Subject: Re: [RFC PATCH 25/27] mtd: nand: Add helpers to manage ECC engines and configurations

From: Miquel Raynal <[email-masked]>
Date: Wed, 27 Feb 2019 15:07:13 +0100

Hi Boris,

Boris Brezillon <[email-masked]> wrote on Fri, 22 Feb 2019
15:44:31 +0100:

> On Thu, 21 Feb 2019 13:58:04 +0100
> Miquel Raynal <[email-masked]> wrote:
> 
> > Add the logic in the NAND core to find the right ECC engine depending
> > on the NAND chip requirements and the user desires. Right now, the
> > choice may be made between (more will come):
> > * software Hamming
> > * software BCH
> > * on-die (SPI-NAND devices only)
> > 
> > Once the ECC engine has been found, the ECC engine must be
> > configured.
> > 
> > Signed-off-by: Miquel Raynal <[email-masked]>
> > ---
> >  drivers/mtd/nand/core.c  | 107 +++++++++++++++++++++++++++++++++++++++
> >  include/linux/mtd/nand.h |   4 ++
> >  2 files changed, 111 insertions(+)
> > 
> > diff --git a/drivers/mtd/nand/core.c b/drivers/mtd/nand/core.c
> > index 872d46b5fc0f..9feb118c9f68 100644
> > --- a/drivers/mtd/nand/core.c
> > +++ b/drivers/mtd/nand/core.c
> > @@ -207,6 +207,113 @@ int nanddev_mtd_max_bad_blocks(struct mtd_info *mtd, loff_t offs, size_t len)
> >  }
> >  EXPORT_SYMBOL_GPL(nanddev_mtd_max_bad_blocks);
> >  
> > +/**
> > + * nanddev_find_ecc_engine() - Find a suitable ECC engine
> > + * @nand: NAND device
> > + */
> > +static int nanddev_find_ecc_engine(struct nand_device *nand)  
> 
> Can we pass the conf in argument instead of reading it from
> nand->ecc.user_conf?

I just changed the root structures, there is a

       struct nand_ecc_conf defaults;

entry now in the nand_ecc_engine structure, which every layer/driver
can overwrite to inform the NAND core of a default value.

Later in this function, I check user_conf (which I might call
"desires" now that we have a "defaults" and a "requirements" entries),
but if the value is missing I fallback to the "defaults" one if filled.
Having all these structures being passed as parameters does not make
sense to me so I would prefer sticking to the single "nand" parameter.

> 
> > +{
> > +	bool is_spinand = mtd_type_is_spinand(&nand->mtd);  
> 
> And here is the reason for the SPINAND type.

This does not exist anymore as you suggested.

> 
> > +
> > +	/* Read the user desires in terms of ECC engine/configuration */
> > +	nand_ecc_read_user_conf(nand);
> > +
> > +	/* No ECC engine requestedn, let's return without error */
> > +	if (nand->ecc.user_conf.mode == NAND_ECC_NONE)
> > +		return 0;
> > +
> > +	/* Raw NAND default mode is hardware */
> > +	if (!is_spinand && nand->ecc.user_conf.mode < 0)
> > +		nand->ecc.user_conf.mode = NAND_ECC_HW;  
> 
> We should let the raw NAND layer take this decision (actually, it's
> even a raw NAND controller driver decision). Please complain if
> user_conf.mode is invalid.
> This way you won't need the SPINAND type you added in one of your
> previous patch.

See above for the solution I choose.

> 
> > +
> > +	/* SPI-NAND default mode is on-die */
> > +	if (is_spinand && nand->ecc.user_conf.mode < 0)
> > +		nand->ecc.user_conf.mode = NAND_ECC_ON_DIE;
> > +
> > +	switch (nand->ecc.user_conf.mode) {
> > +	case NAND_ECC_SOFT:
> > +		nand->ecc.engine = nand_ecc_sw_get_engine(nand);
> > +		break;
> > +	case NAND_ECC_ON_DIE:
> > +		if (is_spinand)
> > +			nand->ecc.engine = spinand_ondie_ecc_get_engine();  
> 
> So, maybe it's worth having the ondie ECC engine instance directly
> embedded in nand_device instead of spinand, or maybe just a pointer, so
> that you don't reserve extra space when the NAND device does not have
> on-die ECC.

It is now: the nand_ecc_engine structure features a *engine and a
*ondie_engine pointer.

The nand_ecc_get_ondie_engine(nand) helper just
returns nand->ecc.ondie_engine.

> 
> > +		else
> > +			pr_err("On-die ECC engines for non SPI devices not supported yet\n");
> > +		break;
> > +	case NAND_ECC_HW:
> > +		pr_err("Hardware ECC engines not supported yet\n");
> > +		break;
> > +	default:
> > +		pr_err("Missing ECC engine property\n");
> > +	}
> > +
> > +	if (!nand->ecc.engine)
> > +		return  -EINVAL;
> > +
> > +	return 0;
> > +}  
> 

Thanks,
Miquèl

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