lib: add crc7.

Used as a checksum on command messages when talking with MMC cards.

Implemented using the unwound bytewise implementation from
https://en.wikipedia.org/wiki/Computation_of_cyclic_redundancy_checks
which is a good mix of size and speed.

The API and naming matches lib/crc7.c in Linux.

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

include/crc7.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2018 Google LLC.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef __CRC7_H_
+#define __CRC7_H_
+
+#include <zephyr/types.h>
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Compute the CRC-7 checksum of a buffer.
+ *
+ * See JESD84-A441.  Used by the MMC protocol.  Uses 0x09 as the
+ * polynomial with no reflection.  The CRC is left
+ * justified, so bit 7 of the result is bit 6 of the CRC.
+ *
+ * @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 CRC7 value
+ */
+u8_t crc7_be(u8_t seed, const u8_t *src, size_t len);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

lib/crc/CMakeLists.txt

@@ -1 +1 @@
-zephyr_sources(crc32_sw.c crc16_sw.c crc8_sw.c)
+zephyr_sources(crc32_sw.c crc16_sw.c crc8_sw.c crc7_sw.c)

lib/crc/crc7_sw.c

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2018 Google LLC.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <crc7.h>
+
+u8_t crc7_be(u8_t seed, const u8_t *src, size_t len)
+{
+	while (len--) {
+		u8_t e = seed ^ *src++;
+		u8_t f = e ^ (e >> 4) ^ (e >> 7);
+
+		seed = (f << 1) ^ (f << 4);
+	}
+
+	return seed;
+}

tests/unit/lib/crc/main.c

@@ -9,6 +9,7 @@
 #include <lib/crc/crc32_sw.c>
 #include <lib/crc/crc16_sw.c>
 #include <lib/crc/crc8_sw.c>
+#include <lib/crc/crc7_sw.c>
 
 void test_crc32_ieee(void)
 {
@@ -109,6 +110,17 @@ void test_crc8_ccitt(void)
 			   sizeof(test2)) == 0xFB, "pass", "fail");
 }
 
+void test_crc7_be(void)
+{
+	u8_t test0[] = { 0 };
+	u8_t test1[] = { 'A' };
+	u8_t test2[] = { '1', '2', '3', '4', '5', '6', '7', '8', '9' };
+
+	zassert_equal(crc7_be(0, test0, sizeof(test0)), 0, NULL);
+	zassert_equal(crc7_be(0, test1, sizeof(test1)), 0xDA, NULL);
+	zassert_equal(crc7_be(0, test2, sizeof(test2)), 0xEA, NULL);
+}
+
 void test_main(void)
 {
 	ztest_test_suite(test_crc,
@@ -118,6 +130,7 @@ void test_main(void)
 			 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_unit_test(test_crc8_ccitt),
+			 ztest_unit_test(test_crc7_be));
 	ztest_run_test_suite(test_crc);
 }