net: dsa: tag_ocelot_8021q: calculate TX checksum in software for deferred packets

DSA inherits NETIF_F_CSUM_MASK from master->vlan_features, and the
expectation is that TX checksumming is offloaded and not done in
software.

Normally the DSA master takes care of this, but packets handled by
ocelot_defer_xmit() are a very special exception, because they are
actually injected into the switch through register-based MMIO. So the
DSA master is not involved at all for these packets => no one calculates
the checksum.

This allows PTP over UDP to work using the ocelot-8021q tagging
protocol.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/tag_ocelot_8021q.c

@@ -32,6 +32,13 @@ static struct sk_buff *ocelot_defer_xmit(struct dsa_port *dp,
 	if (!xmit_work_fn || !xmit_worker)
 		return NULL;
 
+	/* PTP over IP packets need UDP checksumming. We may have inherited
+	 * NETIF_F_HW_CSUM from the DSA master, but these packets are not sent
+	 * through the DSA master, so calculate the checksum here.
+	 */
+	if (skb->ip_summed == CHECKSUM_PARTIAL && skb_checksum_help(skb))
+		return NULL;
+
 	xmit_work = kzalloc(sizeof(*xmit_work), GFP_ATOMIC);
 	if (!xmit_work)
 		return NULL;