net: bcmgenet: use hardware padding of runt frames

When commit 474ea9ca ("net: bcmgenet: correctly pad short
packets") added the call to skb_padto() it should have been
located before the nr_frags parameter was read since that value
could be changed when padding packets with lengths between 55
and 59 bytes (inclusive).

The use of a stale nr_frags value can cause corruption of the
pad data when tx-scatter-gather is enabled. This corruption of
the pad can cause invalid checksum computation when hardware
offload of tx-checksum is also enabled.

Since the original reason for the padding was corrected by
commit 7dd39913 ("net: bcmgenet: fix skb_len in
bcmgenet_xmit_single()") we can remove the software padding all
together and make use of hardware padding of short frames as
long as the hardware also always appends the FCS value to the
frame.

Fixes: 474ea9ca ("net: bcmgenet: correctly pad short packets")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/genet/bcmgenet.c

@@ -2042,11 +2042,6 @@ static netdev_tx_t bcmgenet_xmit(struct sk_buff *skb, struct net_device *dev)
 		goto out;
 	}
 
-	if (skb_padto(skb, ETH_ZLEN)) {
-		ret = NETDEV_TX_OK;
-		goto out;
-	}
-
 	/* Retain how many bytes will be sent on the wire, without TSB inserted
 	 * by transmit checksum offload
 	 */
@@ -2093,6 +2088,9 @@ static netdev_tx_t bcmgenet_xmit(struct sk_buff *skb, struct net_device *dev)
 		len_stat = (size << DMA_BUFLENGTH_SHIFT) |
 			   (priv->hw_params->qtag_mask << DMA_TX_QTAG_SHIFT);
 
+		/* Note: if we ever change from DMA_TX_APPEND_CRC below we
+		 * will need to restore software padding of "runt" packets
+		 */
 		if (!i) {
 			len_stat |= DMA_TX_APPEND_CRC | DMA_SOP;
 			if (skb->ip_summed == CHECKSUM_PARTIAL)