Re: [PATCH v1 4/5] mtd: rawnand: meson: clear OOB buffer before read

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

Hi Dmitry,

ddrokosov@sberdevices.ru wrote on Wed, 12 Apr 2023 22:15:48 +0300:

> On Wed, Apr 12, 2023 at 10:04:28PM +0800, Liang Yang wrote:
> > Hi Miquel and Arseniy,
> > 
> > On 2023/4/12 20:57, Miquel Raynal wrote:  
> > > [ EXTERNAL EMAIL ]
> > > 
> > > Hi Arseniy,
> > > 
> > > avkrasnov@sberdevices.ru wrote on Wed, 12 Apr 2023 15:22:26 +0300:
> > >   
> > > > On 12.04.2023 15:18, Miquel Raynal wrote:  
> > > > > Hi Arseniy,
> > > > > 
> > > > > avkrasnov@sberdevices.ru wrote on Wed, 12 Apr 2023 13:14:52 +0300:  
> > > > > > On 12.04.2023 12:36, Miquel Raynal wrote:  
> > > > > > > Hi Arseniy,
> > > > > > > 
> > > > > > > avkrasnov@sberdevices.ru wrote on Wed, 12 Apr 2023 12:20:55 +0300:  
> > > > > > > > On 12.04.2023 10:44, Miquel Raynal wrote:  
> > > > > > > > > Hi Arseniy,
> > > > > > > > > 
> > > > > > > > > AVKrasnov@sberdevices.ru wrote on Wed, 12 Apr 2023 09:16:58 +0300:  
> > > > > > > > > > This NAND reads only few user's bytes in ECC mode (not full OOB), so  
> > > > > > > > > 
> > > > > > > > > "This NAND reads" does not look right, do you mean "Subpage reads do
> > > > > > > > > not retrieve all the OOB bytes,"?  
> > > > > > > > > > fill OOB buffer with zeroes to not return garbage from previous reads
> > > > > > > > > > to user.
> > > > > > > > > > Otherwise 'nanddump' utility prints something like this for just erased
> > > > > > > > > > page:
> > > > > > > > > > 
> > > > > > > > > > ...
> > > > > > > > > > 0x000007f0: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > > > > > > > > >    OOB Data: ff ff ff ff 00 00 ff ff 80 cf 22 99 cb ad d3 be
> > > > > > > > > >    OOB Data: 63 27 ae 06 16 0a 2f eb bb dd 46 74 41 8e 88 6e
> > > > > > > > > >    OOB Data: 38 a1 2d e6 77 d4 05 06 f2 a5 7e 25 eb 34 7c ff
> > > > > > > > > >    OOB Data: 38 ea de 14 10 de 9b 40 33 16 6a cc 9d aa 2f 5e
> > > > > > > > > > 
> > > > > > > > > > Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
> > > > > > > > > > ---
> > > > > > > > > >   drivers/mtd/nand/raw/meson_nand.c | 5 +++++
> > > > > > > > > >   1 file changed, 5 insertions(+)
> > > > > > > > > > 
> > > > > > > > > > diff --git a/drivers/mtd/nand/raw/meson_nand.c b/drivers/mtd/nand/raw/meson_nand.c
> > > > > > > > > > index f84a10238e4d..f2f2472cb511 100644
> > > > > > > > > > --- a/drivers/mtd/nand/raw/meson_nand.c
> > > > > > > > > > +++ b/drivers/mtd/nand/raw/meson_nand.c
> > > > > > > > > > @@ -858,9 +858,12 @@ static int meson_nfc_read_page_sub(struct nand_chip *nand,
> > > > > > > > > >   static int meson_nfc_read_page_raw(struct nand_chip *nand, u8 *buf,
> > > > > > > > > >   				   int oob_required, int page)
> > > > > > > > > >   {
> > > > > > > > > > +	struct mtd_info *mtd = nand_to_mtd(nand);
> > > > > > > > > >   	u8 *oob_buf = nand->oob_poi;
> > > > > > > > > >   	int ret;
> > > > > > > > > > +	memset(oob_buf, 0, mtd->oobsize);  
> > > > > > > > > 
> > > > > > > > > I'm surprised raw reads do not read the entire OOB?  
> > > > > > > > 
> > > > > > > > Yes! Seems in case of raw access (what i see in this driver) number of OOB bytes read
> > > > > > > > still depends on ECC parameters: for each portion of data covered with ECC code we can
> > > > > > > > read it's ECC code and "user bytes" from OOB - it is what i see by dumping DMA buffer by
> > > > > > > > printk(). For example I'm working with 2K NAND pages, each page has 2 x 1K ECC blocks.
> > > > > > > > For each ECC block I have 16 OOB bytes which I can access by read/write. Each 16 bytes
> > > > > > > > contains 2 bytes of user's data and 14 bytes ECC codes. So when I read page in raw mode
> > > > > > > > controller returns 32 bytes (2 x (2 + 14)) of OOB. While OOB is reported as 64 bytes.  
> > > > > > > 
> > > > > > > In all modes, when you read OOB, you should get the full OOB. The fact
> > > > > > > that ECC correction is enabled or disabled does not matter. If the NAND
> > > > > > > features OOB sections of 64 bytes, you should get the 64 bytes.
> > > > > > > 
> > > > > > > What happens sometimes, is that some of the bytes are not protected
> > > > > > > against bitflips, but the policy is to return the full buffer.  
> > > > > > 
> > > > > > Ok, so to clarify case for this NAND controller:
> > > > > > 1) In both ECC and raw modes i need to return the same raw OOB data (e.g. user bytes
> > > > > >     + ECC codes)?  
> > > > > 
> > > > > Well, you need to cover the same amount of data, yes. But in the ECC
> > > > > case the data won't be raw (at least not all of it).  
> > > > 
> > > > So "same amount of data", in ECC mode current implementation returns only user OOB bytes (e.g.
> > > > OOB data excluding ECC codes), in raw it returns user bytes + ECC codes. IIUC correct
> > > > behaviour is to always return user bytes + ECC codes as OOB data even in ECC mode ?  
> > > 
> > > If the page are 2k+64B you should read 2k+64B when OOB are requested.
> > > 
> > > If the controller only returns 2k+32B, then perform a random read to
> > > just move the read pointer to mtd->size + mtd->oobsize - 32 and
> > > retrieve the missing 32 bytes?  
> > 
> > 1) raw read can read out the whole page data 2k+64B, decided by the len in
> > the controller raw read command:
> > 	cmd = (len & GENMASK(5, 0)) | scrambler | DMA_DIR(dir);
> > after that, the missing oob bytes(not used) can be copied from
> > meson_chip->data_buf. so the implementation of meson_nfc_read_page_raw() is
> > like this if need.
> > 	{
> > 		......
> > 		meson_nfc_read_page_sub(nand, page, 1);
> > 		meson_nfc_get_data_oob(nand, buf, oob_buf);
> > 		oob_len = (nand->ecc.bytes + 2) * nand->ecc.steps;
> > 		memcpy(oob_buf + oob_len, meson_chip->data_buf + oob_len, mtd->oobsize -
> > oob_len);
> > 
> > 	}
> > 2) In ECC mode, the controller can't bring back the missing OOB bytes. it
> > can read out the user bytes and ecc bytes per meson_ooblayout_ops define.
> >   
> 
> How does the Meson controller know the actual NAND flash layout when the
> OOB is split into protected and unprotected areas, such as Free and ECC
> areas? If the controller has a static OOB layout, where is the mapping
> located?

It's usually a set of values hardcoded in the driver. It's a per
geometry set.

> 
> > > 
> > > This applies to the two modes, the only difference is:
> > > - with correction (commonly named "ECC mode"): the user bytes and ECC
> > >    bytes should be fixed if there are any bitflips
> > > - without correction (commonly referred as "raw mode"): no correction
> > >    applies, if there are bitflips, give them
> > > 
> > > Please mind the raw mode can be slow, it's meant for debugging and
> > > testing, mainly. Page reads however should be fast, so if just moving
> > > the column pointer works, then do it, otherwise we'll consider
> > > returning FFs.  
> 


Thanks,
Miquèl

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