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Date: Tue, 11 Apr 2017 15:19:21 +0900
From: Masahiro Yamada <yamada.masahiro@...ionext.com>
To: Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc: linux-mtd@...ts.infradead.org, Enrico Jorns <ejo@...gutronix.de>,
Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
Dinh Nguyen <dinguyen@...nel.org>,
Marek Vasut <marek.vasut@...il.com>,
Graham Moore <grmoore@...nsource.altera.com>,
David Woodhouse <dwmw2@...radead.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Chuanxiao Dong <chuanxiao.dong@...el.com>,
Jassi Brar <jaswinder.singh@...aro.org>,
devicetree@...r.kernel.org,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Brian Norris <computersforpeace@...il.com>,
Richard Weinberger <richard@....at>,
Cyrille Pitchen <cyrille.pitchen@...el.com>,
Rob Herring <robh+dt@...nel.org>,
Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v3 14/37] mtd: nand: denali: support "nand-ecc-strength"
DT property
Hi Boris,
2017-04-10 1:33 GMT+09:00 Boris Brezillon <boris.brezillon@...e-electrons.com>:
> On Mon, 3 Apr 2017 12:16:34 +0900
> Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:
>
>> Hi Boris,
>>
>>
>>
>> 2017-03-31 18:46 GMT+09:00 Boris Brezillon <boris.brezillon@...e-electrons.com>:
>>
>> > 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 wonder if we really want over-implementation
for covering _theoretically_ possible cases.
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),
so anyway we end up with using a fixed setting specified by DT.
--
Best Regards
Masahiro Yamada
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