diff --git a/sys/arch/x86/pci/pci_intr_machdep.c b/sys/arch/x86/pci/pci_intr_machdep.c
index 2a1c0b0ac0b8..6f8af7152cdc 100644
--- a/sys/arch/x86/pci/pci_intr_machdep.c
+++ b/sys/arch/x86/pci/pci_intr_machdep.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: pci_intr_machdep.c,v 1.24 2012/06/15 14:02:41 yamt Exp $	*/
+/*	$NetBSD: pci_intr_machdep.c,v 1.25 2012/06/15 14:07:44 yamt Exp $	*/
 
 /*-
  * Copyright (c) 1997, 1998, 2009 The NetBSD Foundation, Inc.
@@ -73,7 +73,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: pci_intr_machdep.c,v 1.24 2012/06/15 14:02:41 yamt Exp $");
+__KERNEL_RCSID(0, "$NetBSD: pci_intr_machdep.c,v 1.25 2012/06/15 14:07:44 yamt Exp $");
 
 #include <sys/types.h>
 #include <sys/param.h>
@@ -144,6 +144,10 @@ pci_intr_map(const struct pci_attach_args *pa, pci_intr_handle_t *ihp)
 	KASSERT(rawpin <= PCI_INTERRUPT_PIN_D);
 	pci_decompose_tag(pc, pa->pa_tag, &bus, &dev, &func);
 	if (mp_busses != NULL) {
+		/*
+		 * Note: PCI_INTERRUPT_PIN_A == 1 where intr_find_mpmapping
+		 * wants pci bus_pin encoding which uses INT_A == 0.
+		 */
 		if (intr_find_mpmapping(bus,
 		    (dev << 2) | (rawpin - PCI_INTERRUPT_PIN_A), ihp) == 0) {
 			if (APIC_IRQ_LEGACY_IRQ(*ihp) == 0)