1 ==========================================
2 Reed-Solomon Library Programming Interface
3 ==========================================
4
5 :Author: Thomas Gleixner
6
7 Introduction
8 ============
9
10 The generic Reed-Solomon Library provides encoding, decoding and error
11 correction functions.
12
13 Reed-Solomon codes are used in communication and storage applications to
14 ensure data integrity.
15
16 This documentation is provided for developers who want to utilize the
17 functions provided by the library.
18
19 Known Bugs And Assumptions
20 ==========================
21
22 None.
23
24 Usage
25 =====
26
27 This chapter provides examples of how to use the library.
28
29 Initializing
30 ------------
31
32 The init function init_rs returns a pointer to an rs decoder structure,
33 which holds the necessary information for encoding, decoding and error
34 correction with the given polynomial. It either uses an existing
35 matching decoder or creates a new one. On creation all the lookup tables
36 for fast en/decoding are created. The function may take a while, so make
37 sure 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
53 Encoding
54 --------
55
56 The encoder calculates the Reed-Solomon code over the given data length
57 and stores the result in the parity buffer. Note that the parity buffer
58 must be initialized before calling the encoder.
59
60 The expanded data can be inverted on the fly by providing a non-zero
61 inversion mask. The expanded data is XOR'ed with the mask. This is used
62 e.g. for FLASH ECC, where the all 0xFF is inverted to an all 0x00. The
63 Reed-Solomon code for all 0x00 is all 0x00. The code is inverted before
64 storing to FLASH so it is 0xFF too. This prevents that reading from an
65 erased FLASH results in ECC errors.
66
67 The databytes are expanded to the given symbol size on the fly. There is
68 no support for encoding continuous bitstreams with a symbol size != 8 at
69 the moment. If it is necessary it should be not a big deal to implement
70 such 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
82 Decoding
83 --------
84
85 The decoder calculates the syndrome over the given data length and the
86 received parity symbols and corrects errors in the data.
87
88 If a syndrome is available from a hardware decoder then the syndrome
89 calculation is skipped.
90
91 The correction of the data buffer can be suppressed by providing a
92 correction pattern buffer and an error location buffer to the decoder.
93 The decoder stores the calculated error location and the correction
94 bitmask in the given buffers. This is useful for hardware decoders which
95 use a weird bit ordering scheme.
96
97 The databytes are expanded to the given symbol size on the fly. There is
98 no support for decoding continuous bitstreams with a symbolsize != 8 at
99 the moment. If it is necessary it should be not a big deal to implement
100 such functionality.
101
102 Decoding 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
119 Decoding 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
138 Decoding with syndrome given by hardware decoder, no direct data correction.
139 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
140
141 Note: It's not necessary to give data and received parity to the
142 decoder.
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
163 Cleanup
164 -------
165
166 The function free_rs frees the allocated resources, if the caller is
167 the last user of the decoder.
168
169 ::
170
171 /* Release resources */
172 free_rs(rs_decoder);
173
174
175 Structures
176 ==========
177
178 This chapter contains the autogenerated documentation of the structures
179 which are used in the Reed-Solomon Library and are relevant for a
180 developer.
181
182 .. kernel-doc:: include/linux/rslib.h
183 :internal:
184
185 Public Functions Provided
186 =========================
187
188 This chapter contains the autogenerated documentation of the
189 Reed-Solomon functions which are exported.
190
191 .. kernel-doc:: lib/reed_solomon/reed_solomon.c
192 :export:
193
194 Credits
195 =======
196
197 The 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
205 The wrapper functions and interfaces are written by Thomas Gleixner.
206
207 Many users have provided bugfixes, improvements and helping hands for
208 testing. Thanks a lot.
209
210 The following people have contributed to this document:
211
212 Thomas Gleixner\ tglx@linutronix.de
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