net: Add a separate file for IP checksumming

Move the checksum code out into its own file so it can be used elsewhere.
Also use a new version which supports a length which is not a multiple of
2 and add a new function to add two checksums.

Signed-off-by: Simon Glass <sjg@chromium.org>

net/checksum.c

@@ -0,0 +1,60 @@
+/*
+ * This file was originally taken from the FreeBSD project.
+ *
+ * Copyright (c) 2001 Charles Mott <cm@linktel.net>
+ * Copyright (c) 2008 coresystems GmbH
+ * All rights reserved.
+ *
+ * SPDX-License-Identifier:	BSD-2-Clause
+ */
+
+#include <common.h>
+#include <net.h>
+
+unsigned compute_ip_checksum(const void *vptr, unsigned nbytes)
+{
+	int sum, oddbyte;
+	const unsigned short *ptr = vptr;
+
+	sum = 0;
+	while (nbytes > 1) {
+		sum += *ptr++;
+		nbytes -= 2;
+	}
+	if (nbytes == 1) {
+		oddbyte = 0;
+		((u8 *)&oddbyte)[0] = *(u8 *)ptr;
+		((u8 *)&oddbyte)[1] = 0;
+		sum += oddbyte;
+	}
+	sum = (sum >> 16) + (sum & 0xffff);
+	sum += (sum >> 16);
+	sum = ~sum & 0xffff;
+
+	return sum;
+}
+
+unsigned add_ip_checksums(unsigned offset, unsigned sum, unsigned new)
+{
+	unsigned long checksum;
+
+	sum = ~sum & 0xffff;
+	new = ~new & 0xffff;
+	if (offset & 1) {
+		/*
+		 * byte-swap the sum if it came from an odd offset; since the
+		 * computation is endian independant this works.
+		 */
+		new = ((new >> 8) & 0xff) | ((new << 8) & 0xff00);
+	}
+	checksum = sum + new;
+	if (checksum > 0xffff)
+		checksum -= 0xffff;
+
+	return (~checksum) & 0xffff;
+}
+
+int ip_checksum_ok(const void *addr, unsigned nbytes)
+{
+	return !(compute_ip_checksum(addr, nbytes) & 0xfffe);
+}