diff options
Diffstat (limited to 'lib/reed_solomon/decode_rs.c')
| -rw-r--r-- | lib/reed_solomon/decode_rs.c | 115 |
1 files changed, 85 insertions, 30 deletions
diff --git a/lib/reed_solomon/decode_rs.c b/lib/reed_solomon/decode_rs.c index 1db74eb098d0..805de84ae83d 100644 --- a/lib/reed_solomon/decode_rs.c +++ b/lib/reed_solomon/decode_rs.c @@ -22,6 +22,7 @@ uint16_t *index_of = rs->index_of; uint16_t u, q, tmp, num1, num2, den, discr_r, syn_error; int count = 0; + int num_corrected; uint16_t msk = (uint16_t) rs->nn; /* @@ -39,11 +40,21 @@ /* Check length parameter for validity */ pad = nn - nroots - len; - BUG_ON(pad < 0 || pad >= nn); + BUG_ON(pad < 0 || pad >= nn - nroots); /* Does the caller provide the syndrome ? */ - if (s != NULL) - goto decode; + if (s != NULL) { + for (i = 0; i < nroots; i++) { + /* The syndrome is in index form, + * so nn represents zero + */ + if (s[i] != nn) + goto decode; + } + + /* syndrome is zero, no errors to correct */ + return 0; + } /* form the syndromes; i.e., evaluate data(x) at roots of * g(x) */ @@ -88,8 +99,7 @@ /* if syndrome is zero, data[] is a codeword and there are no * errors to correct. So return data[] unmodified */ - count = 0; - goto finish; + return 0; } decode: @@ -99,9 +109,9 @@ if (no_eras > 0) { /* Init lambda to be the erasure locator polynomial */ lambda[1] = alpha_to[rs_modnn(rs, - prim * (nn - 1 - eras_pos[0]))]; + prim * (nn - 1 - (eras_pos[0] + pad)))]; for (i = 1; i < no_eras; i++) { - u = rs_modnn(rs, prim * (nn - 1 - eras_pos[i])); + u = rs_modnn(rs, prim * (nn - 1 - (eras_pos[i] + pad))); for (j = i + 1; j > 0; j--) { tmp = index_of[lambda[j - 1]]; if (tmp != nn) { @@ -175,6 +185,15 @@ if (lambda[i] != nn) deg_lambda = i; } + + if (deg_lambda == 0) { + /* + * deg(lambda) is zero even though the syndrome is non-zero + * => uncorrectable error detected + */ + return -EBADMSG; + } + /* Find roots of error+erasure locator polynomial by Chien search */ memcpy(®[1], &lambda[1], nroots * sizeof(reg[0])); count = 0; /* Number of roots of lambda(x) */ @@ -188,6 +207,12 @@ } if (q != 0) continue; /* Not a root */ + + if (k < pad) { + /* Impossible error location. Uncorrectable error. */ + return -EBADMSG; + } + /* store root (index-form) and error location number */ root[count] = i; loc[count] = k; @@ -202,8 +227,7 @@ * deg(lambda) unequal to number of roots => uncorrectable * error detected */ - count = -EBADMSG; - goto finish; + return -EBADMSG; } /* * Compute err+eras evaluator poly omega(x) = s(x)*lambda(x) (modulo @@ -223,7 +247,9 @@ /* * Compute error values in poly-form. num1 = omega(inv(X(l))), num2 = * inv(X(l))**(fcr-1) and den = lambda_pr(inv(X(l))) all in poly-form + * Note: we reuse the buffer for b to store the correction pattern */ + num_corrected = 0; for (j = count - 1; j >= 0; j--) { num1 = 0; for (i = deg_omega; i >= 0; i--) { @@ -231,6 +257,13 @@ num1 ^= alpha_to[rs_modnn(rs, omega[i] + i * root[j])]; } + + if (num1 == 0) { + /* Nothing to correct at this position */ + b[j] = 0; + continue; + } + num2 = alpha_to[rs_modnn(rs, root[j] * (fcr - 1) + nn)]; den = 0; @@ -242,30 +275,52 @@ i * root[j])]; } } - /* Apply error to data */ - if (num1 != 0 && loc[j] >= pad) { - uint16_t cor = alpha_to[rs_modnn(rs,index_of[num1] + - index_of[num2] + - nn - index_of[den])]; - /* Store the error correction pattern, if a - * correction buffer is available */ - if (corr) { - corr[j] = cor; - } else { - /* If a data buffer is given and the - * error is inside the message, - * correct it */ - if (data && (loc[j] < (nn - nroots))) - data[loc[j] - pad] ^= cor; - } + + b[j] = alpha_to[rs_modnn(rs, index_of[num1] + + index_of[num2] + + nn - index_of[den])]; + num_corrected++; + } + + /* + * We compute the syndrome of the 'error' and check that it matches + * the syndrome of the received word + */ + for (i = 0; i < nroots; i++) { + tmp = 0; + for (j = 0; j < count; j++) { + if (b[j] == 0) + continue; + + k = (fcr + i) * prim * (nn-loc[j]-1); + tmp ^= alpha_to[rs_modnn(rs, index_of[b[j]] + k)]; } + + if (tmp != alpha_to[s[i]]) + return -EBADMSG; } -finish: - if (eras_pos != NULL) { - for (i = 0; i < count; i++) - eras_pos[i] = loc[i] - pad; + /* + * Store the error correction pattern, if a + * correction buffer is available + */ + if (corr && eras_pos) { + j = 0; + for (i = 0; i < count; i++) { + if (b[i]) { + corr[j] = b[i]; + eras_pos[j++] = loc[i] - pad; + } + } + } else if (data && par) { + /* Apply error to data and parity */ + for (i = 0; i < count; i++) { + if (loc[i] < (nn - nroots)) + data[loc[i] - pad] ^= b[i]; + else + par[loc[i] - pad - len] ^= b[i]; + } } - return count; + return num_corrected; } |