tg3: Fix poor tx performance on 57766 after MTU change

GRC reset causes the read DMA engine to go into a mode that breaks up
requests into 256 bytes.  A PHY reset is required to bring it back to
the normal mode.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/tg3.c

@@ -12267,7 +12267,7 @@ static inline void tg3_set_mtu(struct net_device *dev, struct tg3 *tp,
 static int tg3_change_mtu(struct net_device *dev, int new_mtu)
 {
 	struct tg3 *tp = netdev_priv(dev);
-	int err;
+	int err, reset_phy = 0;
 
 	if (new_mtu < TG3_MIN_MTU || new_mtu > TG3_MAX_MTU(tp))
 		return -EINVAL;
@@ -12290,7 +12290,13 @@ static int tg3_change_mtu(struct net_device *dev, int new_mtu)
 
 	tg3_set_mtu(dev, tp, new_mtu);
 
-	err = tg3_restart_hw(tp, 0);
+	/* Reset PHY, otherwise the read DMA engine will be in a mode that
+	 * breaks all requests to 256 bytes.
+	 */
+	if (GET_ASIC_REV(tp->pci_chip_rev_id) == ASIC_REV_57766)
+		reset_phy = 1;
+
+	err = tg3_restart_hw(tp, reset_phy);
 
 	if (!err)
 		tg3_netif_start(tp);