net: dsa: hellcreek: Calculate checksums in tagger

In case the checksum calculation is offloaded to the DSA master network
interface, it will include the switch trailing tag. As soon as the switch strips
that tag on egress, the calculated checksum is wrong.

Therefore, add the checksum calculation to the tagger (if required) before
adding the switch tag. This way, the hellcreek code works with all DSA master
interfaces regardless of their declared feature set.

Fixes: 01ef09caad ("net: dsa: Add tag handling for Hirschmann Hellcreek switches")
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220415103320.90657-1-kurt@linutronix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/dsa/tag_hellcreek.c

@@ -21,6 +21,14 @@ static struct sk_buff *hellcreek_xmit(struct sk_buff *skb,
 	struct dsa_port *dp = dsa_slave_to_port(dev);
 	u8 *tag;
 
+	/* Calculate checksums (if required) before adding the trailer tag to
+	 * avoid including it in calculations. That would lead to wrong
+	 * checksums after the switch strips the tag.
+	 */
+	if (skb->ip_summed == CHECKSUM_PARTIAL &&
+	    skb_checksum_help(skb))
+		return NULL;
+
 	/* Tag encoding */
 	tag  = skb_put(skb, HELLCREEK_TAG_LEN);
 	*tag = BIT(dp->index);