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

[Liang Yang]

Hi Miquel and Arseniy,

On 2023/4/12 20:57, Miquel Raynal wrote:
> [ EXTERNAL EMAIL ]
> 
> Hi Arseniy,
> 
> avkrasnov at sberdevices.ru wrote on Wed, 12 Apr 2023 15:22:26 +0300:
> 
>> On 12.04.2023 15:18, Miquel Raynal wrote:
>>> Hi Arseniy,
>>>
>>> avkrasnov at sberdevices.ru wrote on Wed, 12 Apr 2023 13:14:52 +0300:
>>>    
>>>> On 12.04.2023 12:36, Miquel Raynal wrote:
>>>>> Hi Arseniy,
>>>>>
>>>>> avkrasnov at sberdevices.ru wrote on Wed, 12 Apr 2023 12:20:55 +0300:
>>>>>      
>>>>>> On 12.04.2023 10:44, Miquel Raynal wrote:
>>>>>>> Hi Arseniy,
>>>>>>>
>>>>>>> AVKrasnov at 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 at 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.

> 
> 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
> 

-- 
Thanks,
Liang