linux-kernel - Re: [PATCH -next v2] mtd: nand: Add support for Toshiba BENAND (Built-in ECC NAND)

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Date:   Thu, 5 Oct 2017 09:31:26 +0200
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     KOBAYASHI Yoshitake <yoshitake.kobayashi@...hiba.co.jp>
Cc:     richard@....at, linux-kernel@...r.kernel.org,
        marek.vasut@...il.com, linux-mtd@...ts.infradead.org,
        cyrille.pitchen@...ev4u.fr, computersforpeace@...il.com,
        dwmw2@...radead.org
Subject: Re: [PATCH -next v2] mtd: nand: Add support for Toshiba BENAND
 (Built-in ECC NAND)

On Thu, 5 Oct 2017 16:24:08 +0900
KOBAYASHI Yoshitake <yoshitake.kobayashi@...hiba.co.jp> wrote:

> On 2017/09/27 6:15, Boris Brezillon wrote:
> > On Tue, 26 Sep 2017 18:18:04 +0900
> > KOBAYASHI Yoshitake <yoshitake.kobayashi@...hiba.co.jp> wrote:
> >   
> >> On 2017/09/21 16:28, Boris Brezillon wrote:  
> >>> On Thu, 21 Sep 2017 14:32:02 +0900
> >>> KOBAYASHI Yoshitake <yoshitake.kobayashi@...hiba.co.jp> wrote:
> >>>     
> >>>> This patch enables support for Toshiba BENAND.
> >>>> The current implementation does not support vondor specific command    
> >>>
> >>> 					       ^ vendor
> >>>     
> >>>> TOSHIBA_NAND_CMD_ECC_STATUS. I would like to add the command, when
> >>>> the exec_op() [1] infrastructure is implemented.    
> >>>
> >>> It's not a good idea to reference a branch that is likely to disappear
> >>> in a commit message. Just say that you can't properly support the
> >>> TOSHIBA_NAND_CMD_ECC_STATUS operation right and that it might be
> >>> addressed in the future.    
> >>
> >> Thanks. I'll change the comment.
> >>  
> >>>> +	 */
> >>>> +	if (status & NAND_STATUS_FAIL) {
> >>>> +		/* uncorrectable */
> >>>> +		mtd->ecc_stats.failed++;
> >>>> +	} else if (status & TOSHIBA_NAND_STATUS_REWRITE_RECOMMENDED) {
> >>>> +		/* correctable */
> >>>> +		max_bitflips = mtd->bitflip_threshold;
> >>>> +		mtd->ecc_stats.corrected += max_bitflips;
> >>>> +	}    
> >>>
> >>> Is this working correctly when you read more than one ECC chunk? The
> >>> ECC step size is 512 bytes and the page is bigger than that, which means
> >>> you have more than one ECC chunk per page. What happens to the
> >>> NAND_STATUS_FAIL flag if the first chunk is uncorrectable but
> >>> following ones are correctable (or do not contain bitflips at all)?     
> >>
> >> As you mentioned, the ECC step size is 512 byte. But when 0x70 command is used
> >> a simplified ECC status per page is generated.  
> > 
> > I'm fine with that as long as uncorrectable errors are detected
> > correctly and TOSHIBA_NAND_STATUS_REWRITE_RECOMMENDED is set as soon as
> > one of the ECC chunk exposes too many bitflips.
> >   
> >> In case the first chunk is uncorrectable but the following ones are correctable,
> >> the 0x70 command can only check the status of the uncorrectable one.  
> > 
> > Sounds good.
> >   
> >> Each ECC chunk status can be checked by using the 0x7A command.
> >>  
> >>>> @@ -39,9 +105,43 @@ static void toshiba_nand_decode_id(struct nand_chip *chip)
> >>>>  
> >>>>  static int toshiba_nand_init(struct nand_chip *chip)
> >>>>  {
> >>>> +	struct mtd_info *mtd = nand_to_mtd(chip);
> >>>> +
> >>>>  	if (nand_is_slc(chip))
> >>>>  		chip->bbt_options |= NAND_BBT_SCAN2NDPAGE;
> >>>>  
> >>>> +	if (nand_is_slc(chip) && (chip->id.data[4] & 0x80)) {
> >>>> +		/* BENAND */
> >>>> +
> >>>> +		/*
> >>>> +		 * We can't disable the internal ECC engine, the user
> >>>> +		 * has to use on-die ECC, there is no alternative.
> >>>> +		 */
> >>>> +		if (chip->ecc.mode != NAND_ECC_ON_DIE) {
> >>>> +			pr_err("On-die ECC should be selected.\n");
> >>>> +			return -EINVAL;
> >>>> +		}    
> >>>
> >>> According to your previous explanation that's not exactly true. Since
> >>> ECC bytes are stored in a separate area, the user can decide to use
> >>> another mode without trouble. Just skip the BENAND initialization when
> >>> mode != NAND_ECC_ON_DIE and we should be good, or am I missing something?    
> >>
> >> I am asking to product department to confirm it.  
> > 
> > I'm almost sure this is the case ;-).  
> 
> According to the command sequence written in BENAND's datasheet, the status
> of the internal ECC must be checked after reading. To do that, ecc.mode has been
> set to NAND_ECC_ON_DIE and the status of the internal ECC is checked through 
> the 0x70 or 0x7A command. That's the reason we are returning EINVAL here.

But the status will anyway be retrieved, and what's the point of
checking the ECC flags if the user wants to use its own ECC engine? I
mean, since you have the whole OOB area exposed why would you prevent
existing setup from working (by existing setup I mean those that already
have a BENAND but haven't modified their driver to accept ON_DIE_ECC).

Maybe I'm missing something, but AFAICT it's safe to allow users to
completely ignore the on-die ECC engine and use their own, even if
that means duplicating the work since on-die ECC cannot be disabled on
BENAND devices.