Re: [PATCH v3 14/37] mtd: nand: denali: support "nand-ecc-strength" DT property

[Masahiro Yamada]

Hi Boris,


2017-04-11 16:56 GMT+09:00 Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx>:
> Hi Masahiro,
>
> On Tue, 11 Apr 2017 15:19:21 +0900
> Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> wrote:
>
>> Hi Boris,
>>
>>
>>
>> 2017-04-10 1:33 GMT+09:00 Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx>:
>> > On Mon, 3 Apr 2017 12:16:34 +0900
>> > Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> wrote:
>> >
>> >> Hi Boris,
>> >>
>> >>
>> >>
>> >> 2017-03-31 18:46 GMT+09:00 Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx>:
>> >>
>> >> > You can try something like that when no explicit ecc.strength and
>> >> > ecc.size has been set in the DT and when ECC_MAXIMIZE was not passed.
>> >> >
>> >> > static int
>> >> > denali_get_closest_ecc_strength(struct denali_nand_info *denali,
>> >> >                                 int strength)
>> >> > {
>> >> >         /*
>> >> >          * Whatever you need to select a strength that is greater than
>> >> >          * or equal to strength.
>> >> >          */
>> >> >
>> >> >         return X;
>> >> > }
>> >>
>> >>
>> >> Is here anything specific to Denali?
>> >
>> > Well, only the denali driver knows what the hardware supports, though
>> > having a generic function that takes a table of supported strengths
>> > would work.
>> >
>> >>
>> >>
>> >> > static int denali_try_to_match_ecc_req(struct denali_nand_info *denali)
>> >> > {
>> >> >         struct nand_chip *chip = &denali->nand;
>> >> >         struct mtd_info *mtd = nand_to_mtd(chip);
>> >> >         int max_ecc_bytes = mtd->oobsize - denali->bbtskipbytes;
>> >> >         int ecc_steps, ecc_strength, ecc_bytes;
>> >> >         int ecc_size = chip->ecc_step_ds;
>> >> >         int ecc_strength = chip->ecc_strength_ds;
>> >> >
>> >> >         /*
>> >> >          * No information provided by the NAND chip, let the core
>> >> >          * maximize the strength.
>> >> >          */
>> >> >         if (!ecc_size || !ecc_strength)
>> >> >                 return -ENOTSUPP;
>> >> >
>> >> >         if (ecc_size > 512)
>> >> >                 ecc_size = 1024;
>> >> >         else
>> >> >                 ecc_size = 512;
>> >> >
>> >> >         /* Adjust ECC step size based on hardware support. */
>> >> >         if (ecc_size == 1024 &&
>> >> >             !(denali->caps & DENALI_CAP_ECC_SIZE_1024))
>> >> >                 ecc_size = 512;
>> >> >         else if(ecc_size == 512 &&
>> >> >                 !(denali->caps & DENALI_CAP_ECC_SIZE_512))
>> >> >                 ecc_size = 1024;
>> >> >
>> >> >         if (ecc_size < chip->ecc_size_ds) {
>> >> >                 /*
>> >> >                  * When the selected size if smaller than the expected
>> >> >                  * one we try to use the same strength but on 512 blocks
>> >> >                  * so that we can still fix the same number of errors
>> >> >                  * even if they are concentrated in the first 512bytes
>> >> >                  * of a 1024bytes portion.
>> >> >                  */
>> >> >                 ecc_strength = chip->ecc_strength_ds;
>> >> >                 ecc_strength = denali_get_closest_ecc_strength(denali,
>> >> >                                                                ecc_strength);
>> >> >         } else {
>> >> >                 /* Always prefer 1024bytes ECC blocks when possible. */
>> >> >                 if (ecc_size != 1024 &&
>> >> >                     (denali->caps & DENALI_CAP_ECC_SIZE_1024) &&
>> >> >                     mtd->writesize > 1024)
>> >> >                         ecc_size = 1024;
>> >> >
>> >> >                 /*
>> >> >                  * Adjust the strength based on the selected ECC step
>> >> >                  * size.
>> >> >                  */
>> >> >                 ecc_strength = DIV_ROUND_UP(ecc_size,
>> >> >                                             chip->ecc_step_ds) *
>> >> >                                chip->ecc_strength_ds;
>> >> >         }
>> >> >
>> >> >         ecc_bytes = denali_calc_ecc_bytes(ecc_size,
>> >> >                                           ecc_strength);
>> >> >         ecc_bytes *= mtd->writesize / ecc_size;
>> >> >
>> >> >         /*
>> >> >          * If we don't have enough space, let the core maximize
>> >> >          * the strength.
>> >> >          */
>> >> >         if (ecc_bytes > max_ecc_bytes)
>> >> >                 return -ENOTSUPP;
>> >> >
>> >> >         chip->ecc.strength = ecc_strength;
>> >> >         chip->ecc.size = ecc_size;
>> >> >
>> >> >         return 0;
>> >> > }
>> >>
>> >>
>> >> As a whole, this does not seem to driver-specific.
>> >
>> > It's almost controller-agnostic, except for the denali_calc_ecc_bytes()
>> > function, but I guess we could ask drivers to implement a hook that is
>> > passed the ECC step size and strength and returns the associated
>> > number of ECC bytes.
>> >
>> >>
>> >>
>> >> [1] A driver provides some pairs of (ecc_strength, ecc_size)
>> >>     it can support.
>> >>
>> >> [2] The core framework knows the chip's requirement
>> >>     (ecc_strength_ds, ecc_size_ds).
>> >>
>> >>
>> >> Then, the core framework provides a function
>> >> to return a most recommended (ecc_strength, ecc_size).
>> >>
>> >>
>> >>
>> >> struct nand_ecc_spec {
>> >>        int ecc_strength;
>> >>        int ecc_size;
>> >> };
>> >>
>> >> /*
>> >>  * This function choose the most recommented (ecc_str, ecc_size)
>> >>  * "recommended" means: minimum ecc stregth that meets
>> >>  * the chip's requirment.
>> >>  *
>> >>  *
>> >>  * @chip   - nand_chip
>> >>  * @controller_ecc_spec - Array of (ecc_str, ecc_size) supported by the
>> >>                           controller. (terminated by NULL as sentinel)
>> >>  */
>> >> struct nand_ecc_spec * nand_try_to_match_ecc_req(struct nand_chip *chip,
>> >>                                                  struct nand_ecc_spec
>> >> *controller_ecc_spec)
>> >> {
>> >>       /*
>> >>        * Return the pointer to the most recommended
>> >>        * struct nand_ecc_spec.
>> >>        * If nothing suitable found, return NULL.
>> >>        */
>> >> }
>> >>
>> >
>> > I like the idea, except I would do this slightly differently to avoid
>> > declaring all combinations of stepsize and strengths
>> >
>> > struct nand_ecc_stepsize_info {
>> >         int stepsize;
>> >         int nstrengths;
>> >         int *strengths;
>> > };
>> >
>> > struct nand_ecc_engine_caps {
>> >         int nstepsizes;
>> >         struct nand_ecc_stepsize_info *stepsizes;
>> >         int (*calc_ecc_bytes)(int stepsize, int strength);
>> > };
>> >
>> > int nand_try_to_match_ecc_req(struct nand_chip *chip,
>> >                               const struct nand_ecc_engine_caps *caps,
>> >                               struct nand_ecc_spec *spec)
>> > {
>> >         /*
>> >          * Find the most appropriate setting based on the ECC engine
>> >          * caps and fill the spec object accordingly.
>> >          * Returns 0 in case of success and a negative error code
>> >          * otherwise.
>> >          */
>> > }
>> >
>> > Note that nand_try_to_match_ecc_req() has to be more generic than
>> > denali_try_to_match_ecc_req() WRT step sizes, which will probably
>> > complexify the logic.
>>
>>
>> After I fiddle with this generic approach for a while,
>> I started to feel like giving up.
>
> I don't get it. What was the problem with my initial suggestion (the
> denali specific one, not the generic approach)? You proposed to make it
> generic, which, I agree, is a bit more complicated.
>
>>
>> I wonder if we really want over-implementation
>> for covering _theoretically_ possible cases.
>
> Okay, one more theoretical case I'd like to expose: you have board
> design with different NAND parts which have different ECC requirements.
> If you were about to describe the exact ECC strength you want for each
> board you'll have to have different DTs.

In this case, fixed ecc-strength in DT is not feasible.

> Maximizing the ECC strength
> would still work, but what if the MTD user needs some OOB bytes (like
> is the case with JFFS2) and ECC maximization reserved all of the
> available bytes?

JFFS2 needs some bytes in oob-free area for the clean marker.
You are right.
This implies NAND_ECC_MAXIMIZE is not very useful.
We do not know whether we have enough space left in oob, or not.



> The other reason I prefer to have the drivers automatically guessing
> what's appropriate is because then you don't have to care when writing
> your DT.
>
>>
>> In practice, there are not so many ECC settings possible
>> on a single controller.
>>
>> As for Denali IP, it would be theoretically possible to instantiate
>> multiple ECC engines.  However, in practice, there is no sensible
>> reason to do so.  At least, I do not know any real chip to support that.
>>
>> So, I'd like to simplify the logic for Denali.
>>
>>   - Support either 512 or 1024 ECC size.
>>     If there is (ever) a controller that supports both,
>>     1024 should be chosen.
>>
>>   - ECC strength is not specified via DT, it is simply maximized.
>>
>> This simplifies the logic much and I believe this is enough.
>>
>> One more reason is, as we talked before,
>> we need to match ECC setting between Linux and firmware (boot-loader),
>
> If the bootloader implements the same logic it should match.
>
>> so anyway we end up with using a fixed setting specified by DT.
>>
>
> Really, I don't see what's the problem with the function I proposed,
> but I'm willing to make a concession.
> Make the nand-ecc-strength+nand-ecc-step-size or nand-ecc-maximize
> mandatory so that if someone ever needs to support the 'match NAND
> requirements' feature we won't have to add a vendor specific property
> like this one [1].
>
> Are you fine with that?

No.  This requirement seems too strong.
At least, it is a problem for non-DT platforms.


If a driver provides ECC engine caps info,
perhaps ECC maximizing could be a generalized helper function as well.

I am trying this still.




-- 
Best Regards
Masahiro Yamada