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>

drivers/net/usb/smsc95xx.c

@@ -1189,10 +1189,22 @@ static struct sk_buff *smsc95xx_tx_fixup(struct usbnet *dev,
 	}
 
 	if (csum) {
+		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);
 	tx_cmd_b = (u32)(skb->len - 4);