net: qualcomm: rmnet: clarify a bit of code

In rmnet_map_ipv6_dl_csum_trailer() there is an especially involved
line of code that determines the ones' complement sum of the IPv6
packet header (in host byte order).  Simplify that by storing the
result of computing just the header checksum in a local variable,
then using that in the original assignment.

Use the size of the IPv6 header structure as the number of bytes to
checksum, rather than computing the offset to the transport header.
And use ip_fast_csum() rather than ipa_compute_csum(), knowing that
the size of an IPv6 header (40 bytes) is a multiple of 4 bytes
greater than 16.

Add some comments to match rmnet_map_ipv4_dl_csum_trailer().
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c

@@ -120,27 +120,33 @@ rmnet_map_ipv6_dl_csum_trailer(struct sk_buff *skb,
 			       struct rmnet_map_dl_csum_trailer *csum_trailer,
 			       struct rmnet_priv *priv)
 {
-	__sum16 *csum_field, ip6_payload_csum, pseudo_csum, csum_temp;
+	struct ipv6hdr *ip6h = (struct ipv6hdr *)skb->data;
+	void *txporthdr = skb->data + sizeof(*ip6h);
+	__sum16 *csum_field, pseudo_csum, csum_temp;
 	u16 csum_value, csum_value_final;
 	__be16 ip6_hdr_csum, addend;
-	struct ipv6hdr *ip6h;
-	void *txporthdr;
+	__sum16 ip6_payload_csum;
+	__be16 ip_header_csum;
 	u32 length;
 
-	ip6h = (struct ipv6hdr *)(skb->data);
-
-	txporthdr = skb->data + sizeof(struct ipv6hdr);
+	/* Checksum offload is only supported for UDP and TCP protocols;
+	 * the packet cannot include any IPv6 extension headers
+	 */
 	csum_field = rmnet_map_get_csum_field(ip6h->nexthdr, txporthdr);
-
 	if (!csum_field) {
 		priv->stats.csum_err_invalid_transport++;
 		return -EPROTONOSUPPORT;
 	}
 
+	/* The checksum value in the trailer is computed over the entire
+	 * IP packet, including the IP header and payload.  To derive the
+	 * transport checksum from this, we first subract the contribution
+	 * of the IP header from the trailer checksum.  We then add the
+	 * checksum computed over the pseudo header.
+	 */
 	csum_value = ~ntohs(csum_trailer->csum_value);
-	ip6_hdr_csum = (__force __be16)
-			~ntohs((__force __be16)ip_compute_csum(ip6h,
-			       (int)(txporthdr - (void *)(skb->data))));
+	ip_header_csum = (__force __be16)ip_fast_csum(ip6h, sizeof(*ip6h) / 4);
+	ip6_hdr_csum = (__force __be16)~ntohs(ip_header_csum);
 	ip6_payload_csum = csum16_sub((__force __sum16)csum_value,
 				      ip6_hdr_csum);