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

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

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).

> 2) If I have access to only 32 bytes of OOB (in case above), I must report that size
>    of OOB is only 32 bytes during initialization?

I believe it's just an implementation error in the controller driver.
This controller can be used on NAND chips with other geometries, please
don't play with it.

Thanks,
Miquèl

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