2 * Reed-Solomon ECC handling for the Marvell Kirkwood SOC
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3 * Copyright (C) 2009 Marvell Semiconductor, Inc.
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5 * Authors: Lennert Buytenhek <buytenh@wantstofly.org>
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6 * Nicolas Pitre <nico@cam.org>
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8 * This file is free software; you can redistribute it and/or modify it
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9 * under the terms of the GNU General Public License as published by the
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10 * Free Software Foundation; either version 2 or (at your option) any
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13 * This file is distributed in the hope that it will be useful, but WITHOUT
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14 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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15 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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19 #ifdef HAVE_CONFIG_H
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23 #include <sys/types.h>
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27 /*****************************************************************************
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28 * Arithmetic in GF(2^10) ("F") modulo x^10 + x^3 + 1.
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30 * For multiplication, a discrete log/exponent table is used, with
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31 * primitive element x (F is a primitive field, so x is primitive).
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33 #define MODPOLY 0x409 /* x^10 + x^3 + 1 in binary */
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36 * Maps an integer a [0..1022] to a polynomial b = gf_exp[a] in
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37 * GF(2^10) mod x^10 + x^3 + 1 such that b = x ^ a. There's two
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38 * identical copies of this array back-to-back so that we can save
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39 * the mod 1023 operation when doing a GF multiplication.
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41 static uint16_t gf_exp[1023 + 1023];
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44 * Maps a polynomial b in GF(2^10) mod x^10 + x^3 + 1 to an index
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45 * a = gf_log[b] in [0..1022] such that b = x ^ a.
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47 static uint16_t gf_log[1024];
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49 static void gf_build_log_exp_table(void)
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57 * Initialise to 1 for i = 0.
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61 for (i = 0; i < 1023; i++) {
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63 gf_exp[i + 1023] = p_i;
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70 if (p_i & (1 << 10))
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76 /*****************************************************************************
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79 * This implements a (1023,1015) Reed-Solomon ECC code over GF(2^10)
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80 * mod x^10 + x^3 + 1, shortened to (520,512). The ECC data consists
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81 * of 8 10-bit symbols, or 10 8-bit bytes.
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83 * Given 512 bytes of data, computes 10 bytes of ECC.
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85 * This is done by converting the 512 bytes to 512 10-bit symbols
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86 * (elements of F), interpreting those symbols as a polynomial in F[X]
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87 * by taking symbol 0 as the coefficient of X^8 and symbol 511 as the
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88 * coefficient of X^519, and calculating the residue of that polynomial
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89 * divided by the generator polynomial, which gives us the 8 ECC symbols
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90 * as the remainder. Finally, we convert the 8 10-bit ECC symbols to 10
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93 * The generator polynomial is hardcoded, as that is faster, but it
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94 * can be computed by taking the primitive element a = x (in F), and
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95 * constructing a polynomial in F[X] with roots a, a^2, a^3, ..., a^8
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96 * by multiplying the minimal polynomials for those roots (which are
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97 * just 'x - a^i' for each i).
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99 * Note: due to unfortunate circumstances, the bootrom in the Kirkwood SOC
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100 * expects the ECC to be computed backward, i.e. from the last byte down
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101 * to the first one.
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103 int nand_calculate_ecc_kw(struct nand_device_s *device, const u8 *data, u8 *ecc)
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105 unsigned int r7, r6, r5, r4, r3, r2, r1, r0;
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107 static int tables_initialized = 0;
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109 if (!tables_initialized) {
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110 gf_build_log_exp_table();
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111 tables_initialized = 1;
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115 * Load bytes 504..511 of the data into r.
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128 * Shift bytes 503..0 (in that order) into r0, followed
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129 * by eight zero bytes, while reducing the polynomial by the
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130 * generator polynomial in every step.
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132 for (i = 503; i >= -8; i--) {
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140 u16 *t = gf_exp + gf_log[r7];
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142 r7 = r6 ^ t[0x21c];
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143 r6 = r5 ^ t[0x181];
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144 r5 = r4 ^ t[0x18e];
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145 r4 = r3 ^ t[0x25f];
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146 r3 = r2 ^ t[0x197];
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147 r2 = r1 ^ t[0x193];
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148 r1 = r0 ^ t[0x237];
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163 ecc[1] = (r0 >> 8) | (r1 << 2);
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164 ecc[2] = (r1 >> 6) | (r2 << 4);
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165 ecc[3] = (r2 >> 4) | (r3 << 6);
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166 ecc[4] = (r3 >> 2);
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168 ecc[6] = (r4 >> 8) | (r5 << 2);
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169 ecc[7] = (r5 >> 6) | (r6 << 4);
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170 ecc[8] = (r6 >> 4) | (r7 << 6);
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171 ecc[9] = (r7 >> 2);
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