net/mlx4_en: optimizes get_fixed_ipv6_csum()

While trying to support CHECKSUM_COMPLETE for IPV6 fragments,
I had to experiments various hacks in get_fixed_ipv6_csum().
I must admit I could not find how to implement this :/

However, get_fixed_ipv6_csum() does a lot of redundant operations,
calling csum_partial() twice.

First csum_partial() computes the checksum of saddr and daddr,
put in @csum_pseudo_hdr. Undone later in the second csum_partial()
computed on whole ipv6 header.

Then nexthdr is added once, added a second time, then substracted.

payload_len is added once, then substracted.

Really all this can be reduced to two add_csum(), to add back 6 bytes
that were removed by mlx4 when providing hw_checksum in RX descriptor.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Saeed Mahameed <saeedm@mellanox.com>
Cc: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Acked-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_rx.c

@@ -593,30 +593,25 @@ static int get_fixed_ipv4_csum(__wsum hw_checksum, struct sk_buff *skb,
 }
 
 #if IS_ENABLED(CONFIG_IPV6)
-/* In IPv6 packets, besides subtracting the pseudo header checksum,
- * we also compute/add the IP header checksum which
- * is not added by the HW.
+/* In IPv6 packets, hw_checksum lacks 6 bytes from IPv6 header:
+ * 4 first bytes : priority, version, flow_lbl
+ * and 2 additional bytes : nexthdr, hop_limit.
  */
 static int get_fixed_ipv6_csum(__wsum hw_checksum, struct sk_buff *skb,
 			       struct ipv6hdr *ipv6h)
 {
 	__u8 nexthdr = ipv6h->nexthdr;
-	__wsum csum_pseudo_hdr = 0;
+	__wsum temp;
 
 	if (unlikely(nexthdr == IPPROTO_FRAGMENT ||
 		     nexthdr == IPPROTO_HOPOPTS ||
 		     nexthdr == IPPROTO_SCTP))
 		return -1;
-	hw_checksum = csum_add(hw_checksum, (__force __wsum)htons(nexthdr));
 
-	csum_pseudo_hdr = csum_partial(&ipv6h->saddr,
-				       sizeof(ipv6h->saddr) + sizeof(ipv6h->daddr), 0);
-	csum_pseudo_hdr = csum_add(csum_pseudo_hdr, (__force __wsum)ipv6h->payload_len);
-	csum_pseudo_hdr = csum_add(csum_pseudo_hdr,
-				   (__force __wsum)htons(nexthdr));
-
-	skb->csum = csum_sub(hw_checksum, csum_pseudo_hdr);
-	skb->csum = csum_add(skb->csum, csum_partial(ipv6h, sizeof(struct ipv6hdr), 0));
+	/* priority, version, flow_lbl */
+	temp = csum_add(hw_checksum, *(__wsum *)ipv6h);
+	/* nexthdr and hop_limit */
+	skb->csum = csum_add(temp, (__force __wsum)*(__be16 *)&ipv6h->nexthdr);
 	return 0;
 }
 #endif