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

[Boris Brezillon]

On Fri, 24 Mar 2017 11:43:43 +0900
Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> wrote:

> Hi Boris,
> 
> 
> 2017-03-23 16:56 GMT+09:00 Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx>:
> > On Thu, 23 Mar 2017 14:04:44 +0900
> > Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> wrote:
> >  
> >> 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;
> >>                 }  
> >
> > 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.  
> 
> 
> OK.  I can fix it for software ECC fixup.
> 
> 
> What should I do for hardware ECC fixup case?
> http://patchwork.ozlabs.org/patch/742321/
> 
> 
> If at least one ECC sector fails to correct bit-flips,
> the controller sets INTR__ECC_UNCOR_ERR flag.
> 
> 
> In this case, we can not know the number of uncorrectable errors.
> 
> Possible solutions are:
> 
>   - Increment ecc_stats.failed only by one  (compromised solution)

Let's go for this solution.

> 
> 
> > 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.  
> 
> 
> Yes, I can do this for denali_sw_ecc_fixup().
> 
> Can I ask what disadvantage would happen
> if ecc_stats.failed / .corrected is incremented only by one,
> where actually errors happen in multiple sectors.

Reporting wrong stats, which is not such a big deal, but let's try to
keep them correct when we can (the SW ECC fixup case).