diff --git a/drivers/net/mdio.c b/drivers/net/mdio.c
index 16fbb11d92ac..8403316eb02b 100644
--- a/drivers/net/mdio.c
+++ b/drivers/net/mdio.c
@@ -190,6 +190,9 @@ void mdio45_ethtool_gset_npage(const struct mdio_if_info *mdio,
 	int reg;
 	u32 speed;
 
+	BUILD_BUG_ON(MDIO_SUPPORTS_C22 != ETH_MDIO_SUPPORTS_C22);
+	BUILD_BUG_ON(MDIO_SUPPORTS_C45 != ETH_MDIO_SUPPORTS_C45);
+
 	ecmd->transceiver = XCVR_INTERNAL;
 	ecmd->phy_address = mdio->prtad;
 	ecmd->mdio_support =
diff --git a/drivers/net/mii.c b/drivers/net/mii.c
index c70c2332d15e..4a99c3919037 100644
--- a/drivers/net/mii.c
+++ b/drivers/net/mii.c
@@ -31,7 +31,7 @@
 #include <linux/module.h>
 #include <linux/netdevice.h>
 #include <linux/ethtool.h>
-#include <linux/mdio.h>
+#include <linux/mii.h>
 
 static u32 mii_get_an(struct mii_if_info *mii, u16 addr)
 {
@@ -74,7 +74,7 @@ int mii_ethtool_gset(struct mii_if_info *mii, struct ethtool_cmd *ecmd)
 
 	/* this isn't fully supported at higher layers */
 	ecmd->phy_address = mii->phy_id;
-	ecmd->mdio_support = MDIO_SUPPORTS_C22;
+	ecmd->mdio_support = ETH_MDIO_SUPPORTS_C22;
 
 	ecmd->advertising = ADVERTISED_TP | ADVERTISED_MII;
 
diff --git a/include/linux/ethtool.h b/include/linux/ethtool.h
index da5b2de99ae4..e1d9e0ede309 100644
--- a/include/linux/ethtool.h
+++ b/include/linux/ethtool.h
@@ -30,10 +30,15 @@ struct ethtool_cmd {
 				 * access it */
 	__u8	duplex;		/* Duplex, half or full */
 	__u8	port;		/* Which connector port */
-	__u8	phy_address;
+	__u8	phy_address;	/* MDIO PHY address (PRTAD for clause 45).
+				 * May be read-only or read-write
+				 * depending on the driver.
+				 */
 	__u8	transceiver;	/* Which transceiver to use */
 	__u8	autoneg;	/* Enable or disable autonegotiation */
-	__u8	mdio_support;
+	__u8	mdio_support;	/* MDIO protocols supported.  Read-only.
+				 * Not set by all drivers.
+				 */
 	__u32	maxtxpkt;	/* Tx pkts before generating tx int */
 	__u32	maxrxpkt;	/* Rx pkts before generating rx int */
 	__u16	speed_hi;       /* The forced speed (upper
@@ -59,6 +64,20 @@ static inline __u32 ethtool_cmd_speed(const struct ethtool_cmd *ep)
 	return (ep->speed_hi << 16) | ep->speed;
 }
 
+/* Device supports clause 22 register access to PHY or peripherals
+ * using the interface defined in <linux/mii.h>.  This should not be
+ * set if there are known to be no such peripherals present or if
+ * the driver only emulates clause 22 registers for compatibility.
+ */
+#define ETH_MDIO_SUPPORTS_C22	1
+
+/* Device supports clause 45 register access to PHY or peripherals
+ * using the interface defined in <linux/mii.h> and <linux/mdio.h>.
+ * This should not be set if there are known to be no such peripherals
+ * present.
+ */
+#define ETH_MDIO_SUPPORTS_C45	2
+
 #define ETHTOOL_FWVERS_LEN	32
 #define ETHTOOL_BUSINFO_LEN	32
 /* these strings are set to whatever the driver author decides... */