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Date:   Thu, 13 Apr 2017 07:29:32 -0700
From:   Andrey Smirnov <andrew.smirnov@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Andrey Vostrikov <andrey.vostrikov@...entembedded.com>,
        cphealy@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Smirnov <andrew.smirnov@...il.com>
Subject: [PATCH] lib/crc-ccitt: add CCITT-FALSE CRC16 variant

From: Andrey Vostrikov <andrey.vostrikov@...entembedded.com>

Cc: cphealy@...il.com
Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Andrey Vostrikov <andrey.vostrikov@...entembedded.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@...il.com>
---
 include/linux/crc-ccitt.h |  7 ++++++
 lib/crc-ccitt.c           | 58 ++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 64 insertions(+), 1 deletion(-)

diff --git a/include/linux/crc-ccitt.h b/include/linux/crc-ccitt.h
index f52696a..7cd45a4 100644
--- a/include/linux/crc-ccitt.h
+++ b/include/linux/crc-ccitt.h
@@ -4,12 +4,19 @@
 #include <linux/types.h>
 
 extern u16 const crc_ccitt_table[256];
+extern u16 const crc_ccitt_false_table[256];
 
 extern u16 crc_ccitt(u16 crc, const u8 *buffer, size_t len);
+extern u16 crc_ccitt_false(u16 crc, const u8 *buffer, size_t len);
 
 static inline u16 crc_ccitt_byte(u16 crc, const u8 c)
 {
 	return (crc >> 8) ^ crc_ccitt_table[(crc ^ c) & 0xff];
 }
 
+static inline u16 crc_ccitt_false_byte(u16 crc, const u8 c)
+{
+    return (crc << 8) ^ crc_ccitt_false_table[(crc >> 8) ^ c];
+}
+
 #endif /* _LINUX_CRC_CCITT_H */
diff --git a/lib/crc-ccitt.c b/lib/crc-ccitt.c
index 7f6dd68..d873b34 100644
--- a/lib/crc-ccitt.c
+++ b/lib/crc-ccitt.c
@@ -51,8 +51,49 @@ u16 const crc_ccitt_table[256] = {
 };
 EXPORT_SYMBOL(crc_ccitt_table);
 
+/*
+ * Similar table to calculate CRC16 variant known as CRC-CCITT-FALSE
+ * Reflected bits order, does not augment final value.
+ */
+u16 const crc_ccitt_false_table[256] = {
+    0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50A5, 0x60C6, 0x70E7,
+    0x8108, 0x9129, 0xA14A, 0xB16B, 0xC18C, 0xD1AD, 0xE1CE, 0xF1EF,
+    0x1231, 0x0210, 0x3273, 0x2252, 0x52B5, 0x4294, 0x72F7, 0x62D6,
+    0x9339, 0x8318, 0xB37B, 0xA35A, 0xD3BD, 0xC39C, 0xF3FF, 0xE3DE,
+    0x2462, 0x3443, 0x0420, 0x1401, 0x64E6, 0x74C7, 0x44A4, 0x5485,
+    0xA56A, 0xB54B, 0x8528, 0x9509, 0xE5EE, 0xF5CF, 0xC5AC, 0xD58D,
+    0x3653, 0x2672, 0x1611, 0x0630, 0x76D7, 0x66F6, 0x5695, 0x46B4,
+    0xB75B, 0xA77A, 0x9719, 0x8738, 0xF7DF, 0xE7FE, 0xD79D, 0xC7BC,
+    0x48C4, 0x58E5, 0x6886, 0x78A7, 0x0840, 0x1861, 0x2802, 0x3823,
+    0xC9CC, 0xD9ED, 0xE98E, 0xF9AF, 0x8948, 0x9969, 0xA90A, 0xB92B,
+    0x5AF5, 0x4AD4, 0x7AB7, 0x6A96, 0x1A71, 0x0A50, 0x3A33, 0x2A12,
+    0xDBFD, 0xCBDC, 0xFBBF, 0xEB9E, 0x9B79, 0x8B58, 0xBB3B, 0xAB1A,
+    0x6CA6, 0x7C87, 0x4CE4, 0x5CC5, 0x2C22, 0x3C03, 0x0C60, 0x1C41,
+    0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49,
+    0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13, 0x2E32, 0x1E51, 0x0E70,
+    0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78,
+    0x9188, 0x81A9, 0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F,
+    0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046, 0x6067,
+    0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E,
+    0x02B1, 0x1290, 0x22F3, 0x32D2, 0x4235, 0x5214, 0x6277, 0x7256,
+    0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D,
+    0x34E2, 0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
+    0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E, 0xC71D, 0xD73C,
+    0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634,
+    0xD94C, 0xC96D, 0xF90E, 0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB,
+    0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,
+    0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A,
+    0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1, 0x1AD0, 0x2AB3, 0x3A92,
+    0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9,
+    0x7C26, 0x6C07, 0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1,
+    0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9, 0x9FF8,
+    0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0
+};
+EXPORT_SYMBOL(crc_ccitt_false_table);
+
 /**
- *	crc_ccitt - recompute the CRC for the data buffer
+ *	crc_ccitt - recompute the CRC (CRC-CCITT variant) for the data
+ *	buffer
  *	@crc: previous CRC value
  *	@buffer: data pointer
  *	@len: number of bytes in the buffer
@@ -65,5 +106,20 @@ u16 crc_ccitt(u16 crc, u8 const *buffer, size_t len)
 }
 EXPORT_SYMBOL(crc_ccitt);
 
+/**
+ *	crc_ccitt_false - recompute the CRC (CRC-CCITT-FALSE variant)
+ *	for the data buffer
+ *	@crc: previous CRC value
+ *	@buffer: data pointer
+ *	@len: number of bytes in the buffer
+ */
+u16 crc_ccitt_false(u16 crc, u8 const *buffer, size_t len)
+{
+	while (len--)
+		crc = crc_ccitt_false_byte(crc, *buffer++);
+	return crc;
+}
+EXPORT_SYMBOL(crc_ccitt_false);
+
 MODULE_DESCRIPTION("CRC-CCITT calculations");
 MODULE_LICENSE("GPL");
-- 
2.9.3

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