diff --git a/arch/ia64/kernel/acpi.c b/arch/ia64/kernel/acpi.c
index 73ef4a85b861df136c132c7b3ab4df76d54b8eb5..ef2fe474f107319c90af67317de180162d22ed2b 100644
--- a/arch/ia64/kernel/acpi.c
+++ b/arch/ia64/kernel/acpi.c
@@ -590,6 +590,9 @@ void __init acpi_numa_arch_fixup(void)
  */
 int acpi_register_gsi(u32 gsi, int triggering, int polarity)
 {
+	if (acpi_irq_model == ACPI_IRQ_MODEL_PLATFORM)
+		return gsi;
+
 	if (has_8259 && gsi < 16)
 		return isa_irq_to_vector(gsi);
 
diff --git a/arch/ia64/sn/kernel/io_acpi_init.c b/arch/ia64/sn/kernel/io_acpi_init.c
index 99d7f278612a7be48645168bddccbd759604937b..cb96b4ea7df6970825151bf3b5f0dd014a43c063 100644
--- a/arch/ia64/sn/kernel/io_acpi_init.c
+++ b/arch/ia64/sn/kernel/io_acpi_init.c
@@ -223,6 +223,9 @@ sn_io_acpi_init(void)
 	u64 result;
 	s64 status;
 
+	/* SN Altix does not follow the IOSAPIC IRQ routing model */
+	acpi_irq_model = ACPI_IRQ_MODEL_PLATFORM;
+
 	acpi_bus_register_driver(&acpi_sn_hubdev_driver);
 	status = sal_ioif_init(&result);
 	if (status || result)
diff --git a/drivers/acpi/bus.c b/drivers/acpi/bus.c
index 279c4bac92e553aaec80c4dc1181f682852e2c41..766332e4559212aa9c253ddf3723e65367487b96 100644
--- a/drivers/acpi/bus.c
+++ b/drivers/acpi/bus.c
@@ -561,6 +561,9 @@ static int __init acpi_bus_init_irq(void)
 	case ACPI_IRQ_MODEL_IOSAPIC:
 		message = "IOSAPIC";
 		break;
+	case ACPI_IRQ_MODEL_PLATFORM:
+		message = "platform specific model";
+		break;
 	default:
 		printk(KERN_WARNING PREFIX "Unknown interrupt routing model\n");
 		return -ENODEV;
diff --git a/include/linux/acpi.h b/include/linux/acpi.h
index 2b0c955590fec1dacf875c4e0dd21d4bc0269c5e..91f1f23638700b92ec687279ea2ced21cee974c5 100644
--- a/include/linux/acpi.h
+++ b/include/linux/acpi.h
@@ -47,6 +47,7 @@ enum acpi_irq_model_id {
 	ACPI_IRQ_MODEL_PIC = 0,
 	ACPI_IRQ_MODEL_IOAPIC,
 	ACPI_IRQ_MODEL_IOSAPIC,
+	ACPI_IRQ_MODEL_PLATFORM,
 	ACPI_IRQ_MODEL_COUNT
 };