diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_mdio.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_mdio.c
index 093a223fe408..073078c7a7d0 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_mdio.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_mdio.c
@@ -34,11 +34,27 @@
 
 #define MII_BUSY 0x00000001
 #define MII_WRITE 0x00000002
+#define MII_DATA_MASK GENMASK(15, 0)
 
 /* GMAC4 defines */
 #define MII_GMAC4_GOC_SHIFT		2
+#define MII_GMAC4_REG_ADDR_SHIFT	16
 #define MII_GMAC4_WRITE			(1 << MII_GMAC4_GOC_SHIFT)
 #define MII_GMAC4_READ			(3 << MII_GMAC4_GOC_SHIFT)
+#define MII_GMAC4_C45E			BIT(1)
+
+static void stmmac_mdio_c45_setup(struct stmmac_priv *priv, int phyreg,
+				  u32 *val, u32 *data)
+{
+	unsigned int reg_shift = priv->hw->mii.reg_shift;
+	unsigned int reg_mask = priv->hw->mii.reg_mask;
+
+	*val |= MII_GMAC4_C45E;
+	*val &= ~reg_mask;
+	*val |= ((phyreg >> MII_DEVADDR_C45_SHIFT) << reg_shift) & reg_mask;
+
+	*data |= (phyreg & MII_REGADDR_C45_MASK) << MII_GMAC4_REG_ADDR_SHIFT;
+}
 
 /* XGMAC defines */
 #define MII_XGMAC_SADDR			BIT(18)
@@ -165,22 +181,26 @@ static int stmmac_mdio_read(struct mii_bus *bus, int phyaddr, int phyreg)
 	struct stmmac_priv *priv = netdev_priv(ndev);
 	unsigned int mii_address = priv->hw->mii.addr;
 	unsigned int mii_data = priv->hw->mii.data;
-	u32 v;
-	int data;
 	u32 value = MII_BUSY;
+	int data = 0;
+	u32 v;
 
 	value |= (phyaddr << priv->hw->mii.addr_shift)
 		& priv->hw->mii.addr_mask;
 	value |= (phyreg << priv->hw->mii.reg_shift) & priv->hw->mii.reg_mask;
 	value |= (priv->clk_csr << priv->hw->mii.clk_csr_shift)
 		& priv->hw->mii.clk_csr_mask;
-	if (priv->plat->has_gmac4)
+	if (priv->plat->has_gmac4) {
 		value |= MII_GMAC4_READ;
+		if (phyreg & MII_ADDR_C45)
+			stmmac_mdio_c45_setup(priv, phyreg, &value, &data);
+	}
 
 	if (readl_poll_timeout(priv->ioaddr + mii_address, v, !(v & MII_BUSY),
 			       100, 10000))
 		return -EBUSY;
 
+	writel(data, priv->ioaddr + mii_data);
 	writel(value, priv->ioaddr + mii_address);
 
 	if (readl_poll_timeout(priv->ioaddr + mii_address, v, !(v & MII_BUSY),
@@ -188,7 +208,7 @@ static int stmmac_mdio_read(struct mii_bus *bus, int phyaddr, int phyreg)
 		return -EBUSY;
 
 	/* Read the data from the MII data register */
-	data = (int)readl(priv->ioaddr + mii_data);
+	data = (int)readl(priv->ioaddr + mii_data) & MII_DATA_MASK;
 
 	return data;
 }
@@ -208,8 +228,9 @@ static int stmmac_mdio_write(struct mii_bus *bus, int phyaddr, int phyreg,
 	struct stmmac_priv *priv = netdev_priv(ndev);
 	unsigned int mii_address = priv->hw->mii.addr;
 	unsigned int mii_data = priv->hw->mii.data;
-	u32 v;
 	u32 value = MII_BUSY;
+	int data = phydata;
+	u32 v;
 
 	value |= (phyaddr << priv->hw->mii.addr_shift)
 		& priv->hw->mii.addr_mask;
@@ -217,10 +238,13 @@ static int stmmac_mdio_write(struct mii_bus *bus, int phyaddr, int phyreg,
 
 	value |= (priv->clk_csr << priv->hw->mii.clk_csr_shift)
 		& priv->hw->mii.clk_csr_mask;
-	if (priv->plat->has_gmac4)
+	if (priv->plat->has_gmac4) {
 		value |= MII_GMAC4_WRITE;
-	else
+		if (phyreg & MII_ADDR_C45)
+			stmmac_mdio_c45_setup(priv, phyreg, &value, &data);
+	} else {
 		value |= MII_WRITE;
+	}
 
 	/* Wait until any existing MII operation is complete */
 	if (readl_poll_timeout(priv->ioaddr + mii_address, v, !(v & MII_BUSY),
@@ -228,7 +252,7 @@ static int stmmac_mdio_write(struct mii_bus *bus, int phyaddr, int phyreg,
 		return -EBUSY;
 
 	/* Set the MII address register to write */
-	writel(phydata, priv->ioaddr + mii_data);
+	writel(data, priv->ioaddr + mii_data);
 	writel(value, priv->ioaddr + mii_address);
 
 	/* Wait until any existing MII operation is complete */
diff --git a/include/linux/phy.h b/include/linux/phy.h
index dc4b51060ebc..a2c25a975661 100644
--- a/include/linux/phy.h
+++ b/include/linux/phy.h
@@ -200,6 +200,8 @@ static inline const char *phy_modes(phy_interface_t interface)
 /* Or MII_ADDR_C45 into regnum for read/write on mii_bus to enable the 21 bit
    IEEE 802.3ae clause 45 addressing mode used by 10GIGE phy chips. */
 #define MII_ADDR_C45 (1<<30)
+#define MII_DEVADDR_C45_SHIFT	16
+#define MII_REGADDR_C45_MASK	GENMASK(15, 0)
 
 struct device;
 struct phylink;
