Date:   Wed, 30 May 2018 16:38:08 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Abhishek Sahu <absahu@...eaurora.org>
Cc:     Archit Taneja <architt@...eaurora.org>,
        Marek Vasut <marek.vasut@...il.com>,
        Richard Weinberger <richard@....at>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>,
        Andy Gross <andy.gross@...aro.org>,
        Brian Norris <computersforpeace@...il.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v3 01/16] mtd: rawnand: helper function for setting up ECC configuration

2018-05-30 15:21 GMT+09:00 Abhishek Sahu <absahu@...eaurora.org>:
> On 2018-05-30 05:58, Masahiro Yamada wrote:
>>
>> Hi.
>>
>> 2018-05-30 4:30 GMT+09:00 Boris Brezillon <boris.brezillon@...tlin.com>:
>>>
>>> On Sat, 26 May 2018 10:42:47 +0200
>>> Miquel Raynal <miquel.raynal@...tlin.com> wrote:
>>>
>>>> Hi Abhishek,
>>>>
>>>> On Fri, 25 May 2018 17:51:29 +0530, Abhishek Sahu
>>>> <absahu@...eaurora.org> wrote:
>>>>
>>>> > commit 2c8f8afa7f92 ("mtd: nand: add generic helpers to check,
>>>> > match, maximize ECC settings") provides generic helpers which
>>>> > drivers can use for setting up ECC parameters.
>>>> >
>>>> > Since same board can have different ECC strength nand chips so
>>>> > following is the logic for setting up ECC strength and ECC step
>>>> > size, which can be used by most of the drivers.
>>>> >
>>>> > 1. If both ECC step size and ECC strength are already set
>>>> >    (usually by DT) then just check whether this setting
>>>> >    is supported by NAND controller.
>>>> > 2. If NAND_ECC_MAXIMIZE is set, then select maximum ECC strength
>>>> >    supported by NAND controller.
>>>> > 3. Otherwise, try to match the ECC step size and ECC strength closest
>>>> >    to the chip's requirement. If available OOB size can't fit the chip
>>>> >    requirement then select maximum ECC strength which can be fit with
>>>> >    available OOB size.
>>>> >
>>>> > This patch introduces nand_ecc_choose_conf function which calls the
>>>> > required helper functions for the above logic. The drivers can use
>>>> > this single function instead of calling the 3 helper functions
>>>> > individually.
>>>> >
>>>> > CC: Masahiro Yamada <yamada.masahiro@...ionext.com>
>>>> > Signed-off-by: Abhishek Sahu <absahu@...eaurora.org>
>>>> > ---
>>>> > * Changes from v2:
>>>> >
>>>> >   1. Renamed function to nand_ecc_choose_conf.
>>>> >   2. Minor code reorganization to remove warning and 2 function calls
>>>> >      for nand_maximize_ecc.
>>>> >
>>>> > * Changes from v1:
>>>> >   NEW PATCH
>>>> >
>>>> >  drivers/mtd/nand/raw/nand_base.c | 42
>>>> > ++++++++++++++++++++++++++++++++++++++++
>>>> >  drivers/mtd/nand/raw/nand_base.c | 31 +++++++++++++++++++++++++++++++
>>>> >  include/linux/mtd/rawnand.h      |  3 +++
>>>> >  2 files changed, 34 insertions(+)
>>>> >
>>>> > diff --git a/drivers/mtd/nand/raw/nand_base.c
>>>> > b/drivers/mtd/nand/raw/nand_base.c
>>>> > index 72f3a89..e52019d 100644
>>>> > --- a/drivers/mtd/nand/raw/nand_base.c
>>>> > +++ b/drivers/mtd/nand/raw/nand_base.c
>>>> > @@ -6249,6 +6249,37 @@ int nand_maximize_ecc(struct nand_chip *chip,
>>>> >  }
>>>> >  EXPORT_SYMBOL_GPL(nand_maximize_ecc);
>>>> >
>>>> > +/**
>>>> > + * nand_ecc_choose_conf - Set the ECC strength and ECC step size
>>>> > + * @chip: nand chip info structure
>>>> > + * @caps: ECC engine caps info structure
>>>> > + * @oobavail: OOB size that the ECC engine can use
>>>> > + *
>>>> > + * Choose the ECC configuration according to following logic
>>>> > + *
>>>> > + * 1. If both ECC step size and ECC strength are already set (usually
>>>> > by DT)
>>>> > + *    then check if it is supported by this controller.
>>>> > + * 2. If NAND_ECC_MAXIMIZE is set, then select maximum ECC strength.
>>>> > + * 3. Otherwise, try to match the ECC step size and ECC strength
>>>> > closest
>>>> > + *    to the chip's requirement. If available OOB size can't fit the
>>>> > chip
>>>> > + *    requirement then fallback to the maximum ECC step size and ECC
>>>> > strength.
>>>> > + *
>>>> > + * On success, the chosen ECC settings are set.
>>>> > + */
>>>> > +int nand_ecc_choose_conf(struct nand_chip *chip,
>>>> > +                    const struct nand_ecc_caps *caps, int oobavail)
>>>> > +{
>>>> > +   if (chip->ecc.size && chip->ecc.strength)
>>>> > +           return nand_check_ecc_caps(chip, caps, oobavail);
>>>> > +
>>>> > +   if (!(chip->ecc.options & NAND_ECC_MAXIMIZE) &&
>>>> > +       !nand_match_ecc_req(chip, caps, oobavail))
>>>> > +           return 0;
>>>> > +
>>>> > +   return nand_maximize_ecc(chip, caps, oobavail);
>>>>
>>>> I personally don't mind if nand_maximize_ecc() is called twice in
>>>> the function if it clarifies the logic. Maybe the following will be
>>>> more clear for the user?
>>>>
>>>>       if (chip->ecc.size && chip->ecc.strength)
>>>>               return nand_check_ecc_caps(chip, caps, oobavail);
>>>>
>>>>       if (chip->ecc.options & NAND_ECC_MAXIMIZE)
>>>>               return nand_maximize_ecc(chip, caps, oobavail);
>>>>
>>>>       if (!nand_match_ecc_req(chip, caps, oobavail))
>>>>               return 0;
>>>>
>>>>       return nand_maximize_ecc(chip, caps, oobavail);
>>>
>>>
>>> I personally don't mind, and it seems Masahiro wanted to keep the logic
>>> he had used in the denali driver.
>>>
>>>>
>>>> Also, I'm not sure we should just error out when nand_check_ecc_caps()
>>>> fails. What about something more robust, like:
>>>>
>>>>       int ret;
>>>>
>>>>       if (chip->ecc.size && chip->ecc.strength) {
>>>>               ret = nand_check_ecc_caps(chip, caps, oobavail);
>>>>               if (ret)
>>>>                       goto maximize_ecc;
>>>
>>>
>>> Nope. When someone asked for a specific ECC config by passing the
>>> nand-ecc-xxx props we should apply it or return an erro if it's not
>>> supported. People passing those props should now what the ECC engine
>>> supports and pick one valid values.
>>>
>>>>
>>>>               return 0;
>>>>       }
>>>>
>>>>       if (chip->ecc.options & NAND_ECC_MAXIMIZE)
>>>>               goto maximize_ecc;
>>>>
>>>>       ret = nand_match_ecc_req(chip, caps, oobavail);
>>>>       if (ret)
>>>>               goto maximize_ecc;
>>>>
>>>>       return 0;
>>>>
>>>> maximize_ecc:
>>>>       return nand_maximize_ecc(chip, caps, oobavail);
>>>>
>>>
>>>
>>> ______________________________________________________
>>> Linux MTD discussion mailing list
>>> http://lists.infradead.org/mailman/listinfo/linux-mtd/
>>
>>
>>
>>
>>
>>
>>
>> This version looks good to me.
>>
>> If you want to check the error code more precisely,
>> how about something like follows?
>>
>>
>>
>> int nand_ecc_choose_conf(struct nand_chip *chip,
>>                         const struct nand_ecc_caps *caps, int oobavail)
>> {
>>        int ret;
>>
>>        if (chip->ecc.size && chip->ecc.strength)
>>                return nand_check_ecc_caps(chip, caps, oobavail);
>>
>>        if (!(chip->ecc.options & NAND_ECC_MAXIMIZE)) {
>>                ret = nand_match_ecc_req(chip, caps, oobavail);
>>                if (ret != -ENOTSUPP)
>>                        return ret;
>>        }
>>
>>        return nand_maximize_ecc(chip, caps, oobavail);
>> }
>>
>>
>> Only the difference is the case
>> where nand_match_ecc_req() returns a different error code
>> than ENOTSUPP.
>> (Currently, this happens only when insane 'oobavail' is passed.)
>>
>
>  We can do that but to me, it will make the helper function
>  more complicated. Currently, nand_match_ecc_req is returning
>  other than ENOTSUPP 'oobavail < 0' is passed.
>  and again in nand_maximize_ecc, we will check for validity
>  of oobavail so nothing wrong will happen in calling
>  nand_maximize_ecc.


Right.  When I added those three helpers,
I supposed they were independent APIs.
That is why I added the 'oobavail < 0' sanity check
in each of the three.


If you make them internal sub-helpers
(i.e. add 'static' instead of EXPORT_SYMBOL_GPL),
you can check 'oobavail < 0'
only in nand_ecc_choose_conf().





>  Anyway we put this under WARN_ON condition
>
>         if (WARN_ON(oobavail < 0))
>                 return -EINVAL;
>
>  so if this is being triggered, then it should be mostly
>  programming error.


Right.  Moreover,

WARN_ON(oobavail < 0 || oobavail > mtd->oobsize)



This is programming error, that is why WARN_ON() is used to
make the log noisy.


>  Thanks,
>  Abhishek
>
>>
>> ENOTSUPP means 'required ECC setting is not supported'.
>> Other error code is more significant, so it is not a good reason
>> to fall back to miximization, IMHO.
>
>
> ______________________________________________________
> Linux MTD discussion mailing list
> http://lists.infradead.org/mailman/listinfo/linux-mtd/



-- 
Best Regards
Masahiro Yamada

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