crc: add a faster, chainable version of CRC16/CCITT.

The existing version of crc16_ccitt() is actually CRC-16/AUG-CCITT and
gives different results to Linux, Contiki, and the CRC unit in the
SAM0 SOC.  This version matches Linux.

Note that this is an incompatible API change.

Signed-off-by: Michael Hope <mlhx@google.com>

include/crc16.h

@@ -43,20 +43,30 @@ u16_t crc16(const u8_t *src, size_t len, u16_t polynomial,
 	    u16_t initial_value, bool pad);
 
 /**
- * @brief Compute CCITT variant of CRC 16
+ * @brief Compute the CRC-16/CCITT checksum of a buffer.
  *
- * CCITT variant of CRC 16 is using 0x1021 as its polynomial with the initial
- * value set to 0xffff.
+ * See ITU-T Recommendation V.41 (November 1988).  Uses 0x1021 as the
+ * polynomial, reflects the input, and reflects the output.
  *
+ * 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/CCITT, 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
+ * - X-25 (used in PPP): seed=0xffff, xor=0xffff, residual=0xf0b8
+ *
+ * @note API changed in Zephyr 1.11.
+ *
+ * @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
  */
-static inline u16_t crc16_ccitt(const u8_t *src, size_t len)
-{
-	return crc16(src, len, 0x1021, 0xffff, true);
-}
+u16_t crc16_ccitt(u16_t seed, const u8_t *src, size_t len);
 
 /**
  * @brief Compute ANSI variant of CRC 16

lib/crc/crc16_sw.c

@@ -34,3 +34,16 @@ u16_t crc16(const u8_t *src, size_t len, u16_t polynomial,
 
 	return crc;
 }
+
+u16_t crc16_ccitt(u16_t seed, const u8_t *src, size_t len)
+{
+	for (; len > 0; len--) {
+		u8_t e, f;
+
+		e = seed ^ *src++;
+		f = e ^ (e << 4);
+		seed = (seed >> 8) ^ (f << 8) ^ (f << 3) ^ (f >> 4);
+	}
+
+	return seed;
+}

tests/unit/lib/crc/main.c

@@ -15,9 +15,12 @@ void test_crc16(void)
 	u8_t test1[] = { 'A' };
 	u8_t test2[] = { '1', '2', '3', '4', '5', '6', '7', '8', '9' };
 
-	zassert(crc16_ccitt(test0, sizeof(test0)) == 0x1d0f, "pass", "fail");
-	zassert(crc16_ccitt(test1, sizeof(test1)) == 0x9479, "pass", "fail");
-	zassert(crc16_ccitt(test2, sizeof(test2)) == 0xe5cc, "pass", "fail");
+	zassert_equal(crc16(test0, sizeof(test0), 0x1021, 0xffff, true),
+		      0x1d0f, NULL);
+	zassert_equal(crc16(test1, sizeof(test1), 0x1021, 0xffff, true),
+		      0x9479, NULL);
+	zassert_equal(crc16(test2, sizeof(test2), 0x1021, 0xffff, true),
+		      0xe5cc, NULL);
 }
 
 void test_crc16_ansi(void)
@@ -31,6 +34,47 @@ void test_crc16_ansi(void)
 	zassert(crc16_ansi(test2, sizeof(test2)) == 0x9ecf, "pass", "fail");
 }
 
+void test_crc16_ccitt(void)
+{
+	u8_t test0[] = { };
+	u8_t test1[] = { 'A' };
+	u8_t test2[] = { '1', '2', '3', '4', '5', '6', '7', '8', '9' };
+	u8_t test3[] = { 'Z', 'e', 'p', 'h', 'y', 'r', 0, 0 };
+	u16_t crc;
+
+	zassert_equal(crc16_ccitt(0, test0, sizeof(test0)), 0x0, NULL);
+	zassert_equal(crc16_ccitt(0, test1, sizeof(test1)), 0x538d, NULL);
+	zassert_equal(crc16_ccitt(0, test2, sizeof(test2)), 0x2189, NULL);
+
+	/* Appending the CRC to a buffer and computing the CRC over
+	 * the extended buffer leaves a residual of zero.
+	 */
+	crc = crc16_ccitt(0, test3, sizeof(test3) - sizeof(u16_t));
+	test3[sizeof(test3)-2] = (u8_t)(crc >> 0);
+	test3[sizeof(test3)-1] = (u8_t)(crc >> 8);
+
+	zassert_equal(crc16_ccitt(0, test3, sizeof(test3)), 0, NULL);
+}
+
+void test_crc16_ccitt_for_ppp(void)
+{
+	/* Example capture including FCS from
+	 * https://www.horo.ch/techno/ppp-fcs/examples_en.html
+	 */
+	u8_t test0[] = {
+		0xff, 0x03, 0xc0, 0x21, 0x01, 0x01, 0x00, 0x17,
+		0x02, 0x06, 0x00, 0x0a, 0x00, 0x00, 0x05, 0x06,
+		0x00, 0x2a, 0x2b, 0x78, 0x07, 0x02, 0x08, 0x02,
+		0x0d, 0x03, 0x06, 0xa5, 0xf8
+	};
+	u8_t test2[] = { '1', '2', '3', '4', '5', '6', '7', '8', '9' };
+
+	zassert_equal(crc16_ccitt(0xffff, test0, sizeof(test0)),
+		      0xf0b8, NULL);
+	zassert_equal(crc16_ccitt(0xffff, test2, sizeof(test2)) ^ 0xFFFF,
+		      0x906e, NULL);
+}
+
 void test_crc8_ccitt(void)
 {
 	u8_t test0[] = { 0 };
@@ -50,6 +94,8 @@ void test_main(void)
 	ztest_test_suite(test_crc,
 			 ztest_unit_test(test_crc16),
 			 ztest_unit_test(test_crc16_ansi),
+			 ztest_unit_test(test_crc16_ccitt),
+			 ztest_unit_test(test_crc16_ccitt_for_ppp),
 			 ztest_unit_test(test_crc8_ccitt));
 	ztest_run_test_suite(test_crc);
 }