[SC92031] Using padto turned driver into an IPv6-only interface

IPv4 would work with this driver only with static arp table entries,
the patch  reverts a padto introduced in

 commit 26a17b7b
 sc92031: start transmit return value bugfix

The padto does not work because the driver code evaluates `len' later on and
there are cases where skb->len is not updated accordingly.

This was observed with ARP frames (skb->len = 42 bytes, !skb_cloned(),
skb_tailroom = 84 bytes). Then in skb_pad(), the first condition is true, where
skb->len is not updated. As a consequence, the driver uses 42 bytes instead of
the 60 bytes, and the ARP frame never makes it onto the wire.
Signed-off-by: NGerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: NJeff Garzik <jgarzik@redhat.com>

drivers/net/sc92031.c

@@ -953,9 +953,6 @@ static int sc92031_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	unsigned entry;
 	u32 tx_status;
 
-	if (skb_padto(skb, ETH_ZLEN))
-		return NETDEV_TX_OK;
-
 	if (unlikely(skb->len > TX_BUF_SIZE)) {
 		dev->stats.tx_dropped++;
 		goto out;
@@ -975,6 +972,11 @@ static int sc92031_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	skb_copy_and_csum_dev(skb, priv->tx_bufs + entry * TX_BUF_SIZE);
 
 	len = skb->len;
+	if (unlikely(len < ETH_ZLEN)) {
+		memset(priv->tx_bufs + entry * TX_BUF_SIZE + len,
+				0, ETH_ZLEN - len);
+		len = ETH_ZLEN;
+	}
 
 	wmb();