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

[Masahiro Yamada]

Hi Boris,

2017-03-23 5:56 GMT+09:00 Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx>:
> On Wed, 22 Mar 2017 23:07:17 +0900
> Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> 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;
                }





-- 
Best Regards
Masahiro Yamada