213 lines
5.9 KiB
ReStructuredText
213 lines
5.9 KiB
ReStructuredText
==========================================
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Reed-Solomon Library Programming Interface
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==========================================
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:Author: Thomas Gleixner
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Introduction
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============
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The generic Reed-Solomon Library provides encoding, decoding and error
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correction functions.
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Reed-Solomon codes are used in communication and storage applications to
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ensure data integrity.
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This documentation is provided for developers who want to utilize the
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functions provided by the library.
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Known Bugs And Assumptions
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==========================
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None.
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Usage
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=====
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This chapter provides examples of how to use the library.
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Initializing
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------------
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The init function init_rs returns a pointer to an rs decoder structure,
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which holds the necessary information for encoding, decoding and error
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correction with the given polynomial. It either uses an existing
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matching decoder or creates a new one. On creation all the lookup tables
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for fast en/decoding are created. The function may take a while, so make
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sure not to call it in critical code paths.
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::
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/* the Reed Solomon control structure */
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static struct rs_control *rs_decoder;
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/* Symbolsize is 10 (bits)
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* Primitive polynomial is x^10+x^3+1
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* first consecutive root is 0
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* primitive element to generate roots = 1
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* generator polynomial degree (number of roots) = 6
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*/
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rs_decoder = init_rs (10, 0x409, 0, 1, 6);
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Encoding
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--------
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The encoder calculates the Reed-Solomon code over the given data length
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and stores the result in the parity buffer. Note that the parity buffer
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must be initialized before calling the encoder.
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The expanded data can be inverted on the fly by providing a non-zero
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inversion mask. The expanded data is XOR'ed with the mask. This is used
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e.g. for FLASH ECC, where the all 0xFF is inverted to an all 0x00. The
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Reed-Solomon code for all 0x00 is all 0x00. The code is inverted before
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storing to FLASH so it is 0xFF too. This prevents that reading from an
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erased FLASH results in ECC errors.
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The databytes are expanded to the given symbol size on the fly. There is
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no support for encoding continuous bitstreams with a symbol size != 8 at
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the moment. If it is necessary it should be not a big deal to implement
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such functionality.
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::
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/* Parity buffer. Size = number of roots */
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uint16_t par[6];
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/* Initialize the parity buffer */
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memset(par, 0, sizeof(par));
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/* Encode 512 byte in data8. Store parity in buffer par */
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encode_rs8 (rs_decoder, data8, 512, par, 0);
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Decoding
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--------
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The decoder calculates the syndrome over the given data length and the
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received parity symbols and corrects errors in the data.
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If a syndrome is available from a hardware decoder then the syndrome
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calculation is skipped.
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The correction of the data buffer can be suppressed by providing a
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correction pattern buffer and an error location buffer to the decoder.
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The decoder stores the calculated error location and the correction
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bitmask in the given buffers. This is useful for hardware decoders which
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use a weird bit ordering scheme.
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The databytes are expanded to the given symbol size on the fly. There is
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no support for decoding continuous bitstreams with a symbolsize != 8 at
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the moment. If it is necessary it should be not a big deal to implement
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such functionality.
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Decoding with syndrome calculation, direct data correction
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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::
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/* Parity buffer. Size = number of roots */
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uint16_t par[6];
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uint8_t data[512];
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int numerr;
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/* Receive data */
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.....
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/* Receive parity */
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.....
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/* Decode 512 byte in data8.*/
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numerr = decode_rs8 (rs_decoder, data8, par, 512, NULL, 0, NULL, 0, NULL);
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Decoding with syndrome given by hardware decoder, direct data correction
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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::
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/* Parity buffer. Size = number of roots */
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uint16_t par[6], syn[6];
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uint8_t data[512];
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int numerr;
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/* Receive data */
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.....
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/* Receive parity */
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.....
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/* Get syndrome from hardware decoder */
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.....
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/* Decode 512 byte in data8.*/
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numerr = decode_rs8 (rs_decoder, data8, par, 512, syn, 0, NULL, 0, NULL);
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Decoding with syndrome given by hardware decoder, no direct data correction.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Note: It's not necessary to give data and received parity to the
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decoder.
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::
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/* Parity buffer. Size = number of roots */
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uint16_t par[6], syn[6], corr[8];
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uint8_t data[512];
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int numerr, errpos[8];
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/* Receive data */
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.....
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/* Receive parity */
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.....
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/* Get syndrome from hardware decoder */
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.....
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/* Decode 512 byte in data8.*/
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numerr = decode_rs8 (rs_decoder, NULL, NULL, 512, syn, 0, errpos, 0, corr);
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for (i = 0; i < numerr; i++) {
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do_error_correction_in_your_buffer(errpos[i], corr[i]);
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}
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Cleanup
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-------
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The function free_rs frees the allocated resources, if the caller is
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the last user of the decoder.
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::
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/* Release resources */
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free_rs(rs_decoder);
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Structures
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==========
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This chapter contains the autogenerated documentation of the structures
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which are used in the Reed-Solomon Library and are relevant for a
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developer.
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.. kernel-doc:: include/linux/rslib.h
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:internal:
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Public Functions Provided
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=========================
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This chapter contains the autogenerated documentation of the
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Reed-Solomon functions which are exported.
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.. kernel-doc:: lib/reed_solomon/reed_solomon.c
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:export:
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Credits
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=======
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The library code for encoding and decoding was written by Phil Karn.
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::
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Copyright 2002, Phil Karn, KA9Q
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May be used under the terms of the GNU General Public License (GPL)
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The wrapper functions and interfaces are written by Thomas Gleixner.
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Many users have provided bugfixes, improvements and helping hands for
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testing. Thanks a lot.
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The following people have contributed to this document:
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Thomas Gleixner\ tglx@linutronix.de
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