1==========================================
  2Reed-Solomon Library Programming Interface
  3==========================================
  4
  5:Author: Thomas Gleixner
  6
  7Introduction
  8============
  9
 10The generic Reed-Solomon Library provides encoding, decoding and error
 11correction functions.
 12
 13Reed-Solomon codes are used in communication and storage applications to
 14ensure data integrity.
 15
 16This documentation is provided for developers who want to utilize the
 17functions provided by the library.
 18
 19Known Bugs And Assumptions
 20==========================
 21
 22None.
 23
 24Usage
 25=====
 26
 27This chapter provides examples of how to use the library.
 28
 29Initializing
 30------------
 31
 32The init function init_rs returns a pointer to an rs decoder structure,
 33which holds the necessary information for encoding, decoding and error
 34correction with the given polynomial. It either uses an existing
 35matching decoder or creates a new one. On creation all the lookup tables
 36for fast en/decoding are created. The function may take a while, so make
 37sure not to call it in critical code paths.
 38
 39::
 40
 41    /* the Reed Solomon control structure */
 42    static struct rs_control *rs_decoder;
 43
 44    /* Symbolsize is 10 (bits)
 45     * Primitive polynomial is x^10+x^3+1
 46     * first consecutive root is 0
 47     * primitive element to generate roots = 1
 48     * generator polynomial degree (number of roots) = 6
 49     */
 50    rs_decoder = init_rs (10, 0x409, 0, 1, 6);
 51
 52
 53Encoding
 54--------
 55
 56The encoder calculates the Reed-Solomon code over the given data length
 57and stores the result in the parity buffer. Note that the parity buffer
 58must be initialized before calling the encoder.
 59
 60The expanded data can be inverted on the fly by providing a non-zero
 61inversion mask. The expanded data is XOR'ed with the mask. This is used
 62e.g. for FLASH ECC, where the all 0xFF is inverted to an all 0x00. The
 63Reed-Solomon code for all 0x00 is all 0x00. The code is inverted before
 64storing to FLASH so it is 0xFF too. This prevents that reading from an
 65erased FLASH results in ECC errors.
 66
 67The databytes are expanded to the given symbol size on the fly. There is
 68no support for encoding continuous bitstreams with a symbol size != 8 at
 69the moment. If it is necessary it should be not a big deal to implement
 70such functionality.
 71
 72::
 73
 74    /* Parity buffer. Size = number of roots */
 75    uint16_t par[6];
 76    /* Initialize the parity buffer */
 77    memset(par, 0, sizeof(par));
 78    /* Encode 512 byte in data8. Store parity in buffer par */
 79    encode_rs8 (rs_decoder, data8, 512, par, 0);
 80
 81
 82Decoding
 83--------
 84
 85The decoder calculates the syndrome over the given data length and the
 86received parity symbols and corrects errors in the data.
 87
 88If a syndrome is available from a hardware decoder then the syndrome
 89calculation is skipped.
 90
 91The correction of the data buffer can be suppressed by providing a
 92correction pattern buffer and an error location buffer to the decoder.
 93The decoder stores the calculated error location and the correction
 94bitmask in the given buffers. This is useful for hardware decoders which
 95use a weird bit ordering scheme.
 96
 97The databytes are expanded to the given symbol size on the fly. There is
 98no support for decoding continuous bitstreams with a symbolsize != 8 at
 99the moment. If it is necessary it should be not a big deal to implement
100such functionality.
101
102Decoding with syndrome calculation, direct data correction
103~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
104
105::
106
107    /* Parity buffer. Size = number of roots */
108    uint16_t par[6];
109    uint8_t  data[512];
110    int numerr;
111    /* Receive data */
112    .....
113    /* Receive parity */
114    .....
115    /* Decode 512 byte in data8.*/
116    numerr = decode_rs8 (rs_decoder, data8, par, 512, NULL, 0, NULL, 0, NULL);
117
118
119Decoding with syndrome given by hardware decoder, direct data correction
120~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
121
122::
123
124    /* Parity buffer. Size = number of roots */
125    uint16_t par[6], syn[6];
126    uint8_t  data[512];
127    int numerr;
128    /* Receive data */
129    .....
130    /* Receive parity */
131    .....
132    /* Get syndrome from hardware decoder */
133    .....
134    /* Decode 512 byte in data8.*/
135    numerr = decode_rs8 (rs_decoder, data8, par, 512, syn, 0, NULL, 0, NULL);
136
137
138Decoding with syndrome given by hardware decoder, no direct data correction.
139~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
140
141Note: It's not necessary to give data and received parity to the
142decoder.
143
144::
145
146    /* Parity buffer. Size = number of roots */
147    uint16_t par[6], syn[6], corr[8];
148    uint8_t  data[512];
149    int numerr, errpos[8];
150    /* Receive data */
151    .....
152    /* Receive parity */
153    .....
154    /* Get syndrome from hardware decoder */
155    .....
156    /* Decode 512 byte in data8.*/
157    numerr = decode_rs8 (rs_decoder, NULL, NULL, 512, syn, 0, errpos, 0, corr);
158    for (i = 0; i < numerr; i++) {
159        do_error_correction_in_your_buffer(errpos[i], corr[i]);
160    }
161
162
163Cleanup
164-------
165
166The function free_rs frees the allocated resources, if the caller is
167the last user of the decoder.
168
169::
170
171    /* Release resources */
172    free_rs(rs_decoder);
173
174
175Structures
176==========
177
178This chapter contains the autogenerated documentation of the structures
179which are used in the Reed-Solomon Library and are relevant for a
180developer.
181
182.. kernel-doc:: include/linux/rslib.h
183   :internal:
184
185Public Functions Provided
186=========================
187
188This chapter contains the autogenerated documentation of the
189Reed-Solomon functions which are exported.
190
191.. kernel-doc:: lib/reed_solomon/reed_solomon.c
192   :export:
193
194Credits
195=======
196
197The library code for encoding and decoding was written by Phil Karn.
198
199::
200
201            Copyright 2002, Phil Karn, KA9Q
202            May be used under the terms of the GNU General Public License (GPL)
203
204
205The wrapper functions and interfaces are written by Thomas Gleixner.
206
207Many users have provided bugfixes, improvements and helping hands for
208testing. Thanks a lot.
209
210The following people have contributed to this document:
211
212Thomas Gleixner\ tglx@linutronix.de