smsc95xx: Fix tx checksum offload for small packets

TX checksum offload does not work properly when transmitting
UDP packets with 0, 1 or 2 bytes of data.  This patch works
around the problem by calculating checksums for these packets
in the driver.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 3 deletions

diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
index d222d7e25273..73f9a31cf94d 100644
--- a/drivers/net/usb/smsc95xx.c
+++ b/drivers/net/usb/smsc95xx.c
@@ -1189,9 +1189,21 @@ static struct sk_buff *smsc95xx_tx_fixup(struct usbnet *dev,
         }
 
         if (csum) {
-                u32 csum_preamble = smsc95xx_calc_csum_preamble(skb);
-                skb_push(skb, 4);
-                memcpy(skb->data, &csum_preamble, 4);
+                if (skb->len <= 45) {
+                        /* workaround - hardware tx checksum does not work
+                         * properly with extremely small packets */
+                        long csstart = skb->csum_start - skb_headroom(skb);
+                        __wsum calc = csum_partial(skb->data + csstart,
+                                skb->len - csstart, 0);
+                        *((__sum16 *)(skb->data + csstart
+                                + skb->csum_offset)) = csum_fold(calc);
+
+                        csum = false;
+                } else {
+                        u32 csum_preamble = smsc95xx_calc_csum_preamble(skb);
+                        skb_push(skb, 4);
+                        memcpy(skb->data, &csum_preamble, 4);
+                }
         }
 
         skb_push(skb, 4);