net: qualcomm: rmnet: avoid unnecessary byte-swapping

Internet checksums are used for IPv4 header checksum, as well as TCP
segment and UDP datagram checksums.  Such a checksum represents the
negated sum of adjacent pairs of bytes, using ones' complement
arithmetic.

One property of the Internet checkum is byte order independence [1].
Specifically, the sum of byte-swapped pairs is equal to the result
of byte swapping the sum of those same pairs when not byte-swapped.

So for example if a, b, c, d, y, and z are hexadecimal digits, and
PLUS represents ones' complement addition:
    If:		ab PLUS cd = yz
    Then:	ba PLUS dc = zy

For this reason, there is no need to swap the order of bytes in the
checksum value held in a message header, nor the one in the QMAPv4
trailer, in order to operate on them.

In other words, we can determine whether the hardware-computed
checksum matches the one in the message header without any byte
swaps.

(This patch leaves in place all existing type casts.)

[1] https://tools.ietf.org/html/rfc1071Signed-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

@@ -78,15 +78,15 @@ rmnet_map_ipv4_dl_csum_trailer(struct sk_buff *skb,
 	 * trailer checksum.  Therefore the checksum in the trailer is
 	 * just the checksum computed over the IP payload.
 	 */
-	ip_payload_csum = (__force __sum16)~ntohs(csum_trailer->csum_value);
+	ip_payload_csum = (__force __sum16)~csum_trailer->csum_value;
 
 	pseudo_csum = ~csum_tcpudp_magic(ip4h->saddr, ip4h->daddr,
 					 ntohs(ip4h->tot_len) - ip4h->ihl * 4,
 					 ip4h->protocol, 0);
-	addend = (__force __be16)ntohs((__force __be16)pseudo_csum);
+	addend = (__force __be16)pseudo_csum;
 	pseudo_csum = csum16_add(ip_payload_csum, addend);
 
-	addend = (__force __be16)ntohs((__force __be16)*csum_field);
+	addend = (__force __be16)*csum_field;
 	csum_temp = ~csum16_sub(pseudo_csum, addend);
 	csum_value_final = (__force u16)csum_temp;
 
@@ -105,7 +105,7 @@ rmnet_map_ipv4_dl_csum_trailer(struct sk_buff *skb,
 		}
 	}
 
-	if (csum_value_final == ntohs((__force __be16)*csum_field)) {
+	if (csum_value_final == (__force u16)*csum_field) {
 		priv->stats.csum_ok++;
 		return 0;
 	} else {