net: phy: add genphy_c45_loopback

Add generic code to enable C45 PHY loopback into the common phy-c45.c
file. This will allow C45 PHY drivers aceess this by setting
.set_loopback.
Suggested-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phy-c45.c

@@ -560,6 +560,14 @@ int gen10g_config_aneg(struct phy_device *phydev)
 }
 EXPORT_SYMBOL_GPL(gen10g_config_aneg);
 
+int genphy_c45_loopback(struct phy_device *phydev, bool enable)
+{
+	return phy_modify_mmd(phydev, MDIO_MMD_PCS, MDIO_CTRL1,
+			      MDIO_PCS_CTRL1_LOOPBACK,
+			      enable ? MDIO_PCS_CTRL1_LOOPBACK : 0);
+}
+EXPORT_SYMBOL_GPL(genphy_c45_loopback);
+
 struct phy_driver genphy_c45_driver = {
 	.phy_id         = 0xffffffff,
 	.phy_id_mask    = 0xffffffff,

include/linux/phy.h

@@ -1532,6 +1532,7 @@ int genphy_c45_read_mdix(struct phy_device *phydev);
 int genphy_c45_pma_read_abilities(struct phy_device *phydev);
 int genphy_c45_read_status(struct phy_device *phydev);
 int genphy_c45_config_aneg(struct phy_device *phydev);
+int genphy_c45_loopback(struct phy_device *phydev, bool enable);
 
 /* Generic C45 PHY driver */
 extern struct phy_driver genphy_c45_driver;