sh_eth: Ensure proper ordering of descriptor active bit write/read

When submitting a DMA descriptor, the active bit must be written last.
When reading a completed DMA descriptor, the active bit must be read
first.

Add memory barriers to ensure that this ordering is maintained.
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/sh_eth.c

@@ -1410,6 +1410,8 @@ static int sh_eth_txfree(struct net_device *ndev)
 		txdesc = &mdp->tx_ring[entry];
 		if (txdesc->status & cpu_to_edmac(mdp, TD_TACT))
 			break;
+		/* TACT bit must be checked before all the following reads */
+		rmb();
 		/* Free the original skb. */
 		if (mdp->tx_skbuff[entry]) {
 			dma_unmap_single(&ndev->dev, txdesc->addr,
@@ -1447,6 +1449,8 @@ static int sh_eth_rx(struct net_device *ndev, u32 intr_status, int *quota)
 	limit = boguscnt;
 	rxdesc = &mdp->rx_ring[entry];
 	while (!(rxdesc->status & cpu_to_edmac(mdp, RD_RACT))) {
+		/* RACT bit must be checked before all the following reads */
+		rmb();
 		desc_status = edmac_to_cpu(mdp, rxdesc->status);
 		pkt_len = rxdesc->frame_length;
 
@@ -1526,6 +1530,7 @@ static int sh_eth_rx(struct net_device *ndev, u32 intr_status, int *quota)
 			skb_checksum_none_assert(skb);
 			rxdesc->addr = dma_addr;
 		}
+		wmb(); /* RACT bit must be set after all the above writes */
 		if (entry >= mdp->num_rx_ring - 1)
 			rxdesc->status |=
 				cpu_to_edmac(mdp, RD_RACT | RD_RFP | RD_RDEL);
@@ -2195,6 +2200,7 @@ static int sh_eth_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 	}
 	txdesc->buffer_length = skb->len;
 
+	wmb(); /* TACT bit must be set after all the above writes */
 	if (entry >= mdp->num_tx_ring - 1)
 		txdesc->status |= cpu_to_edmac(mdp, TD_TACT | TD_TDLE);
 	else