diff --git a/arch/powerpc/sysdev/dart_iommu.c b/arch/powerpc/sysdev/dart_iommu.c
index 61d3174..38087bd 100644
--- a/arch/powerpc/sysdev/dart_iommu.c
+++ b/arch/powerpc/sysdev/dart_iommu.c
@@ -49,6 +49,7 @@
 
 #include "dart.h"
 
+extern int iommu_is_off;
 extern int iommu_force_on;
 
 /* Physical base address and size of the DART table */
@@ -329,10 +330,17 @@
 
 void __init alloc_dart_table(void)
 {
-	/* Only reserve DART space if machine has more than 2GB of RAM
+	/* Only reserve DART space if machine has more than 1GB of RAM
 	 * or if requested with iommu=on on cmdline.
+	 *
+	 * 1GB of RAM is picked as limit because some default devices
+	 * (i.e. Airport Extreme) have 30 bit address range limits.
 	 */
-	if (lmb_end_of_DRAM() <= 0x80000000ull && !iommu_force_on)
+
+	if (iommu_is_off)
+		return;
+
+	if (!iommu_force_on && lmb_end_of_DRAM() <= 0x40000000ull)
 		return;
 
 	/* 512 pages (2MB) is max DART tablesize. */