net: macb: initialize checksum when using checksum offloading

I'm still struggling to get this fix right..

Changes since v2:
 - do not blindly modify SKB contents according to Dave's legitimate
   objection

Changes since v1:
 - dropped disabling HW checksum offload for Zynq
 - initialize checksum similar to net/ethernet/freescale/fec_main.c

-- >8 --
MACB/GEM needs the checksum field initialized to 0 to get correct
results on transmit in all cases, e.g. on Zynq, UDP packets with
payload <= 2 otherwise contain a wrong checksums.
Signed-off-by: Helmut Buchsbaum <helmut.buchsbaum@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cadence/macb.c

@@ -1323,6 +1323,24 @@ static unsigned int macb_tx_map(struct macb *bp,
 	return 0;
 }
 
+static inline int macb_clear_csum(struct sk_buff *skb)
+{
+	/* no change for packets without checksum offloading */
+	if (skb->ip_summed != CHECKSUM_PARTIAL)
+		return 0;
+
+	/* make sure we can modify the header */
+	if (unlikely(skb_cow_head(skb, 0)))
+		return -1;
+
+	/* initialize checksum field
+	 * This is required - at least for Zynq, which otherwise calculates
+	 * wrong UDP header checksums for UDP packets with UDP data len <=2
+	 */
+	*(__sum16 *)(skb_checksum_start(skb) + skb->csum_offset) = 0;
+	return 0;
+}
+
 static int macb_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	u16 queue_index = skb_get_queue_mapping(skb);
@@ -1362,6 +1380,11 @@ static int macb_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		return NETDEV_TX_BUSY;
 	}
 
+	if (macb_clear_csum(skb)) {
+		dev_kfree_skb_any(skb);
+		return NETDEV_TX_OK;
+	}
+
 	/* Map socket buffer for DMA transfer */
 	if (!macb_tx_map(bp, queue, skb)) {
 		dev_kfree_skb_any(skb);