diff --git a/include/linux/ioport.h b/include/linux/ioport.h
index bfee885791c2..85ac9b9b72a2 100644
--- a/include/linux/ioport.h
+++ b/include/linux/ioport.h
@@ -29,7 +29,7 @@ struct resource {
 #define IORESOURCE_BITS		0x000000ff	/* Bus-specific bits */
 
 #define IORESOURCE_TYPE_BITS	0x00001f00	/* Resource type */
-#define IORESOURCE_IO		0x00000100
+#define IORESOURCE_IO		0x00000100	/* PCI/ISA I/O ports */
 #define IORESOURCE_MEM		0x00000200
 #define IORESOURCE_REG		0x00000300	/* Register offsets */
 #define IORESOURCE_IRQ		0x00000400