net: checksum: Move from32to16() to generic header

from32to16() is used by lib/checksum.c and also by
arch/parisc/lib/checksum.c. The next patch will use it in the
bpf_csum_diff helper.

Move from32to16() to the include/net/checksum.h as csum_from32to16() and
remove other implementations.

Signed-off-by: Puranjay Mohan <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Toke Høiland-Jørgensen <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

arch/parisc/lib/checksum.c

···
 	: "=r"(_t)                      \
 	: "r"(_r), "0"(_t));
 
-static inline unsigned short from32to16(unsigned int x)
-{
-	/* 32 bits --> 16 bits + carry */
-	x = (x & 0xffff) + (x >> 16);
-	/* 16 bits + carry --> 16 bits including carry */
-	x = (x & 0xffff) + (x >> 16);
-	return (unsigned short)x;
-}
-
 static inline unsigned int do_csum(const unsigned char * buff, int len)
 {
 	int odd, count;
···
 	}
 	if (len & 1)
 		result += le16_to_cpu(*buff);
-	result = from32to16(result);
+	result = csum_from32to16(result);
 	if (odd)
 		result = swab16(result);
 out:
···
 {
 	unsigned int result = do_csum(buff, len);
 	addc(result, sum);
-	return (__force __wsum)from32to16(result);
+	return (__force __wsum)csum_from32to16(result);
 }
 
 EXPORT_SYMBOL(csum_partial);

include/net/checksum.h

···
 	*csum = csum_add(csum_sub(*csum, old), new);
 }
 
+static inline unsigned short csum_from32to16(unsigned int sum)
+{
+	sum += (sum >> 16) | (sum << 16);
+	return (unsigned short)(sum >> 16);
+}
+
 struct sk_buff;
 void inet_proto_csum_replace4(__sum16 *sum, struct sk_buff *skb,
 			      __be32 from, __be32 to, bool pseudohdr);

lib/checksum.c

···
 #include <asm/byteorder.h>
 
 #ifndef do_csum
-static inline unsigned short from32to16(unsigned int x)
-{
-	/* add up 16-bit and 16-bit for 16+c bit */
-	x = (x & 0xffff) + (x >> 16);
-	/* add up carry.. */
-	x = (x & 0xffff) + (x >> 16);
-	return x;
-}
-
 static unsigned int do_csum(const unsigned char *buff, int len)
 {
 	int odd;
···
 #else
 		result += (*buff << 8);
 #endif
-	result = from32to16(result);
+	result = csum_from32to16(result);
 	if (odd)
 		result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
 out: