diff --git a/include/linux/phy.h b/include/linux/phy.h
index bfdbdc538910..a2bfae80c449 100644
--- a/include/linux/phy.h
+++ b/include/linux/phy.h
@@ -292,6 +292,7 @@ static inline long rgmii_clock(int speed)
 
 /* Used when trying to connect to a specific phy (mii bus id:phy device id) */
 #define PHY_ID_FMT "%s:%02x"
+#define PHY_ID_SIZE	(MII_BUS_ID_SIZE + 3)
 
 #define MII_BUS_ID_SIZE	61