net: qualcomm: rmnet: trailer value is a checksum

The csum_value field in the rmnet_map_dl_csum_trailer structure is a
"real" Internet checksum.  It is a 16 bit value, in big endian format,
which represents an inverted ones' complement sum over pairs of bytes.

Make that clear by changing its type to __sum16.

This makes a typecast in rmnet_map_ipv4_dl_csum_trailer() and
another in rmnet_map_ipv6_dl_csum_trailer() unnecessary.
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

@@ -87,7 +87,7 @@ rmnet_map_ipv4_dl_csum_trailer(struct sk_buff *skb,
 	 * header checksum value; it is already accounted for in the
 	 * checksum value found in the trailer.
 	 */
-	ip_payload_csum = (__force __sum16)~csum_trailer->csum_value;
+	ip_payload_csum = ~csum_trailer->csum_value;
 
 	pseudo_csum = ~csum_tcpudp_magic(ip4h->saddr, ip4h->daddr,
 					 ntohs(ip4h->tot_len) - ip4h->ihl * 4,
@@ -132,8 +132,7 @@ rmnet_map_ipv6_dl_csum_trailer(struct sk_buff *skb,
 	 * checksum computed over the pseudo header.
 	 */
 	ip_header_csum = (__force __be16)ip_fast_csum(ip6h, sizeof(*ip6h) / 4);
-	ip6_payload_csum = ~csum16_sub((__force __sum16)csum_trailer->csum_value,
-				       ip_header_csum);
+	ip6_payload_csum = ~csum16_sub(csum_trailer->csum_value, ip_header_csum);
 
 	length = (ip6h->nexthdr == IPPROTO_UDP) ?
 		 ntohs(((struct udphdr *)txporthdr)->len) :

include/linux/if_rmnet.h

@@ -25,7 +25,7 @@ struct rmnet_map_dl_csum_trailer {
 	u8 flags;			/* MAP_CSUM_DL_VALID_FLAG */
 	__be16 csum_start_offset;
 	__be16 csum_length;
-	__be16 csum_value;
+	__sum16 csum_value;
 } __aligned(1);
 
 /* rmnet_map_dl_csum_trailer flags field: