Commit a2918a16 authored by Alex Elder's avatar Alex Elder Committed by David S. Miller
Browse files

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/rfc1071



Signed-off-by: default avatarAlex Elder <elder@linaro.org>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent e5adbbdf

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

+4 −4
Original line number Diff line number Diff line
@@ -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 {