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/*
* 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");
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