Subject: [PATCH v2 1/1] net: macb: initialize checksum when using checksum offloading

From: Helmut Buchsbaum <[email-masked]>
Date: Mon, 29 Aug 2016 15:57:25 +0200

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 <[email-masked]>
---
 drivers/net/ethernet/cadence/macb.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/drivers/net/ethernet/cadence/macb.c b/drivers/net/ethernet/cadence/macb.c
index 89c0cfa..de2f791 100644
--- a/drivers/net/ethernet/cadence/macb.c
+++ b/drivers/net/ethernet/cadence/macb.c
@@ -1323,6 +1323,19 @@ dma_error:
 	return 0;
 }
 
+static inline void macb_clear_csum(struct sk_buff *skb)
+{
+	/* no change for packets without checksum offloading */
+	if (skb->ip_summed != CHECKSUM_PARTIAL)
+		return;
+
+	/* 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->head + skb->csum_start + skb->csum_offset) = 0;
+}
+
 static int macb_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	u16 queue_index = skb_get_queue_mapping(skb);
@@ -1362,6 +1375,8 @@ static int macb_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		return NETDEV_TX_BUSY;
 	}
 
+	macb_clear_csum(skb);
+
 	/* Map socket buffer for DMA transfer */
 	if (!macb_tx_map(bp, queue, skb)) {
 		dev_kfree_skb_any(skb);
-- 
2.1.4