Date:   Fri, 20 Oct 2017 09:09:25 +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 Fri, 20 Oct 2017 13:52:32 +0900
KOBAYASHI Yoshitake <yoshitake.kobayashi@...hiba.co.jp> wrote:

> On 2017/10/12 22:26, Boris Brezillon wrote:
> > On Thu, 12 Oct 2017 22:03:23 +0900
> > KOBAYASHI Yoshitake <yoshitake.kobayashi@...hiba.co.jp> wrote:
> >   
> >> On 2017/10/05 16:31, Boris Brezillon wrote:  
> >>> On Thu, 5 Oct 2017 16:24:08 +0900
> >>> KOBAYASHI Yoshitake <yoshitake.kobayashi@...hiba.co.jp> wrote:    
> >>>>>>>> @@ -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.    
> >>
> >> If user host controller ECC engine can support 8bit ECC or more ,
> >> Toshiba offers 24nm SLC NAND products (not BENAND).  If user host
> >> controller ECC engine is less that 8bit ECC (for example: 1bit or
> >> 4bit ECC) Toshiba offers BENAND.  When using BENAND, checking
> >> BENAND own ECC status (ECC flag) is required as per BENAND
> >> product datasheet. Ignoring BENAND on-die ECC operation status,
> >> and rely only on host 1 bit ECC or 4 bit ECC status, is not
> >> recommended because the host ECC capability is inferior to BENAND
> >> 8bit ECC and data refresh or other operations may not work
> >> properly.  
> > 
> > Well, that's not really your problem. The framework already complains
> > if someone tries to use an ECC that is weaker than the chip
> > requirement. On the other hand, it's perfectly valid to use on
> > host-side ECC engine that meets NAND requirements (8bit/xxxbytes).  
> 
> I have assumed to specify the ecc strength and size by devicetree.
> Before BENAND patch updated, I would like to submit a patch which read
> the ECC strength and size from the nand using extended-id like the
> Samsung nand patch[1].

Sure.

>  [1] https://patchwork.ozlabs.org/patch/712549/
> 
> > The use case I'm trying to gracefully handle here is: your NAND
> > controller refuses to use anything but the host-side ECC engine and you
> > have a BENAND connected to this controller.
> > Before your patch this use case worked just fine, and the user didn't
> > even notice it was using a NAND chip that was capable of correcting
> > bitflips. After your patch it fails to probe the NAND chip and users
> > will have to patch their controller driver to make it work again. Sorry
> > but this is not really an option: we have to keep existing setup in a
> > working state, and that means allowing people to use their BENAND in a
> > degraded state where they'll just ignore the on-die ECC and use their
> > own ECC engine instead.
> > 
> > I really don't see the problem here. It's not worse than it was before
> > your patch, and those wanting to activate on-die ECC support will have
> > to patch their controller driver anyway.  
> 
> If the above approach is acceptable, I will update BENAND patch according to
> your idea.

Okay. BTW, an ->exec_op() implementation has been posted earlier
this week [1], and since you are depending on it to support your custom
TOSHIBA_NAND_CMD_ECC_STATUS command, that'd be great to have a review
from you.

[1]http://patchwork.ozlabs.org/project/linux-mtd/list/?series=8923

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