Prepared for checksum-on-copy (task #6849):

- Added option LWIP_CHECKSUM_ON_COPY;
- Added function + define lwip_chksum_copy to create checksum when copying data

src/include/ipv4/lwip/inet_chksum.h

@@ -37,6 +37,35 @@
 #include "lwip/pbuf.h"
 #include "lwip/ip_addr.h"
 
+/** Swap the bytes in an u16_t: much like htons() for little-endian */
+#ifndef SWAP_BYTES_IN_WORD
+#if LWIP_PLATFORM_BYTESWAP && (BYTE_ORDER == LITTLE_ENDIAN)
+/* little endian and PLATFORM_BYTESWAP defined */
+#define SWAP_BYTES_IN_WORD(w) LWIP_PLATFORM_HTONS(w)
+#else /* LWIP_PLATFORM_BYTESWAP && (BYTE_ORDER == LITTLE_ENDIAN) */
+/* can't use htons on big endian (or PLATFORM_BYTESWAP not defined)... */
+#define SWAP_BYTES_IN_WORD(w) (((w) & 0xff) << 8) | (((w) & 0xff00) >> 8)
+#endif /* LWIP_PLATFORM_BYTESWAP && (BYTE_ORDER == LITTLE_ENDIAN)*/
+#endif /* SWAP_BYTES_IN_WORD */
+
+/** Split an u32_t in two u16_ts and add them up */
+#ifndef FOLD_U32T
+#define FOLD_U32T(u)          (((u) >> 16) + ((u) & 0x0000ffffUL))
+#endif
+
+#if LWIP_CHECKSUM_ON_COPY
+/** Function-like macro: same as MEMCPY but returns the checksum of copied data
+    as u16_t */
+#ifndef LWIP_CHKSUM_COPY
+#define LWIP_CHKSUM_COPY(dst, src, len) lwip_chksum_copy(dst, src, len)
+#ifndef LWIP_CHKSUM_COPY_ALGORITHM
+#define LWIP_CHKSUM_COPY_ALGORITHM 1
+#endif /* LWIP_CHKSUM_COPY_ALGORITHM */
+#endif /* LWIP_CHKSUM_COPY */
+#else /* LWIP_CHECKSUM_ON_COPY */
+#define LWIP_CHKSUM_COPY_ALGORITHM 0
+#endif /* LWIP_CHECKSUM_ON_COPY */
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -46,11 +75,12 @@ u16_t inet_chksum_pbuf(struct pbuf *p);
 u16_t inet_chksum_pseudo(struct pbuf *p,
        ip_addr_t *src, ip_addr_t *dest,
        u8_t proto, u16_t proto_len);
-#if LWIP_UDPLITE
 u16_t inet_chksum_pseudo_partial(struct pbuf *p,
        ip_addr_t *src, ip_addr_t *dest,
        u8_t proto, u16_t proto_len, u16_t chksum_len);
-#endif
+#if LWIP_CHKSUM_COPY_ALGORITHM
+u16_t lwip_chksum_copy(void *dst, const void *src, u16_t len);
+#endif /* LWIP_CHKSUM_COPY_ALGORITHM */
 
 #ifdef __cplusplus
 }

src/include/lwip/opt.h

@@ -444,7 +444,6 @@
 #define ETH_PAD_SIZE                    0
 #endif
 
-
 /*
    --------------------------------
    ---------- IP options ----------
@@ -1708,6 +1707,14 @@
 #define CHECKSUM_CHECK_TCP              1
 #endif
 
+/**
+ * LWIP_CHECKSUM_ON_COPY==1: Calculate checksum when copying data from
+ * application buffers to pbufs.
+ */
+#ifndef LWIP_CHECKSUM_ON_COPY
+#define LWIP_CHECKSUM_ON_COPY           0
+#endif
+
 /*
    ---------------------------------------
    ---------- Debugging options ----------