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

From: Boris Brezillon <[email-masked]>
Date: Wed, 27 Feb 2019 15:30:13 +0100

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

> 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.

Ack to all of this.

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