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>

diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
index d222d7e..73f9a31 100644 (file)
--- 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);