lan78xx: workaround of forced 100 Full/Half duplex mode error

At forced 100 Full & Half duplex mode, chip may fail to set mode correctly
when cable is switched between long(~50+m) and short one.
As workaround, set to 10 before setting to 100 at forced 100 F/H mode.
Signed-off-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/lan78xx.c

@@ -1804,7 +1804,34 @@ static void lan78xx_remove_mdio(struct lan78xx_net *dev)
 
 static void lan78xx_link_status_change(struct net_device *net)
 {
-	/* nothing to do */
+	struct phy_device *phydev = net->phydev;
+	int ret, temp;
+
+	/* At forced 100 F/H mode, chip may fail to set mode correctly
+	 * when cable is switched between long(~50+m) and short one.
+	 * As workaround, set to 10 before setting to 100
+	 * at forced 100 F/H mode.
+	 */
+	if (!phydev->autoneg && (phydev->speed == 100)) {
+		/* disable phy interrupt */
+		temp = phy_read(phydev, LAN88XX_INT_MASK);
+		temp &= ~LAN88XX_INT_MASK_MDINTPIN_EN_;
+		ret = phy_write(phydev, LAN88XX_INT_MASK, temp);
+
+		temp = phy_read(phydev, MII_BMCR);
+		temp &= ~(BMCR_SPEED100 | BMCR_SPEED1000);
+		phy_write(phydev, MII_BMCR, temp); /* set to 10 first */
+		temp |= BMCR_SPEED100;
+		phy_write(phydev, MII_BMCR, temp); /* set to 100 later */
+
+		/* clear pending interrupt generated while workaround */
+		temp = phy_read(phydev, LAN88XX_INT_STS);
+
+		/* enable phy interrupt back */
+		temp = phy_read(phydev, LAN88XX_INT_MASK);
+		temp |= LAN88XX_INT_MASK_MDINTPIN_EN_;
+		ret = phy_write(phydev, LAN88XX_INT_MASK, temp);
+	}
 }
 
 static int lan78xx_phy_init(struct lan78xx_net *dev)