slab.h>
+#include <linux/acpi.h>
 #include "pci.h"
 
 unsigned int pci_flags;
@@ -1852,8 +1853,10 @@ void __init pci_assign_unassigned_resources(void)
 {
 	struct pci_bus *root_bus;
 
-	list_for_each_entry(root_bus, &pci_root_buses, node)
+	list_for_each_entry(root_bus, &pci_root_buses, node) {
 		pci_assign_unassigned_root_bus_resources(root_bus);
+		acpi_ioapic_add(ACPI_HANDLE(root_bus->bridge));
+	}
 }
 
 void pci_assign_unassigned_bridge_resources(struct pci_dev *bridge)