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

From: Boris Brezillon <[email-masked]>
Date: Mon, 25 Feb 2019 17:34:22 +0100

On Mon, 25 Feb 2019 17:01:18 +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?
> >   
> > > +{
> > > +	bool is_spinand = mtd_type_is_spinand(&nand->mtd);    
> > 
> > And here is the reason for the SPINAND type.
> >   
> > > +
> > > +	/* 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.
> >   
> > > +
> > > +	/* 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.
> >   
> > > +		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;
> > > +}    
> >   
> 
> I think this is the root patch were our ideas diverge. For me, each
> 'ECC engine' has a _get() helper and the NAND core decides which one to
> call to retrieve the right engine. Can you please explain what was your
> idea if this one does not fit?

Each class of ECC engine has its own way of getting a pointer to an ECC
engine instance:

- ondie: the engine is directly attached to the device and can be
  retrieved by accessing a nand_device field (nanddev->ondie_ecc?).
- sw ECC: you can create a new instance for each device and possibly
  pass the OOB layout you want to use (assuming you don't want to use
  the default one)
- HW-controller-side engine: refers to the controller device (parent
  node) or the device pointed by the ecc-engine DT prop (we'll probably
  need a way to pass this info when for non-DT platforms). For this one
  we'll add a nanddev_get_hw_ecc_engine() which will search for all
  registered HW ECC engines and try to match with some search keys (in
  case of DT, it's the ->of_node pointer address)

Clearly, for SW-based ECC, you'll need more than just the nanddev
object, as layouts can differ depending on the controller driver. So,
what I'm suggesting is to have a

	nand_create_sw_ecc_engine(algo, layout) 

funtion that returns this ECC engine instance which the driver will
then attach to the NAND device.

> 
> Also, the parsing of the DT (in nand_ecc_read_user_conf()) gives me the
> user ECC mode and algo, so I cannot let the raw NAND core (or a raw
> NAND controller driver) or the SPI NAND core decide which mode is the
> default if not provided by the user.

Except this prop is optional in most cases, and the default value is
not always the same, which is why I think this ECC engine retrieval step
should be left to each sub-layer (and sometimes to the controller driver
behind it). Maybe you can provide helpers to help with that, but I
don't think taking this decision here, based on the bus type, is a good
idea. And I also don't like the idea of adding a new SPINAND type.

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