docecc.c

linux下的MTD设备驱动源代码,配合jffs2 yaffss2文件系统.

	}
      if (q != 0)
	continue;
      /* store root and error location number indices */
      root[count] = i;
      loc[count] = k;
      count++;
    }
    if (count != no_eras) {
      printf("\n lambda(x) is WRONG\n");
      count = -1;
      goto finish;
    }
#if DEBUG >= 2
    printf("\n Erasure positions as determined by roots of Eras Loc Poly:\n");
    for (i = 0; i < count; i++)
      printf("%d ", loc[i]);
    printf("\n");
#endif
#endif
  }
  for(i=0;i<NN-KK+1;i++)
    b[i] = Index_of[lambda[i]];
  
  /*
   * Begin Berlekamp-Massey algorithm to determine error+erasure
   * locator polynomial
   */
  r = no_eras;
  el = no_eras;
  while (++r <= NN-KK) {	/* r is the step number */
    /* Compute discrepancy at the r-th step in poly-form */
    discr_r = 0;
    for (i = 0; i < r; i++){
      if ((lambda[i] != 0) && (s[r - i] != A0)) {
	discr_r ^= Alpha_to[modnn(Index_of[lambda[i]] + s[r - i])];
      }
    }
    discr_r = Index_of[discr_r];	/* Index form */
    if (discr_r == A0) {
      /* 2 lines below: B(x) <-- x*B(x) */
      COPYDOWN(&b[1],b,NN-KK);
      b[0] = A0;
    } else {
      /* 7 lines below: T(x) <-- lambda(x) - discr_r*x*b(x) */
      t[0] = lambda[0];
      for (i = 0 ; i < NN-KK; i++) {
	if(b[i] != A0)
	  t[i+1] = lambda[i+1] ^ Alpha_to[modnn(discr_r + b[i])];
	else
	  t[i+1] = lambda[i+1];
      }
      if (2 * el <= r + no_eras - 1) {
	el = r + no_eras - el;
	/*
	 * 2 lines below: B(x) <-- inv(discr_r) *
	 * lambda(x)
	 */
	for (i = 0; i <= NN-KK; i++)
	  b[i] = (lambda[i] == 0) ? A0 : modnn(Index_of[lambda[i]] - discr_r + NN);
      } else {
	/* 2 lines below: B(x) <-- x*B(x) */
	COPYDOWN(&b[1],b,NN-KK);
	b[0] = A0;
      }
      COPY(lambda,t,NN-KK+1);
    }
  }

  /* Convert lambda to index form and compute deg(lambda(x)) */
  deg_lambda = 0;
  for(i=0;i<NN-KK+1;i++){
    lambda[i] = Index_of[lambda[i]];
    if(lambda[i] != A0)
      deg_lambda = i;
  }
  /*
   * Find roots of the error+erasure locator polynomial by Chien
   * Search
   */
  COPY(&reg[1],&lambda[1],NN-KK);
  count = 0;		/* Number of roots of lambda(x) */
  for (i = 1,k=NN-Ldec; i <= NN; i++,k = modnn(NN+k-Ldec)) {
    q = 1;
    for (j = deg_lambda; j > 0; j--){
      if (reg[j] != A0) {
	reg[j] = modnn(reg[j] + j);
	q ^= Alpha_to[reg[j]];
      }
    }
    if (q != 0)
      continue;
    /* store root (index-form) and error location number */
    root[count] = i;
    loc[count] = k;
    /* If we've already found max possible roots,
     * abort the search to save time
     */
    if(++count == deg_lambda)
      break;
  }
  if (deg_lambda != count) {
    /*
     * deg(lambda) unequal to number of roots => uncorrectable
     * error detected
     */
    count = -1;
    goto finish;
  }
  /*
   * Compute err+eras evaluator poly omega(x) = s(x)*lambda(x) (modulo
   * x**(NN-KK)). in index form. Also find deg(omega).
   */
  deg_omega = 0;
  for (i = 0; i < NN-KK;i++){
    tmp = 0;
    j = (deg_lambda < i) ? deg_lambda : i;
    for(;j >= 0; j--){
      if ((s[i + 1 - j] != A0) && (lambda[j] != A0))
	tmp ^= Alpha_to[modnn(s[i + 1 - j] + lambda[j])];
    }
    if(tmp != 0)
      deg_omega = i;
    omega[i] = Index_of[tmp];
  }
  omega[NN-KK] = A0;
  
  /*
   * Compute error values in poly-form. num1 = omega(inv(X(l))), num2 =
   * inv(X(l))**(B0-1) and den = lambda_pr(inv(X(l))) all in poly-form
   */
  for (j = count-1; j >=0; j--) {
    num1 = 0;
    for (i = deg_omega; i >= 0; i--) {
      if (omega[i] != A0)
	num1  ^= Alpha_to[modnn(omega[i] + i * root[j])];
    }
    num2 = Alpha_to[modnn(root[j] * (B0 - 1) + NN)];
    den = 0;
    
    /* lambda[i+1] for i even is the formal derivative lambda_pr of lambda[i] */
    for (i = min(deg_lambda,NN-KK-1) & ~1; i >= 0; i -=2) {
      if(lambda[i+1] != A0)
	den ^= Alpha_to[modnn(lambda[i+1] + i * root[j])];
    }
    if (den == 0) {
#if DEBUG >= 1
      printf("\n ERROR: denominator = 0\n");
#endif
      /* Convert to dual- basis */
      count = -1;
      goto finish;
    }
    /* Apply error to data */
    if (num1 != 0) {
        eras_val[j] = Alpha_to[modnn(Index_of[num1] + Index_of[num2] + NN - Index_of[den])];
    } else {
        eras_val[j] = 0;
    }
  }
 finish:
  for(i=0;i<count;i++)
      eras_pos[i] = loc[i];
  return count;
}

/***************************************************************************/
/* The DOC specific code begins here */

#define SECTOR_SIZE 512
/* The sector bytes are packed into NB_DATA MM bits words */
#define NB_DATA (((SECTOR_SIZE + 1) * 8 + 6) / MM)

/* 
 * Correct the errors in 'sector[]' by using 'ecc1[]' which is the
 * content of the feedback shift register applyied to the sector and
 * the ECC. Return the number of errors corrected (and correct them in
 * sector), or -1 if error 
 */
int doc_decode_ecc(unsigned char sector[SECTOR_SIZE], unsigned char ecc1[6])
{
    int parity, i, nb_errors;
    gf bb[NN - KK + 1];
    gf error_val[NN-KK];
    int error_pos[NN-KK], pos, bitpos, index, val;
    dtype *Alpha_to, *Index_of;

    /* init log and exp tables here to save memory. However, it is slower */
    Alpha_to = kmalloc((NN + 1) * sizeof(dtype), GFP_KERNEL);
    if (!Alpha_to)
        return -1;
    
    Index_of = kmalloc((NN + 1) * sizeof(dtype), GFP_KERNEL);
    if (!Index_of) {
        kfree(Alpha_to);
        return -1;
    }

    generate_gf(Alpha_to, Index_of);

    parity = ecc1[1];

    bb[0] =  (ecc1[4] & 0xff) | ((ecc1[5] & 0x03) << 8);
    bb[1] = ((ecc1[5] & 0xfc) >> 2) | ((ecc1[2] & 0x0f) << 6);
    bb[2] = ((ecc1[2] & 0xf0) >> 4) | ((ecc1[3] & 0x3f) << 4);
    bb[3] = ((ecc1[3] & 0xc0) >> 6) | ((ecc1[0] & 0xff) << 2);

    nb_errors = eras_dec_rs(Alpha_to, Index_of, bb, 
                            error_val, error_pos, 0);
    if (nb_errors <= 0)
        goto the_end;

    /* correct the errors */
    for(i=0;i<nb_errors;i++) {
        pos = error_pos[i];
        if (pos >= NB_DATA && pos < KK) {
            nb_errors = -1;
            goto the_end;
        }
        if (pos < NB_DATA) {
            /* extract bit position (MSB first) */
            pos = 10 * (NB_DATA - 1 - pos) - 6;
            /* now correct the following 10 bits. At most two bytes
               can be modified since pos is even */
            index = (pos >> 3) ^ 1;
            bitpos = pos & 7;
            if ((index >= 0 && index < SECTOR_SIZE) || 
                index == (SECTOR_SIZE + 1)) {
                val = error_val[i] >> (2 + bitpos);
                parity ^= val;
                if (index < SECTOR_SIZE)
                    sector[index] ^= val;
            }
            index = ((pos >> 3) + 1) ^ 1;
            bitpos = (bitpos + 10) & 7;
            if (bitpos == 0)
                bitpos = 8;
            if ((index >= 0 && index < SECTOR_SIZE) || 
                index == (SECTOR_SIZE + 1)) {
                val = error_val[i] << (8 - bitpos);
                parity ^= val;
                if (index < SECTOR_SIZE)
                    sector[index] ^= val;
            }
        }
    }
    
    /* use parity to test extra errors */
    if ((parity & 0xff) != 0)
        nb_errors = -1;

 the_end:
    kfree(Alpha_to);
    kfree(Index_of);
    return nb_errors;
}

EXPORT_SYMBOL_GPL(doc_decode_ecc);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Fabrice Bellard <[e-mail]>");
MODULE_DESCRIPTION("ECC code for correcting errors detected by DiskOnChip 2000 and Millennium ECC hardware");