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

[Liang Yang <liang.yang@amlogic.com>]

Hi Miquel,

On 2023/4/12 22:32, Miquel Raynal wrote:
> [ EXTERNAL EMAIL ]
> 
> Hello,
> 
> liang.yang@amlogic.com wrote on Wed, 12 Apr 2023 22:04:28 +0800:
> 
>> 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.
> 
> And then (if oob_required) you can bring the missing bytes with
> something along:
> nand_change_read_column_op(chip, mtd->writesize + oob_len,
> 			   oob_buf + oob_len,
> 			   mtd->oobsize - oob_len,
> 			   false);
> Should not be a huge performance hit.

After finishing ECC mode reading, the column address internal in NAND 
device should be the right pos; it doesn't need to change the column 
again. so adding controller raw read for the missing bytes after ECC 
reading may works.

> 
>>
>>>
>>> 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,
> Miquèl
> 

-- 
Thanks,
Liang

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