Re: [PATCH 4/4] mtd: nand: omap2: Add data correction support

[Ivan Djelic]

On Wed, Oct 03, 2012 at 03:29:49PM +0100, Philip, Avinash wrote:
> ELM module can be used for error correction of BCH 4 & 8 bit. Also
> support read & write page in one shot by adding custom read_page &
> write_page methods. This helps in optimizing code.
> 
> New structure member "is_elm_used" is added to know the status of
> whether the ELM module is used for error correction or not.
> 
> Note:
> ECC layout of BCH8 uses 14 bytes for 512 byte of data to make compatible
> with RBL ECC layout, even though the requirement was only 13 byte. This
> results a common ecc layout across RBL, U-boot & Linux.
> 

See a few comments below,

(...)
> +static int omap_elm_correct_data(struct mtd_info *mtd, u_char *dat,
> +                               u_char *read_ecc, u_char *calc_ecc)
> +{
> +       struct omap_nand_info *info = container_of(mtd, struct omap_nand_info,
> +                       mtd);
> +       int eccsteps = info->nand.ecc.steps;
> +       int i , j, stat = 0;
> +       int eccsize, eccflag, size;
> +       struct elm_errorvec err_vec[ERROR_VECTOR_MAX];
> +       u_char *ecc_vec = calc_ecc;
> +       enum bch_ecc type;
> +       bool is_error_reported = false;
> +
> +       /* initialize elm error vector to zero */
> +       memset(err_vec, 0, sizeof(err_vec));
> +       if (info->nand.ecc.strength == BCH8_MAX_ERROR) {
> +               size = BCH8_SIZE;
> +               eccsize = BCH8_ECC_OOB_BYTES;
> +               type = BCH8_ECC;
> +       } else {
> +               size = BCH4_SIZE;
> +               eccsize = BCH4_SIZE;
> +               type = BCH4_ECC;
> +       }
> +
> +       for (i = 0; i < eccsteps ; i++) {
> +               eccflag = 0;    /* initialize eccflag */
> +
> +               for (j = 0; (j < eccsize); j++) {
> +                       if (read_ecc[j] != 0xFF) {
> +                               eccflag = 1;    /* data area is flashed */

Just a reminder: this way of checking if a page has been programmed is not robust to bitflips,
so you may get into trouble with UBIFS on a fairly recent device.

(...)
> @@ -1039,14 +1180,45 @@ static void omap3_enable_hwecc_bch(struct mtd_info *mtd, int mode)
> 
>         nerrors = info->nand.ecc.strength;
>         dev_width = (chip->options & NAND_BUSWIDTH_16) ? 1 : 0;
> +#ifdef CONFIG_MTD_NAND_OMAP_BCH
> +       if (info->is_elm_used) {
> +               /*
> +                * Program GPMC to perform correction on (steps * 512) byte
> +                * sector at a time.
> +                */
> +               gpmc_enable_hwecc_bch(info->gpmc_cs, mode, dev_width,
> +                               info->nand.ecc.steps, nerrors);
> +               return;
> +       }
> +#endif
>         /*
> -        * Program GPMC to perform correction on one 512-byte sector at a time.
> -        * Using 4 sectors at a time (i.e. ecc.size = 2048) is also possible and
> -        * gives a slight (5%) performance gain (but requires additional code).
> +        * Program GPMC to perform correction on one 512-byte sector at
> +        * a time.

Why removing the comment about 4-sector perf gain ? :-)

(...)
> @@ -1146,35 +1402,62 @@ static int omap3_init_bch(struct mtd_info *mtd, int ecc_opt)
>                 goto fail;
>         }
> 
> -       /* initialize GPMC BCH engine */
> -       ret = gpmc_init_hwecc_bch(info->gpmc_cs, 1, max_errors);
> -       if (ret)
> -               goto fail;
> -
> -       /* software bch library is only used to detect and locate errors */
> -       info->bch = init_bch(13, max_errors, 0x201b /* hw polynomial */);
> -       if (!info->bch)
> -               goto fail;
> +       info->nand.ecc.size = 512;
> +       info->nand.ecc.hwctl = omap3_enable_hwecc_bch;
> +       info->nand.ecc.mode = NAND_ECC_HW;
> +       info->nand.ecc.strength = hw_errors;
> 
> -       info->nand.ecc.size    = 512;
> -       info->nand.ecc.hwctl   = omap3_enable_hwecc_bch;
> -       info->nand.ecc.correct = omap3_correct_data_bch;
> -       info->nand.ecc.mode    = NAND_ECC_HW;
> +       if (info->is_elm_used && (mtd->writesize <= 4096)) {
> +               enum bch_ecc bch_type;
> 
> -       /*
> -        * The number of corrected errors in an ecc block that will trigger
> -        * block scrubbing defaults to the ecc strength (4 or 8).
> -        * Set mtd->bitflip_threshold here to define a custom threshold.
> -        */
> +               if (hw_errors == BCH8_MAX_ERROR) {
> +                       bch_type = BCH8_ECC;
> +                       info->nand.ecc.bytes = BCH8_SIZE;
> +               } else {
> +                       bch_type = BCH4_ECC;
> +                       info->nand.ecc.bytes = BCH4_SIZE;
> +               }
> 
> -       if (max_errors == 8) {
> -               info->nand.ecc.strength  = 8;
> -               info->nand.ecc.bytes     = 13;
> -               info->nand.ecc.calculate = omap3_calculate_ecc_bch8;
> +               info->nand.ecc.correct = omap_elm_correct_data;
> +               info->nand.ecc.calculate = omap3_calculate_ecc_bch;
> +               info->nand.ecc.read_page = omap_read_page_bch;
> +               info->nand.ecc.write_page = omap_write_page_bch;
> +               info->elm_dev = elm_request(bch_type);
> +               if (!info->elm_dev) {
> +                       pr_err("Request to elm module failed\n");
> +                       goto fail;
> +               }
>         } else {
> -               info->nand.ecc.strength  = 4;
> -               info->nand.ecc.bytes     = 7;
> -               info->nand.ecc.calculate = omap3_calculate_ecc_bch4;
> +
> +               /* initialize GPMC BCH engine */
> +               ret = gpmc_init_hwecc_bch(info->gpmc_cs, 1, max_errors);
> +               if (ret)
> +                       goto fail;
> +
> +               /*
> +                * software bch library is only used to detect and
> +                * locateerrors

s/locateerrors/locate errors/

BTW, did you check that your patch does not break the software BCH case (i.e. no ELM) ?

BR,
--
Ivan
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