crc: add crc16_itu_t which is used in the MMC protocol.

This is the MSB first version of crc16_ccitt.  Use the same name as in
Linux.

Signed-off-by: Michael Hope <[email protected]>

Author: nzmichaelh

include/crc16.h

@@ -55,7 +55,6 @@ u16_t crc16(const u8_t *src, size_t len, u16_t polynomial,
  * the same family can be calculated by changing the seed and/or
  * XORing the final value.  Examples include:
  *
- * - CCIITT-FALSE: seed=0xffff
  * - X-25 (used in PPP): seed=0xffff, xor=0xffff, residual=0xf0b8
  *
  * @note API changed in Zephyr 1.11.
@@ -68,6 +67,30 @@ u16_t crc16(const u8_t *src, size_t len, u16_t polynomial,
  */
 u16_t crc16_ccitt(u16_t seed, const u8_t *src, size_t len);
 
+/**
+ * @brief Compute the CRC-16/XMODEM checksum of a buffer.
+ *
+ * The MSB first version of ITU-T Recommendation V.41 (November 1988).
+ * Uses 0x1021 as the polynomial with no reflection.
+ *
+ * To calculate the CRC across non-contigious blocks use the return
+ * value from block N-1 as the seed for block N.
+ *
+ * For CRC-16/XMODEM, use 0 as the initial seed.  Other checksums in
+ * the same family can be calculated by changing the seed and/or
+ * XORing the final value.  Examples include:
+ *
+ * - CCIITT-FALSE: seed=0xffff
+ * - GSM: seed=0, xorout=0xffff, residue=0x1d0f
+ *
+ * @param seed Value to seed the CRC with
+ * @param src Input bytes for the computation
+ * @param len Length of the input in bytes
+ *
+ * @return The computed CRC16 value
+ */
+u16_t crc16_itu_t(u16_t seed, const u8_t *src, size_t len);
+
 /**
  * @brief Compute ANSI variant of CRC 16
  *

lib/crc/crc16_sw.c

@@ -47,3 +47,16 @@ u16_t crc16_ccitt(u16_t seed, const u8_t *src, size_t len)
 
 	return seed;
 }
+
+u16_t crc16_itu_t(u16_t seed, const u8_t *src, size_t len)
+{
+	for (; len > 0; len--) {
+		seed = (seed >> 8) | (seed << 8);
+		seed ^= *src++;
+		seed ^= (seed & 0xff) >> 4;
+		seed ^= seed << 12;
+		seed ^= (seed & 0xff) << 5;
+	}
+
+	return seed;
+}

tests/unit/lib/crc/main.c

@@ -75,6 +75,14 @@ void test_crc16_ccitt_for_ppp(void)
 		      0x906e, NULL);
 }
 
+void test_crc16_itu_t(void)
+{
+	u8_t test2[] = { '1', '2', '3', '4', '5', '6', '7', '8', '9' };
+
+	zassert_equal(crc16_itu_t(0, test2, sizeof(test2)),
+		      0x31c3, NULL);
+}
+
 void test_crc8_ccitt(void)
 {
 	u8_t test0[] = { 0 };
@@ -96,6 +104,7 @@ void test_main(void)
 			 ztest_unit_test(test_crc16_ansi),
 			 ztest_unit_test(test_crc16_ccitt),
 			 ztest_unit_test(test_crc16_ccitt_for_ppp),
+			 ztest_unit_test(test_crc16_itu_t),
 			 ztest_unit_test(test_crc8_ccitt));
 	ztest_run_test_suite(test_crc);
 }