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

[Miquel Raynal]

Hi Arseniy,

avkrasnov@xxxxxxxxxxxxxx wrote on Wed, 12 Apr 2023 12:20:55 +0300:

> On 12.04.2023 10:44, Miquel Raynal wrote:
> > Hi Arseniy,
> > 
> > AVKrasnov@xxxxxxxxxxxxxx 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@xxxxxxxxxxxxxx>
> >> ---
> >>  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.

> 
> Thanks, Arseniy
> 
> >   
> >> +
> >>  	ret = meson_nfc_read_page_sub(nand, page, 1);
> >>  	if (ret)
> >>  		return ret;
> >> @@ -881,6 +884,8 @@ static int meson_nfc_read_page_hwecc(struct nand_chip *nand, u8 *buf,
> >>  	u8 *oob_buf = nand->oob_poi;
> >>  	int ret, i;
> >>  
> >> +	memset(oob_buf, 0, mtd->oobsize);
> >> +
> >>  	ret = meson_nfc_read_page_sub(nand, page, 0);
> >>  	if (ret)
> >>  		return ret;  
> > 
> > 
> > Thanks,
> > Miquèl  


Thanks,
Miquèl