Re: [PATCH v2 10/53] mtd: nand: denali: fix erased page checking

Boris Brezillon
2017-03-23T07:57:09Z

On Thu, 23 Mar 2017 14:04:44 +0900
Masahiro Yamada <yamada.masahiro@socionext.com> wrote:

> Hi Boris,
>
> 2017-03-23 5:56 GMT+09:00 Boris Brezillon <boris.brezillon@free-electrons.com>:
> > On Wed, 22 Mar 2017 23:07:17 +0900
> > Masahiro Yamada <yamada.masahiro@socionext.com> wrote:
> >> dev_err(denali->dev,
> >> @@ -1148,12 +1136,15 @@ static int denali_read_page(struct mtd_info *mtd, struct nand_chip *chip,
> >> if (check_erased_page) {
> >> read_oob_data(mtd, chip->oob_poi, denali->page);
> >>
> >> - /* check ECC failures that may have occurred on erased pages */
> >> - if (check_erased_page) {
> >> - if (!is_erased(buf, mtd->writesize))
> >> - mtd->ecc_stats.failed++;
> >> - if (!is_erased(buf, mtd->oobsize))
> >> - mtd->ecc_stats.failed++;
> >> + stat = nand_check_erased_ecc_chunk(
> >> + buf, mtd->writesize,
> >> + chip->oob_poi, mtd->oobsize,
> >> + NULL, 0,
> >> + chip->ecc.strength * chip->ecc.steps);
> >
> > That's not how it's supposed to be done. Each chunk should be checked
> > independently. Here is a simple example explaining why this is
> > important:
> >
> > Let's consider the following setup:
> > - 4k pages
> > - 16bits/1024bytes ECC
> >
> > With your approach, you turn this into:
> > - 4k pages
> > - 64bits/4096bytes ECC
> >
> > Now suppose you have 32 bitflips in the first 1024 bytes. The real ECC
> > config is expected to report uncorrectable errors, but your approach
> > will just report that 32 bits have been fixed, which is wrong.
>
>
> OK. How about adding a helper like follows:
>
> static int denali_check_erased_page(struct mtd_info *mtd,
> struct nand_chip *chip, uint8_t *buf)
> {
> uint8_t *ecc_code = chip->buffers->ecccode;
> int ecc_steps = chip->ecc.steps;
> int ecc_size = chip->ecc.size;
> int ecc_bytes = chip->ecc.bytes;
> int i, ret;
>
> ret = mtd_ooblayout_get_eccbytes(mtd, ecc_code, chip->oob_poi, 0,
> chip->ecc.total);
> if (ret)
> return ret;
>
> for (i = 0; i < ecc_steps; i++) {
> ret = nand_check_erased_ecc_chunk(buf, ecc_size,
> ecc_code, ecc_bytes,
> NULL, 0,
> chip->ecc.strength);
> if (ret < 0)
> return ret;
> buf += ecc_size;
> ecc_code += ecc_bytes;
> }
>
> return 0;
> }
>
>
>
> Then,
>
> stat = denali_check_erased_page(mtd, chip, buf);
> if (stat < 0) {
> mtd->ecc_stats.failed++;
> /* return 0 for uncorrectable bitflips */
> stat = 0;
> }

What's the point of checking all ECC chunks if only one contains ECC
errors? I really recommend to put the nand_check_erased_ecc_chunk()
call next to the per-ECC-block correction test.

Also, mtd->ecc_stats.failed is supposed to be incremented each time an
uncorrectable error is detected. In your denali_sw_ecc_fixup()
implementation you can detect errors at the ECC chunk level, so you
should increment ecc_stats.failed for each failure and not once if at
least one chunk is faulty.