Re: [RFC v2] Reed-Solomon Code: Update no_eras to the actual number of errors

[Ferdinand Blomqvist]

Hi!

On 2020-06-25 00:36:01, Aiden Leong wrote:
> Corr and eras_pos are updated to actual correction pattern and erasure
> positions, but no_eras is not.
>
> When this library is used to recover lost bytes, we normally memset the
> lost trunk of bytes to zero as a placeholder. Unfortunately, if the lost
> byte is zero, b[i] is zero too. Without correct no_eras, users won't be
> able to determine the valid length of corr and eras_pos.
>
> Signed-off-by: Aiden Leong <aiden.leong@xxxxxxxxx>

I'm not sure I understand what you try to do. decode_rs* already returns
the number of errors correted (or something negative upon failure). So
your last statment is false. The lengt of corr and eras_pos is returned
by the function. So this change is unnecessary. More comments inline.

> diff --git a/lib/reed_solomon/decode_rs.c b/lib/reed_solomon/decode_rs.c
> index 805de84ae83d..44136ea33d16 100644
> --- a/lib/reed_solomon/decode_rs.c
> +++ b/lib/reed_solomon/decode_rs.c
> @@ -24,6 +24,7 @@
> 	int count = 0;
> 	int num_corrected;
> 	uint16_t msk = (uint16_t) rs->nn;
> +	int no_eras_local = no_eras ? *no_eras : 0;
>
> 	/*
> 	 * The decoder buffers are in the rs control struct. They are
> @@ -106,11 +107,11 @@
> 	memset(&lambda[1], 0, nroots * sizeof(lambda[0]));
> 	lambda[0] = 1;
>
> -	if (no_eras > 0) {
> +	if (no_eras_local > 0) {
> 		/* Init lambda to be the erasure locator polynomial */
> 		lambda[1] = alpha_to[rs_modnn(rs,
> 					prim * (nn - 1 - (eras_pos[0] + pad)))];
> -		for (i = 1; i < no_eras; i++) {
> +		for (i = 1; i < no_eras_local; 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]];
> @@ -129,8 +130,8 @@
> 	 * Begin Berlekamp-Massey algorithm to determine error+erasure
> 	 * locator polynomial
> 	 */
> -	r = no_eras;
> -	el = no_eras;
> +	r = no_eras_local;
> +	el = no_eras_local;
> 	while (++r <= nroots) {	/* r is the step number */
> 		/* Compute discrepancy at the r-th step in poly-form */
> 		discr_r = 0;
> @@ -158,8 +159,8 @@
> 				} else
> 					t[i + 1] = lambda[i + 1];
> 			}
> -			if (2 * el <= r + no_eras - 1) {
> -				el = r + no_eras - el;
> +			if (2 * el <= r + no_eras_local - 1) {
> +				el = r + no_eras_local - el;
> 				/*
> 				 * 2 lines below: B(x) <-- inv(discr_r) *
> 				 * lambda(x)
> @@ -312,14 +313,21 @@
> 				eras_pos[j++] = loc[i] - pad;
> 			}
> 		}
> +		if (no_eras)
> +			*no_eras = j;
At this point j will be equal to num_corrected. So why return this
information in no_eras, when it is already returned by the function?

> 	} else if (data && par) {
> 		/* Apply error to data and parity */
> +		j = 0;
> 		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];
> +			if (b[i])
> +				j++;
> 		}
> +		if (no_eras)
> +			*no_eras = j;

Same as above.

> 2.25.1

Best,
Ferdinand

--
Ferdinand Blomqvist
ferdinand.blomqvist[at]gmail.com
GPG key: 9EFB 7A2C 0432 4EC5 32BA FA61 CFE9 4164 93E8 B9E4