Re: [RFC PATCH 0/3] Fix proposal for the Micron shallow erase issue

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Boris,

Boris Brezillon <boris.brezillon@collabora.com> wrote on Wed, 15 Jan
2020 08:58:06 +0100:

> On Tue, 14 Jan 2020 20:46:17 +0000
> "Wojtaszczyk, Piotr" <WojtaszczykP@cumminsallison.com> wrote:
> 
> > On 1/14/20 3:12 AM, Miquel Raynal wrote:  
> > > Crap. I'll not resend immediately as this is an RFC, I expect
> > > feedback on this proposal before sending an actual patch.
> > > 
> > > 
> > > Thanks,
> > > Miquèl
> > >     
> > 
> > Hi Miquèl, here are some my comments:
> > 
> > +static int micron_nand_avoid_shallow_erase(struct nand_chip *chip,
> > +					   unsigned int eb)
> > +{
> > +	struct micron_nand *micron = nand_get_manufacturer_data(chip);
> > +	unsigned int page = eb * nanddev_pages_per_eraseblock(&chip->base);
> > +	u8 *databuf = nand_get_data_buf(chip);
> > +	int ret, i;
> > +
> > +	memset(databuf, 0xFF, nanddev_page_size(&chip->base));
> > +
> > +	/* Micron advises to only write odd pages */
> > +	for (i = 0; i < MICRON_SHALLOW_ERASE_MIN_PAGE; i += 2, page += 2) {
> > +		if (!(micron->writtenp[eb] & BIT(i))) {
> > +			ret = nand_write_page_raw(chip, databuf, false, page);
> > +			if (ret)
> > +				return ret;
> > +		}
> > +	}
> > +
> > +	return 0;
> > +}
> > 
> > Shouldn't we program only the OOB area of the pages to 0'es? Programming pages to 0xFF 
> > which are already 0xFF takes more time and doesn't make any difference.  
> 
> Hm, I'm pretty sure we should set in-band data to 0, not 0xff.
> Programming only the OOB portion might not be enough for the internal
> "is page written?" logic to return true.

Absolutely, this is a mistake, the idea is to program all cells (to 0).

> > Also after power loss all flags in micron->writtenp are gone so the 
> > micron_nand_avoid_shallow_erase will perform on all PEBs causing performance loss.  
> 
> Yes, that's a performance hit we'll have to accept for now.
> 

This is quite severe issue, this is the best idea we came with to
limit performance hits.

Thanks,
Miquèl

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