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

[Helmut Buchsbaum]

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.buchsb...@gmail.com>
---
 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