From 299e4ca7168a7dd9a289178cd2b4bd313f277e70 Mon Sep 17 00:00:00 2001
From: jack <jack@espressif.com>
Date: Tue, 26 Feb 2019 16:38:20 +0800
Subject: [PATCH] Add CRC APIs usage

---
 components/esp32/include/rom/crc.h | 57 +++++++++++++++++++++++-------
 1 file changed, 45 insertions(+), 12 deletions(-)

diff --git a/components/esp32/include/rom/crc.h b/components/esp32/include/rom/crc.h
index 84e17882de..faa1e8c351 100644
--- a/components/esp32/include/rom/crc.h
+++ b/components/esp32/include/rom/crc.h
@@ -30,15 +30,48 @@ extern "C" {
   */
 
 
-/* Standard CRC8/16/32 algorithms. */
-// CRC-8        x8+x2+x1+1              0x07
-// CRC16-CCITT  x16+x12+x5+1   1021   ISO HDLC, ITU X.25, V.34/V.41/V.42, PPP-FCS
-// CRC32:
-//G(x) = x32 +x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x1 + 1
-//If your buf is not continuous, you can use the first result to be the second parameter.
+/*           Notes about CRC APIs usage
+ * The ESP32 ROM include some CRC tables and CRC APIs to speed up CRC calculation.
+ * The CRC APIs include CRC8, CRC16, CRC32 algorithms for both little endian and big endian modes.
+ * Here are the polynomials for the algorithms:
+ * CRC-8        x8+x2+x1+1                                              0x07
+ * CRC16-CCITT  x16+x12+x5+1                                            0x1021
+ * CRC32        x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x1+1     0x04c11db7
+ * 
+ * These group of CRC APIs are designed to calculate the data in buffers either continuous or not.
+ * To make it easy, we had added a `~` at the beginning and the end of the functions.
+ * To calculate non-continuous buffers, we can write the code like this:
+ *     init = ~init;
+ *     crc = crc32_le(init, buf0, length0);
+ *     crc = crc32_le(crc, buf1, length1);
+ *     crc = ~crc;
+ *
+ * However, it is not easy to select which API to use and give the correct parameters.
+ * A specific CRC algorithm will include this parameters: width, polynomials, init, refin, refout, xorout
+ *     refin and refout show the endian of the algorithm:
+ *         if both of them are true, please use the little endian API.
+ *         if both of them are false, please use the big endian API.
+ *     xorout is the value which you need to be xored to the raw result.
+ * However, these group of APIs need one '~' before and after the APIs.
+ *
+ * Here are some examples for CRC16:
+ * CRC-16/CCITT, poly = 0x1021, init = 0x0000, refin = true, refout = true, xorout = 0x0000
+ *     crc = ~crc16_le((uint16_t)~0x0000, buf, length);
+ * 
+ * CRC-16/CCITT-FALSE, poly = 0x1021, init = 0xffff, refin = false, refout = false, xorout = 0x0000
+ *     crc = ~crc16_be((uint16_t)~0xffff, buf, length);
+ *
+ * CRC-16/X25, poly = 0x1021, init = 0xffff, refin = true, refout = true, xorout = 0xffff
+ *     crc = (~crc16_le((uint16_t)~(0xffff), buf, length))^0xffff;
+ *
+ * CRC-16/XMODEM, poly= 0x1021, init = 0x0000, refin = false, refout = false, xorout = 0x0000
+ *     crc = ~crc16_be((uint16_t)~0x0000, buf, length);
+ *
+ *     
+ */
 
 /**
-  * @brief Crc32 value that is in little endian.
+  * @brief CRC32 value that is in little endian.
   *
   * @param  uint32_t crc : init crc value, use 0 at the first use.
   *
@@ -51,7 +84,7 @@ extern "C" {
 uint32_t crc32_le(uint32_t crc, uint8_t const *buf, uint32_t len);
 
 /**
-  * @brief Crc32 value that is in big endian.
+  * @brief CRC32 value that is in big endian.
   *
   * @param  uint32_t crc : init crc value, use 0 at the first use.
   *
@@ -64,7 +97,7 @@ uint32_t crc32_le(uint32_t crc, uint8_t const *buf, uint32_t len);
 uint32_t crc32_be(uint32_t crc, uint8_t const *buf, uint32_t len);
 
 /**
-  * @brief Crc16 value that is in little endian.
+  * @brief CRC16 value that is in little endian.
   *
   * @param  uint16_t crc : init crc value, use 0 at the first use.
   *
@@ -77,7 +110,7 @@ uint32_t crc32_be(uint32_t crc, uint8_t const *buf, uint32_t len);
 uint16_t crc16_le(uint16_t crc, uint8_t const *buf, uint32_t len);
 
 /**
-  * @brief Crc16 value that is in big endian.
+  * @brief CRC16 value that is in big endian.
   *
   * @param  uint16_t crc : init crc value, use 0 at the first use.
   *
@@ -90,7 +123,7 @@ uint16_t crc16_le(uint16_t crc, uint8_t const *buf, uint32_t len);
 uint16_t crc16_be(uint16_t crc, uint8_t const *buf, uint32_t len);
 
 /**
-  * @brief Crc8 value that is in little endian.
+  * @brief CRC8 value that is in little endian.
   *
   * @param  uint8_t crc : init crc value, use 0 at the first use.
   *
@@ -103,7 +136,7 @@ uint16_t crc16_be(uint16_t crc, uint8_t const *buf, uint32_t len);
 uint8_t crc8_le(uint8_t crc, uint8_t const *buf, uint32_t len);
 
 /**
-  * @brief Crc8 value that is in big endian.
+  * @brief CRC8 value that is in big endian.
   *
   * @param  uint32_t crc : init crc value, use 0 at the first use.
   *