stmmac: Fix for nfs hang on multiple reboot

It was observed that during multiple reboots nfs hangs. The status of
receive descriptors shows that all the descriptors were in control of
CPU, and none were assigned to DMA.
Also the DMA status register confirmed that the Rx buffer is
unavailable.

This patch adds the fix for the same by adding the memory barriers to
ascertain that the all instructions before enabling the Rx or Tx DMA are
completed which involves the proper setting of the ownership bit in DMA
descriptors.
Signed-off-by: Deepak Sikri <deepak.sikri@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -1212,6 +1212,7 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		priv->hw->desc->prepare_tx_desc(desc, 0, len, csum_insertion);
 		wmb();
 		priv->hw->desc->set_tx_owner(desc);
+		wmb();
 	}
 
 	/* Interrupt on completition only for the latest segment */
@@ -1227,6 +1228,7 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	/* To avoid raise condition */
 	priv->hw->desc->set_tx_owner(first);
+	wmb();
 
 	priv->cur_tx++;
 
@@ -1290,6 +1292,7 @@ static inline void stmmac_rx_refill(struct stmmac_priv *priv)
 		}
 		wmb();
 		priv->hw->desc->set_rx_owner(p + entry);
+		wmb();
 	}
 }