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 15:22:26 +0300:

> On 12.04.2023 15:18, Miquel Raynal wrote:
> > Hi Arseniy,
> > 
> > avkrasnov@xxxxxxxxxxxxxx wrote on Wed, 12 Apr 2023 13:14:52 +0300:
> >   
> >> On 12.04.2023 12:36, Miquel Raynal wrote:  
> >>> 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.    
> >>
> >> 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?

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