linux-kernel - Re: [PATCH 15/39] mtd: nand: denali: improve readability of handle

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Date:   Fri, 2 Dec 2016 13:26:27 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:     linux-mtd@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Marek Vasut <marek.vasut@...il.com>,
        Brian Norris <computersforpeace@...il.com>,
        Richard Weinberger <richard@....at>,
        David Woodhouse <dwmw2@...radead.org>,
        Cyrille Pitchen <cyrille.pitchen@...el.com>
Subject: Re: [PATCH 15/39] mtd: nand: denali: improve readability of handle_ecc()

Hi Boris,


2016-11-28 0:42 GMT+09:00 Boris Brezillon <boris.brezillon@...e-electrons.com>:
>> +                     if (err_byte < ECC_SECTOR_SIZE) {
>> +                             struct mtd_info *mtd =
>> +                                     nand_to_mtd(&denali->nand);
>> +                             int offset;
>> +
>> +                             offset = (err_sector * ECC_SECTOR_SIZE + err_byte) *
>> +                                     denali->devnum + err_device;
>> +                             /* correct the ECC error */
>> +                             buf[offset] ^= err_correction_value;
>> +                             mtd->ecc_stats.corrected++;
>> +                             bitflips++;
>
> Hm, bitflips is what is set in max_bitflips, and apparently the
> implementation (which is not yours) is not doing what the core expects.
>
> You should first count bitflips per sector with something like that:
>
>                                 bitflips[err_sector]++;
>
>
> And then once you've iterated over all errors do:
>
>         for (i = 0; i < nsectors; i++)
>                 max_bitflips = max(bitflips[err_sector], max_bitflips);


I see.

For soft ECC fixup, we can calculate bitflips
for each ECC sector, so I can fix the max_bitflips
as the core framework expects.

For hard ECC fixup, the register only reports
the number of corrected bit-flips
in the whole page (sum from all ECC sectors).
We cannot calculate max_bitflips, I think.



BTW, I noticed another problem of the current code.

      buf[offset] ^= err_correction_value;
      mtd->ecc_stats.corrected++;
      bitflips++;

This code is counting the number of corrected bytes,
not the number of corrected bits.


I think multiple bit-flips within one byte can happen.


Perhaps, we should add

  hweight8(buf[offset] ^ err_correction_value)

to ecc_stats.corrected and bitflips.




-- 
Best Regards
Masahiro Yamada