/* * Generic binary BCH encoding/decoding library * * This program is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 as published by * the Free Software Foundation. * * This program is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for * more details. * * You should have received a copy of the GNU General Public License along with * this program; if not, write to the Free Software Foundation, Inc., 51 * Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Copyright © 2011 Parrot S.A. * * Author: Ivan Djelic <ivan.djelic@parrot.com> * * Description: * * This library provides runtime configurable encoding/decoding of binary * Bose-Chaudhuri-Hocquenghem (BCH) codes. * * Call init_bch to get a pointer to a newly allocated bch_control structure for * the given m (Galois field order), t (error correction capability) and * (optional) primitive polynomial parameters. * * Call encode_bch to compute and store ecc parity bytes to a given buffer. * Call decode_bch to detect and locate errors in received data. * * On systems supporting hw BCH features, intermediate results may be provided * to decode_bch in order to skip certain steps. See decode_bch() documentation * for details. * * Option CONFIG_BCH_CONST_PARAMS can be used to force fixed values of * parameters m and t; thus allowing extra compiler optimizations and providing * better (up to 2x) encoding performance. Using this option makes sense when * (m,t) are fixed and known in advance, e.g. when using BCH error correction * on a particular NAND flash device. * * Algorithmic details: * * Encoding is performed by processing 32 input bits in parallel, using 4 * remainder lookup tables. * * The final stage of decoding involves the following internal steps: * a. Syndrome computation * b. Error locator polynomial computation using Berlekamp-Massey algorithm * c. Error locator root finding (by far the most expensive step) * * In this implementation, step c is not performed using the usual Chien search. * Instead, an alternative approach described in [1] is used. It consists in * factoring the error locator polynomial using the Berlekamp Trace algorithm * (BTA) down to a certain degree (4), after which ad hoc low-degree polynomial * solving techniques [2] are used. The resulting algorithm, called BTZ, yields * much better performance than Chien search for usual (m,t) values (typically * m >= 13, t < 32, see [1]). * * [1] B. Biswas, V. Herbert. Efficient root finding of polynomials over fields * of characteristic 2, in: Western European Workshop on Research in Cryptology * - WEWoRC 2009, Graz, Austria, LNCS, Springer, July 2009, to appear. * [2] [Zin96] V.A. Zinoviev. On the solution of equations of degree 10 over * finite fields GF(2^q). In Rapport de recherche INRIA no 2829, 1996. */ #include <linux/kernel.h> #include <linux/errno.h> #include <linux/init.h> #include <linux/module.h> #include <linux/slab.h> #include <linux/bitops.h> #include <asm/byteorder.h> #include <linux/bch.h> #if defined(CONFIG_BCH_CONST_PARAMS) #define GF_M(_p) (CONFIG_BCH_CONST_M) #define GF_T(_p) (CONFIG_BCH_CONST_T) #define GF_N(_p) ((1 << (CONFIG_BCH_CONST_M))-1) #else #define GF_M(_p) ((_p)->m) #define GF_T(_p) ((_p)->t) #define GF_N(_p) ((_p)->n) #endif #define BCH_ECC_WORDS(_p) DIV_ROUND_UP(GF_M(_p)*GF_T(_p), 32) #define BCH_ECC_BYTES(_p) DIV_ROUND_UP(GF_M(_p)*GF_T(_p), 8) #ifndef dbg #define dbg(_fmt, args...) do {} while (0) #endif /* * represent a polynomial over GF(2^m) */ struct gf_poly { unsigned int deg; /* polynomial degree */ unsigned int c[0]; /* polynomial terms */ }; /* given its degree, compute a polynomial size in bytes */ #define GF_POLY_SZ(_d) (sizeof(struct gf_poly)+((_d)+1)*sizeof(unsigned int)) /* polynomial of degree 1 */ struct gf_poly_deg1 { struct gf_poly poly; unsigned int c[2]; }; /* * same as encode_bch(), but process input data one byte at a time */ static void encode_bch_unaligned(struct bch_control *bch, const unsigned char *data, unsigned int len, uint32_t *ecc) { int i; const uint32_t *p; const int l = BCH_ECC_WORDS(bch)-1; while (len--) { p = bch->mod8_tab + (l+1)*(((ecc[0] >> 24)^(*data++)) & 0xff); for (i = 0; i < l; i++) ecc[i] = ((ecc[i] << 8)|(ecc[i+1] >> 24))^(*p++); ecc[l] = (ecc[l] << 8)^(*p); } } /* * convert ecc bytes to aligned, zero-padded 32-bit ecc words */ static void load_ecc8(struct bch_control *bch, uint32_t *dst, const uint8_t *src) { uint8_t pad[4] = {0, 0, 0, 0}; unsigned int i, nwords = BCH_ECC_WORDS(bch)-1; for (i = 0; i < nwords; i++, src += 4) dst[i] = (src[0] << 24)|(src[1] << 16)|(src[2] << 8)|src[3]; memcpy(pad, src, BCH_ECC_BYTES(bch)-4*nwords); dst[nwords] = (pad[0] << 24)|(pad[1] << 16)|(pad[2] << 8)|pad[3]; } /* * convert 32-bit ecc words to ecc bytes */ static void store_ecc8(struct bch_control *bch, uint8_t *dst, const uint32_t *src) { uint8_t pad[4]; unsigned int i, nwords = BCH_ECC_WORDS(bch)-1; for (i = 0; i < nwords; i++) { *dst++ = (src[i] >> 24); *dst++ = (src[i] >> 16) & 0xff; *dst++ = (src[i] >> 8) & 0xff; *dst++ = (src[i] >> 0) & 0xff; } pad[0] = (src[nwords] >> 24); pad[1] = (src[nwords] >> 16) & 0xff; pad[2] = (src[nwords] >> 8) & 0xff; pad[3] = (src[nwords] >> 0) & 0xff; memcpy(dst, pad, BCH_ECC_BYTES(bch)-4*nwords); } /** * encode_bch - calculate BCH ecc parity of data * @bch: BCH control structure * @data: data to encode * @len: data length in bytes * @ecc: ecc parity data, must be initialized by caller * * The @ecc parity array is used both as input and output parameter, in order to * allow incremental computations. It should be of the size indicated by member * @ecc_bytes of @bch, and should be initialized to 0 before the first call. * * The exact number of computed ecc parity bits is given by member @ecc_bits of * @bch; it may be less than m*t for large values of t. */ void encode_bch(struct bch_control *bch, const uint8_t *data, unsigned int len, uint8_t *ecc) { const unsigned int l = BCH_ECC_WORDS(bch)-1; unsigned int i, mlen; unsigned long m; uint32_t w, r[l+1]; const uint32_t * const tab0 = bch->mod8_tab; const uint32_t * const tab1 = tab0 + 256*(l+1); const uint32_t * const tab2 = tab1 + 256*(l+1); const uint32_t * const tab3 = tab2 + 256*(l+1); const uint32_t *pdata, *p0, *p1, *p2, *p3; if (ecc) { /* load ecc parity bytes into internal 32-bit buffer */ load_ecc8(bch, bch->ecc_buf, ecc); } else { memset(bch->ecc_buf, 0, sizeof(r)); } /* process first unaligned data bytes */ m = ((unsigned long)data) & 3; if (m) { mlen = (len < (4-m)) ? len : 4-m; encode_bch_unaligned(bch, data, mlen, bch->ecc_buf); data += mlen; len -= mlen; } /* process 32-bit aligned data words */ pdata = (uint32_t *)data; mlen = len/4; data += 4*mlen; len -= 4*mlen; memcpy(r, bch->ecc_buf, sizeof(r)); /* * split each 32-bit word into 4 polynomials of weight 8 as follows: * * 31 ...24 23 ...16 15 ... 8 7 ... 0 * xxxxxxxx yyyyyyyy zzzzzzzz tttttttt * tttttttt mod g = r0 (precomputed) * zzzzzzzz 00000000 mod g = r1 (precomputed) * yyyyyyyy 00000000 00000000 mod g = r2 (precomputed) * xxxxxxxx 00000000 00000000 00000000 mod g = r3 (precomputed) * xxxxxxxx yyyyyyyy zzzzzzzz tttttttt mod g = r0^r1^r2^r3 */ while (mlen--) { /* input data is read in big-endian format */ w = r[0]^cpu_to_be32(*pdata++); p0 = tab0 + (l+1)*((w >> 0) & 0xff); p1 = tab1 + (l+1)*((w >> 8) & 0xff); p2 = tab2 + (l+1)*((w >> 16) & 0xff); p3 = tab3 + (l+1)*((w >> 24) & 0xff); for (i = 0; i < l; i++) r[i] = r[i+1]^p0[i]^p1[i]^p2[i]^p3[i]; r[l] = p0[l]^p1[l]^p2[l]^p3[l]; } memcpy(bch->ecc_buf, r, sizeof(r)); /* process last unaligned bytes */ if (len) encode_bch_unaligned(bch, data, len, bch->ecc_buf); /* store ecc parity bytes into original parity buffer */ if (ecc) store_ecc8(bch, ecc, bch->ecc_buf); } EXPORT_SYMBOL_GPL(encode_bch); static inline int modulo(struct bch_control *bch, unsigned int v) { const unsigned int n = GF_N(bch); while (v >= n) { v -= n; v = (v & n) + (v >> GF_M(bch)); } return v; } /* * shorter and faster modulo function, only works when v < 2N. */ static inline int mod_s(struct bch_control *bch, unsigned int v) { const unsigned int n = GF_N(bch); return (v < n) ? v : v-n; } static inline int deg(unsigned int poly) { /* polynomial degree is the most-significant bit index */ return fls(poly)-1; } static inline int parity(unsigned int x) { /* * public domain code snippet, lifted from * http://www-graphics.stanford.edu/~seander/bithacks.html */ x ^= x >> 1; x ^= x >> 2; x = (x & 0x11111111U) * 0x11111111U; return (x >> 28) & 1; } /* Galois field basic operations: multiply, divide, inverse, etc. */ static inline unsigned int gf_mul(struct bch_control *bch, unsigned int a, unsigned int b) { return (a && b) ? bch->a_pow_tab[mod_s(bch, bch->a_log_tab[a]+ bch->a_log_tab[b])] : 0; } static inline unsigned int gf_sqr(struct bch_control *bch, unsigned int a) { return a ? bch->a_pow_tab[mod_s(bch, 2*bch->a_log_tab[a])] : 0; } static inline unsigned int gf_div(struct bch_control *bch, unsigned int a, unsigned int b) { return a ? bch->a_pow_tab[mod_s(bch, bch->a_log_tab[a]+ GF_N(bch)-bch->a_log_tab[b])] : 0; } static inline unsigned int gf_inv(struct bch_control *bch, unsigned int a) { return bch->a_pow_tab[GF_N(bch)-bch->a_log_tab[a]]; } static inline unsigned int a_pow(struct bch_control *bch, int i) { return bch->a_pow_tab[modulo(bch, i)]; } static inline int a_log(struct bch_control *bch, unsigned int x) { return bch->a_log_tab[x]; } static inline int a_ilog(struct bch_control *bch, unsigned int x) { return mod_s(bch, GF_N(bch)-bch->a_log_tab[x]); } /* * compute 2t syndromes of ecc polynomial, i.e. ecc(a^j) for j=1..2t */ static void compute_syndromes(struct bch_control *bch, uint32_t *ecc, unsigned int *syn) { int i, j, s; unsigned int m; uint32_t poly; const int t = GF_T(bch); s = bch->ecc_bits; /* make sure extra bits in last ecc word are cleared */ m = ((unsigned int)s) & 31; if (m) ecc[s/32] &= ~((1u << (32-m))-1); memset(syn, 0, 2*t*sizeof(*syn)); /* compute v(a^j) for j=1 .. 2t-1 */ do { poly = *ecc++; s -= 32; while (poly) { i = deg(poly); for (j = 0; j < 2*t; j += 2) syn[j] ^= a_pow(bch, (j+1)*(i+s)); poly ^= (1 << i); } } while (s > 0); /* v(a^(2j)) = v(a^j)^2 */ for (j = 0; j < t; j++) syn[2*j+1] = gf_sqr(bch, syn[j]); } static void gf_poly_copy(struct gf_poly *dst, struct gf_poly *src) { memcpy(dst, src, GF_POLY_SZ(src->deg)); } static int compute_error_locator_polynomial(struct bch_control *bch, const unsigned int *syn) { const unsigned int t = GF_T(bch); const unsigned int n = GF_N(bch); unsigned int i, j, tmp, l, pd = 1, d = syn[0]; struct gf_poly *elp = bch->elp; struct gf_poly *pelp = bch->poly_2t[0]; struct gf_poly *elp_copy = bch->poly_2t[1]; int k, pp = -1; memset(pelp, 0, GF_POLY_SZ(2*t)); memset(elp, 0, GF_POLY_SZ(2*t)); pelp->deg = 0; pelp->c[0] = 1; elp->deg = 0; elp->c[0] = 1; /* use simplified binary Berlekamp-Massey algorithm */ for (i = 0; (i < t) && (elp->deg <= t); i++) { if (d) { k = 2*i-pp; gf_poly_copy(elp_copy, elp); /* e[i+1](X) = e[i](X)+di*dp^-1*X^2(i-p)*e[p](X) */ tmp = a_log(bch, d)+n-a_log(bch, pd); for (j = 0; j <= pelp->deg; j++) { if (pelp->c[j]) { l = a_log(bch, pelp->c[j]); elp->c[j+k] ^= a_pow(bch, tmp+l); } } /* compute l[i+1] = max(l[i]->c[l[p]+2*(i-p]) */ tmp = pelp->deg+k; if (tmp > elp->deg) { elp->deg = tmp; gf_poly_copy(pelp, elp_copy); pd = d; pp = 2*i; } } /* di+1 = S(2i+3)+elp[i+1].1*S(2i+2)+...+elp[i+1].lS(2i+3-l) */ if (i < t-1) { d = syn[2*i+2]; for (j = 1; j <= elp->deg; j++) d ^= gf_mul(bch, elp->c[j], syn[2*i+2-j]); } } dbg("elp=%s\n", gf_poly_str(elp)); return (elp->deg > t) ? -1 : (int)elp->deg; } /* * solve a m x m linear system in GF(2) with an expected number of solutions, * and return the number of found solutions */ static int solve_linear_system(struct bch_control *bch, unsigned int *rows, unsigned int *sol, int nsol) { const int m = GF_M(bch); unsigned int tmp, mask; int rem, c, r, p, k, param[m]; k = 0; mask = 1 << m; /* Gaussian elimination */ for (c = 0; c < m; c++) { rem = 0; p = c-k; /* find suitable row for elimination */ for (r = p; r < m; r++) { if (rows[r] & mask) { if (r != p) { tmp = rows[r]; rows[r] = rows[p]; rows[p] = tmp; } rem = r+1; break; } } if (rem) { /* perform elimination on remaining rows */ tmp = rows[p]; for (r = rem; r < m; r++) { if (rows[r] & mask) rows[r] ^= tmp; } } else { /* elimination not needed, store defective row index */ param[k++] = c; } mask >>= 1; } /* rewrite system, inserting fake parameter rows */ if (k > 0) { p = k; for (r = m-1; r >= 0; r--) { if ((r > m-1-k) && rows[r]) /* system has no solution */ return 0; rows[r] = (p && (r == param[p-1])) ? p--, 1u << (m-r) : rows[r-p]; } } if (nsol != (1 << k)) /* unexpected number of solutions */ return 0; for (p = 0; p < nsol; p++) { /* set parameters for p-th solution */ for (c = 0; c < k; c++) rows[param[c]] = (rows[param[c]] & ~1)|((p >> c) & 1); /* compute unique solution */ tmp = 0; for (r = m-1; r >= 0; r--) { mask = rows[r] & (tmp|1); tmp |= parity(mask) << (m-r); } sol[p] = tmp >> 1; } return nsol; } /* * this function builds and solves a linear system for finding roots of a degree * 4 affine monic polynomial X^4+aX^2+bX+c over GF(2^m). */ static int find_affine4_roots(struct bch_control *bch, unsigned int a, unsigned int b, unsigned int c, unsigned int *roots) { int i, j, k; const int m = GF_M(bch); unsigned int mask = 0xff, t, rows[16] = {0,}; j = a_log(bch, b); k = a_log(bch, a); rows[0] = c; /* buid linear system to solve X^4+aX^2+bX+c = 0 */ for (i = 0; i < m; i++) { rows[i+1] = bch->a_pow_tab[4*i]^ (a ? bch->a_pow_tab[mod_s(bch, k)] : 0)^ (b ? bch->a_pow_tab[mod_s(bch, j)] : 0); j++; k += 2; } /* * transpose 16x16 matrix before passing it to linear solver * warning: this code assumes m < 16 */ for (j = 8; j != 0; j >>= 1, mask ^= (mask << j)) { for (k = 0; k < 16; k = (k+j+1) & ~j) { t = ((rows[k] >> j)^rows[k+j]) & mask; rows[k] ^= (t << j); rows[k+j] ^= t; } } return solve_linear_system(bch, rows, roots, 4); } /* * compute root r of a degree 1 polynomial over GF(2^m) (returned as log(1/r)) */ static int find_poly_deg1_roots(struct bch_control *bch, struct gf_poly *poly, unsigned int *roots) { int n = 0; if (poly->c[0]) /* poly[X] = bX+c with c!=0, root=c/b */ roots[n++] = mod_s(bch, GF_N(bch)-bch->a_log_tab[poly->c[0]]+ bch->a_log_tab[poly->c[1]]); return n; } /* * compute roots of a degree 2 polynomial over GF(2^m) */ static int find_poly_deg2_roots(struct bch_control *bch, struct gf_poly *poly, unsigned int *roots) { int n = 0, i, l0, l1, l2; unsigned int u, v, r; if (poly->c[0] && poly->c[1]) { l0 = bch->a_log_tab[poly->c[0]]; l1 = bch->a_log_tab[poly->c[1]]; l2 = bch->a_log_tab[poly->c[2]]; /* using z=a/bX, transform aX^2+bX+c into z^2+z+u (u=ac/b^2) */ u = a_pow(bch, l0+l2+2*(GF_N(bch)-l1)); /* * let u = sum(li.a^i) i=0..m-1; then compute r = sum(li.xi): * r^2+r = sum(li.(xi^2+xi)) = sum(li.(a^i+Tr(a^i).a^k)) = * u + sum(li.Tr(a^i).a^k) = u+a^k.Tr(sum(li.a^i)) = u+a^k.Tr(u) * i.e. r and r+1 are roots iff Tr(u)=0 */ r = 0; v = u; while (v) { i = deg(v); r ^= bch->xi_tab[i]; v ^= (1 << i); } /* verify root */ if ((gf_sqr(bch, r)^r) == u) { /* reverse z=a/bX transformation and compute log(1/r) */ roots[n++] = modulo(bch, 2*GF_N(bch)-l1- bch->a_log_tab[r]+l2); roots[n++] = modulo(bch, 2*GF_N(bch)-l1- bch->a_log_tab[r^1]+l2); } } return n; } /* * compute roots of a degree 3 polynomial over GF(2^m) */ static int find_poly_deg3_roots(struct bch_control *bch, struct gf_poly *poly, unsigned int *roots) { int i, n = 0; unsigned int a, b, c, a2, b2, c2, e3, tmp[4]; if (poly->c[0]) { /* transform polynomial into monic X^3 + a2X^2 + b2X + c2 */ e3 = poly->c[3]; c2 = gf_div(bch, poly->c[0], e3); b2 = gf_div(bch, poly->c[1], e3); a2 = gf_div(bch, poly->c[2], e3); /* (X+a2)(X^3+a2X^2+b2X+c2) = X^4+aX^2+bX+c (affine) */ c = gf_mul(bch, a2, c2); /* c = a2c2 */ b = gf_mul(bch, a2, b2)^c2; /* b = a2b2 + c2 */ a = gf_sqr(bch, a2)^b2; /* a = a2^2 + b2 */ /* find the 4 roots of this affine polynomial */ if (find_affine4_roots(bch, a, b, c, tmp) == 4) { /* remove a2 from final list of roots */ for (i = 0; i < 4; i++) { if (tmp[i] != a2) roots[n++] = a_ilog(bch, tmp[i]); } } } return n; } /* * compute roots of a degree 4 polynomial over GF(2^m) */ static int find_poly_deg4_roots(struct bch_control *bch, struct gf_poly *poly, unsigned int *roots) { int i, l, n = 0; unsigned int a, b, c, d, e = 0, f, a2, b2, c2, e4; if (poly->c[0] == 0) return 0; /* transform polynomial into monic X^4 + aX^3 + bX^2 + cX + d */ e4 = poly->c[4]; d = gf_div(bch, poly->c[0], e4); c = gf_div(bch, poly->c[1], e4); b = gf_div(bch, poly->c[2], e4); a = gf_div(bch, poly->c[3], e4); /* use Y=1/X transformation to get an affine polynomial */ if (a) { /* first, eliminate cX by using z=X+e with ae^2+c=0 */ if (c) { /* compute e such that e^2 = c/a */ f = gf_div(bch, c, a); l = a_log(bch, f); l += (l & 1) ? GF_N(bch) : 0; e = a_pow(bch, l/2); /* * use transformation z=X+e: * z^4+e^4 + a(z^3+ez^2+e^2z+e^3) + b(z^2+e^2) +cz+ce+d * z^4 + az^3 + (ae+b)z^2 + (ae^2+c)z+e^4+be^2+ae^3+ce+d * z^4 + az^3 + (ae+b)z^2 + e^4+be^2+d * z^4 + az^3 + b'z^2 + d' */ d = a_pow(bch, 2*l)^gf_mul(bch, b, f)^d; b = gf_mul(bch, a, e)^b; } /* now, use Y=1/X to get Y^4 + b/dY^2 + a/dY + 1/d */ if (d == 0) /* assume all roots have multiplicity 1 */ return 0; c2 = gf_inv(bch, d); b2 = gf_div(bch, a, d); a2 = gf_div(bch, b, d); } else { /* polynomial is already affine */ c2 = d; b2 = c; a2 = b; } /* find the 4 roots of this affine polynomial */ if (find_affine4_roots(bch, a2, b2, c2, roots) == 4) { for (i = 0; i < 4; i++) { /* post-process roots (reverse transformations) */ f = a ? gf_inv(bch, roots[i]) : roots[i]; roots[i] = a_ilog(bch, f^e); } n = 4; } return n; } /* * build monic, log-based representation of a polynomial */ static void gf_poly_logrep(struct bch_control *bch, const struct gf_poly *a, int *rep) { int i, d = a->deg, l = GF_N(bch)-a_log(bch, a->c[a->deg]); /* represent 0 values with -1; warning, rep[d] is not set to 1 */ for (i = 0; i < d; i++) rep[i] = a->c[i] ? mod_s(bch, a_log(bch, a->c[i])+l) : -1; } /* * compute polynomial Euclidean division remainder in GF(2^m)[X] */ static void gf_poly_mod(struct bch_control *bch, struct gf_poly *a, const struct gf_poly *b, int *rep) { int la, p, m; unsigned int i, j, *c = a->c; const unsigned int d = b->deg; if (a->deg < d) return; /* reuse or compute log representation of denominator */ if (!rep) { rep = bch->cache; gf_poly_logrep(bch, b, rep); } for (j = a->deg; j >= d; j--) { if (c[j]) { la = a_log(bch, c[j]); p = j-d; for (i = 0; i < d; i++, p++) { m = rep[i]; if (m >= 0) c[p] ^= bch->a_pow_tab[mod_s(bch, m+la)]; } } } a->deg = d-1; while (!c[a->deg] && a->deg) a->deg--; } /* * compute polynomial Euclidean division quotient in GF(2^m)[X] */ static void gf_poly_div(struct bch_control *bch, struct gf_poly *a, const struct gf_poly *b, struct gf_poly *q) { if (a->deg >= b->deg) { q->deg = a->deg-b->deg; /* compute a mod b (modifies a) */ gf_poly_mod(bch, a, b, NULL); /* quotient is stored in upper part of polynomial a */ memcpy(q->c, &a->c[b->deg], (1+q->deg)*sizeof(unsigned int)); } else { q->deg = 0; q->c[0] = 0; } } /* * compute polynomial GCD (Greatest Common Divisor) in GF(2^m)[X] */ static struct gf_poly *gf_poly_gcd(struct bch_control *bch, struct gf_poly *a, struct gf_poly *b) { struct gf_poly *tmp; dbg("gcd(%s,%s)=", gf_poly_str(a), gf_poly_str(b)); if (a->deg < b->deg) { tmp = b; b = a; a = tmp; } while (b->deg > 0) { gf_poly_mod(bch, a, b, NULL); tmp = b; b = a; a = tmp; } dbg("%s\n", gf_poly_str(a)); return a; } /* * Given a polynomial f and an integer k, compute Tr(a^kX) mod f * This is used in Berlekamp Trace algorithm for splitting polynomials */ static void compute_trace_bk_mod(struct bch_control *bch, int k, const struct gf_poly *f, struct gf_poly *z, struct gf_poly *out) { const int m = GF_M(bch); int i, j; /* z contains z^2j mod f */ z->deg = 1; z->c[0] = 0; z->c[1] = bch->a_pow_tab[k]; out->deg = 0; memset(out, 0, GF_POLY_SZ(f->deg)); /* compute f log representation only once */ gf_poly_logrep(bch, f, bch->cache); for (i = 0; i < m; i++) { /* add a^(k*2^i)(z^(2^i) mod f) and compute (z^(2^i) mod f)^2 */ for (j = z->deg; j >= 0; j--) { out->c[j] ^= z->c[j]; z->c[2*j] = gf_sqr(bch, z->c[j]); z->c[2*j+1] = 0; } if (z->deg > out->deg) out->deg = z->deg; if (i < m-1) { z->deg *= 2; /* z^(2(i+1)) mod f = (z^(2^i) mod f)^2 mod f */ gf_poly_mod(bch, z, f, bch->cache); } } while (!out->c[out->deg] && out->deg) out->deg--; dbg("Tr(a^%d.X) mod f = %s\n", k, gf_poly_str(out)); } /* * factor a polynomial using Berlekamp Trace algorithm (BTA) */ static void factor_polynomial(struct bch_control *bch, int k, struct gf_poly *f, struct gf_poly **g, struct gf_poly **h) { struct gf_poly *f2 = bch->poly_2t[0]; struct gf_poly *q = bch->poly_2t[1]; struct gf_poly *tk = bch->poly_2t[2]; struct gf_poly *z = bch->poly_2t[3]; struct gf_poly *gcd; dbg("factoring %s...\n", gf_poly_str(f)); *g = f; *h = NULL; /* tk = Tr(a^k.X) mod f */ compute_trace_bk_mod(bch, k, f, z, tk); if (tk->deg > 0) { /* compute g = gcd(f, tk) (destructive operation) */ gf_poly_copy(f2, f); gcd = gf_poly_gcd(bch, f2, tk); if (gcd->deg < f->deg) { /* compute h=f/gcd(f,tk); this will modify f and q */ gf_poly_div(bch, f, gcd, q); /* store g and h in-place (clobbering f) */ *h = &((struct gf_poly_deg1 *)f)[gcd->deg].poly; gf_poly_copy(*g, gcd); gf_poly_copy(*h, q); } } } /* * find roots of a polynomial, using BTZ algorithm; see the beginning of this * file for details */ static int find_poly_roots(struct bch_control *bch, unsigned int k, struct gf_poly *poly, unsigned int *roots) { int cnt; struct gf_poly *f1, *f2; switch (poly->deg) { /* handle low degree polynomials with ad hoc techniques */ case 1: cnt = find_poly_deg1_roots(bch, poly, roots); break; case 2: cnt = find_poly_deg2_roots(bch, poly, roots); break; case 3: cnt = find_poly_deg3_roots(bch, poly, roots); break; case 4: cnt = find_poly_deg4_roots(bch, poly, roots); break; default: /* factor polynomial using Berlekamp Trace Algorithm (BTA) */ cnt = 0; if (poly->deg && (k <= GF_M(bch))) { factor_polynomial(bch, k, poly, &f1, &f2); if (f1) cnt += find_poly_roots(bch, k+1, f1, roots); if (f2) cnt += find_poly_roots(bch, k+1, f2, roots+cnt); } break; } return cnt; } #if defined(USE_CHIEN_SEARCH) /* * exhaustive root search (Chien) implementation - not used, included only for * reference/comparison tests */ static int chien_search(struct bch_control *bch, unsigned int len, struct gf_poly *p, unsigned int *roots) { int m; unsigned int i, j, syn, syn0, count = 0; const unsigned int k = 8*len+bch->ecc_bits; /* use a log-based representation of polynomial */ gf_poly_logrep(bch, p, bch->cache); bch->cache[p->deg] = 0; syn0 = gf_div(bch, p->c[0], p->c[p->deg]); for (i = GF_N(bch)-k+1; i <= GF_N(bch); i++) { /* compute elp(a^i) */ for (j = 1, syn = syn0; j <= p->deg; j++) { m = bch->cache[j]; if (m >= 0) syn ^= a_pow(bch, m+j*i); } if (syn == 0) { roots[count++] = GF_N(bch)-i; if (count == p->deg) break; } } return (count == p->deg) ? count : 0; } #define find_poly_roots(_p, _k, _elp, _loc) chien_search(_p, len, _elp, _loc) #endif /* USE_CHIEN_SEARCH */ /** * decode_bch - decode received codeword and find bit error locations * @bch: BCH control structure * @data: received data, ignored if @calc_ecc is provided * @len: data length in bytes, must always be provided * @recv_ecc: received ecc, if NULL then assume it was XORed in @calc_ecc * @calc_ecc: calculated ecc, if NULL then calc_ecc is computed from @data * @syn: hw computed syndrome data (if NULL, syndrome is calculated) * @errloc: output array of error locations * * Returns: * The number of errors found, or -EBADMSG if decoding failed, or -EINVAL if * invalid parameters were provided * * Depending on the available hw BCH support and the need to compute @calc_ecc * separately (using encode_bch()), this function should be called with one of * the following parameter configurations - * * by providing @data and @recv_ecc only: * decode_bch(@bch, @data, @len, @recv_ecc, NULL, NULL, @errloc) * * by providing @recv_ecc and @calc_ecc: * decode_bch(@bch, NULL, @len, @recv_ecc, @calc_ecc, NULL, @errloc) * * by providing ecc = recv_ecc XOR calc_ecc: * decode_bch(@bch, NULL, @len, NULL, ecc, NULL, @errloc) * * by providing syndrome results @syn: * decode_bch(@bch, NULL, @len, NULL, NULL, @syn, @errloc) * * Once decode_bch() has successfully returned with a positive value, error * locations returned in array @errloc should be interpreted as follows - * * if (errloc[n] >= 8*len), then n-th error is located in ecc (no need for * data correction) * * if (errloc[n] < 8*len), then n-th error is located in data and can be * corrected with statement data[errloc[n]/8] ^= 1 << (errloc[n] % 8); * * Note that this function does not perform any data correction by itself, it * merely indicates error locations. */ int decode_bch(struct bch_control *bch, const uint8_t *data, unsigned int len, const uint8_t *recv_ecc, const uint8_t *calc_ecc, const unsigned int *syn, unsigned int *errloc) { const unsigned int ecc_words = BCH_ECC_WORDS(bch); unsigned int nbits; int i, err, nroots; uint32_t sum; /* sanity check: make sure data length can be handled */ if (8*len > (bch->n-bch->ecc_bits)) return -EINVAL; /* if caller does not provide syndromes, compute them */ if (!syn) { if (!calc_ecc) { /* compute received data ecc into an internal buffer */ if (!data || !recv_ecc) return -EINVAL; encode_bch(bch, data, len, NULL); } else { /* load provided calculated ecc */ load_ecc8(bch, bch->ecc_buf, calc_ecc); } /* load received ecc or assume it was XORed in calc_ecc */ if (recv_ecc) { load_ecc8(bch, bch->ecc_buf2, recv_ecc); /* XOR received and calculated ecc */ for (i = 0, sum = 0; i < (int)ecc_words; i++) { bch->ecc_buf[i] ^= bch->ecc_buf2[i]; sum |= bch->ecc_buf[i]; } if (!sum) /* no error found */ return 0; } compute_syndromes(bch, bch->ecc_buf, bch->syn); syn = bch->syn; } err = compute_error_locator_polynomial(bch, syn); if (err > 0) { nroots = find_poly_roots(bch, 1, bch->elp, errloc); if (err != nroots) err = -1; } if (err > 0) { /* post-process raw error locations for easier correction */ nbits = (len*8)+bch->ecc_bits; for (i = 0; i < err; i++) { if (errloc[i] >= nbits) { err = -1; break; } errloc[i] = nbits-1-errloc[i]; errloc[i] = (errloc[i] & ~7)|(7-(errloc[i] & 7)); } } return (err >= 0) ? err : -EBADMSG; } EXPORT_SYMBOL_GPL(decode_bch); /* * generate Galois field lookup tables */ static int build_gf_tables(struct bch_control *bch, unsigned int poly) { unsigned int i, x = 1; const unsigned int k = 1 << deg(poly); /* primitive polynomial must be of degree m */ if (k != (1u << GF_M(bch))) return -1; for (i = 0; i < GF_N(bch); i++) { bch->a_pow_tab[i] = x; bch->a_log_tab[x] = i; if (i && (x == 1)) /* polynomial is not primitive (a^i=1 with 0<i<2^m-1) */ return -1; x <<= 1; if (x & k) x ^= poly; } bch->a_pow_tab[GF_N(bch)] = 1; bch->a_log_tab[0] = 0; return 0; } /* * compute generator polynomial remainder tables for fast encoding */ static void build_mod8_tables(struct bch_control *bch, const uint32_t *g) { int i, j, b, d; uint32_t data, hi, lo, *tab; const int l = BCH_ECC_WORDS(bch); const int plen = DIV_ROUND_UP(bch->ecc_bits+1, 32); const int ecclen = DIV_ROUND_UP(bch->ecc_bits, 32); memset(bch->mod8_tab, 0, 4*256*l*sizeof(*bch->mod8_tab)); for (i = 0; i < 256; i++) { /* p(X)=i is a small polynomial of weight <= 8 */ for (b = 0; b < 4; b++) { /* we want to compute (p(X).X^(8*b+deg(g))) mod g(X) */ tab = bch->mod8_tab + (b*256+i)*l; data = i << (8*b); while (data) { d = deg(data); /* subtract X^d.g(X) from p(X).X^(8*b+deg(g)) */ data ^= g[0] >> (31-d); for (j = 0; j < ecclen; j++) { hi = (d < 31) ? g[j] << (d+1) : 0; lo = (j+1 < plen) ? g[j+1] >> (31-d) : 0; tab[j] ^= hi|lo; } } } } } /* * build a base for factoring degree 2 polynomials */ static int build_deg2_base(struct bch_control *bch) { const int m = GF_M(bch); int i, j, r; unsigned int sum, x, y, remaining, ak = 0, xi[m]; /* find k s.t. Tr(a^k) = 1 and 0 <= k < m */ for (i = 0; i < m; i++) { for (j = 0, sum = 0; j < m; j++) sum ^= a_pow(bch, i*(1 << j)); if (sum) { ak = bch->a_pow_tab[i]; break; } } /* find xi, i=0..m-1 such that xi^2+xi = a^i+Tr(a^i).a^k */ remaining = m; memset(xi, 0, sizeof(xi)); for (x = 0; (x <= GF_N(bch)) && remaining; x++) { y = gf_sqr(bch, x)^x; for (i = 0; i < 2; i++) { r = a_log(bch, y); if (y && (r < m) && !xi[r]) { bch->xi_tab[r] = x; xi[r] = 1; remaining--; dbg("x%d = %x\n", r, x); break; } y ^= ak; } } /* should not happen but check anyway */ return remaining ? -1 : 0; } static void *bch_alloc(size_t size, int *err) { void *ptr; ptr = kmalloc(size, GFP_KERNEL); if (ptr == NULL) *err = 1; return ptr; } /* * compute generator polynomial for given (m,t) parameters. */ static uint32_t *compute_generator_polynomial(struct bch_control *bch) { const unsigned int m = GF_M(bch); const unsigned int t = GF_T(bch); int n, err = 0; unsigned int i, j, nbits, r, word, *roots; struct gf_poly *g; uint32_t *genpoly; g = bch_alloc(GF_POLY_SZ(m*t), &err); roots = bch_alloc((bch->n+1)*sizeof(*roots), &err); genpoly = bch_alloc(DIV_ROUND_UP(m*t+1, 32)*sizeof(*genpoly), &err); if (err) { kfree(genpoly); genpoly = NULL; goto finish; } /* enumerate all roots of g(X) */ memset(roots , 0, (bch->n+1)*sizeof(*roots)); for (i = 0; i < t; i++) { for (j = 0, r = 2*i+1; j < m; j++) { roots[r] = 1; r = mod_s(bch, 2*r); } } /* build generator polynomial g(X) */ g->deg = 0; g->c[0] = 1; for (i = 0; i < GF_N(bch); i++) { if (roots[i]) { /* multiply g(X) by (X+root) */ r = bch->a_pow_tab[i]; g->c[g->deg+1] = 1; for (j = g->deg; j > 0; j--) g->c[j] = gf_mul(bch, g->c[j], r)^g->c[j-1]; g->c[0] = gf_mul(bch, g->c[0], r); g->deg++; } } /* store left-justified binary representation of g(X) */ n = g->deg+1; i = 0; while (n > 0) { nbits = (n > 32) ? 32 : n; for (j = 0, word = 0; j < nbits; j++) { if (g->c[n-1-j]) word |= 1u << (31-j); } genpoly[i++] = word; n -= nbits; } bch->ecc_bits = g->deg; finish: kfree(g); kfree(roots); return genpoly; } /** * init_bch - initialize a BCH encoder/decoder * @m: Galois field order, should be in the range 5-15 * @t: maximum error correction capability, in bits * @prim_poly: user-provided primitive polynomial (or 0 to use default) * * Returns: * a newly allocated BCH control structure if successful, NULL otherwise * * This initialization can take some time, as lookup tables are built for fast * encoding/decoding; make sure not to call this function from a time critical * path. Usually, init_bch() should be called on module/driver init and * free_bch() should be called to release memory on exit. * * You may provide your own primitive polynomial of degree @m in argument * @prim_poly, or let init_bch() use its default polynomial. * * Once init_bch() has successfully returned a pointer to a newly allocated * BCH control structure, ecc length in bytes is given by member @ecc_bytes of * the structure. */ struct bch_control *init_bch(int m, int t, unsigned int prim_poly) { int err = 0; unsigned int i, words; uint32_t *genpoly; struct bch_control *bch = NULL; const int min_m = 5; const int max_m = 15; /* default primitive polynomials */ static const unsigned int prim_poly_tab[] = { 0x25, 0x43, 0x83, 0x11d, 0x211, 0x409, 0x805, 0x1053, 0x201b, 0x402b, 0x8003, }; #if defined(CONFIG_BCH_CONST_PARAMS) if ((m != (CONFIG_BCH_CONST_M)) || (t != (CONFIG_BCH_CONST_T))) { printk(KERN_ERR "bch encoder/decoder was configured to support " "parameters m=%d, t=%d only!\n", CONFIG_BCH_CONST_M, CONFIG_BCH_CONST_T); goto fail; } #endif if ((m < min_m) || (m > max_m)) /* * values of m greater than 15 are not currently supported; * supporting m > 15 would require changing table base type * (uint16_t) and a small patch in matrix transposition */ goto fail; /* sanity checks */ if ((t < 1) || (m*t >= ((1 << m)-1))) /* invalid t value */ goto fail; /* select a primitive polynomial for generating GF(2^m) */ if (prim_poly == 0) prim_poly = prim_poly_tab[m-min_m]; bch = kzalloc(sizeof(*bch), GFP_KERNEL); if (bch == NULL) goto fail; bch->m = m; bch->t = t; bch->n = (1 << m)-1; words = DIV_ROUND_UP(m*t, 32); bch->ecc_bytes = DIV_ROUND_UP(m*t, 8); bch->a_pow_tab = bch_alloc((1+bch->n)*sizeof(*bch->a_pow_tab), &err); bch->a_log_tab = bch_alloc((1+bch->n)*sizeof(*bch->a_log_tab), &err); bch->mod8_tab = bch_alloc(words*1024*sizeof(*bch->mod8_tab), &err); bch->ecc_buf = bch_alloc(words*sizeof(*bch->ecc_buf), &err); bch->ecc_buf2 = bch_alloc(words*sizeof(*bch->ecc_buf2), &err); bch->xi_tab = bch_alloc(m*sizeof(*bch->xi_tab), &err); bch->syn = bch_alloc(2*t*sizeof(*bch->syn), &err); bch->cache = bch_alloc(2*t*sizeof(*bch->cache), &err); bch->elp = bch_alloc((t+1)*sizeof(struct gf_poly_deg1), &err); for (i = 0; i < ARRAY_SIZE(bch->poly_2t); i++) bch->poly_2t[i] = bch_alloc(GF_POLY_SZ(2*t), &err); if (err) goto fail; err = build_gf_tables(bch, prim_poly); if (err) goto fail; /* use generator polynomial for computing encoding tables */ genpoly = compute_generator_polynomial(bch); if (genpoly == NULL) goto fail; build_mod8_tables(bch, genpoly); kfree(genpoly); err = build_deg2_base(bch); if (err) goto fail; return bch; fail: free_bch(bch); return NULL; } EXPORT_SYMBOL_GPL(init_bch); /** * free_bch - free the BCH control structure * @bch: BCH control structure to release */ void free_bch(struct bch_control *bch) { unsigned int i; if (bch) { kfree(bch->a_pow_tab); kfree(bch->a_log_tab); kfree(bch->mod8_tab); kfree(bch->ecc_buf); kfree(bch->ecc_buf2); kfree(bch->xi_tab); kfree(bch->syn); kfree(bch->cache); kfree(bch->elp); for (i = 0; i < ARRAY_SIZE(bch->poly_2t); i++) kfree(bch->poly_2t[i]); kfree(bch); } } EXPORT_SYMBOL_GPL(free_bch); MODULE_LICENSE("GPL"); MODULE_AUTHOR("Ivan Djelic <ivan.djelic@parrot.com>"); MODULE_DESCRIPTION("Binary BCH encoder/decoder");