Re: [PATCH v4 2/5] mtd: rawnand: meson: move OOB to non-protected ECC area

[Arseniy Krasnov <avkrasnov@sberdevices.ru>]

On 30.05.2023 11:21, Miquel Raynal wrote:
> Hi Arseniy,
> 
> avkrasnov@sberdevices.ru wrote on Tue, 30 May 2023 11:09:10 +0300:
> 
>> Hi Miquel,
>>
>> On 30.05.2023 10:44, Miquel Raynal wrote:
>>> Hi Arseniy,
>>>   
>>>>>>>> -static void meson_nfc_get_user_byte(struct nand_chip *nand, u8 *oob_buf)
>>>>>>>> -{
>>>>>>>> -	struct meson_nfc_nand_chip *meson_chip = to_meson_nand(nand);
>>>>>>>> -	__le64 *info;
>>>>>>>> -	int i, count;
>>>>>>>> +	int i;
>>>>>>>>  
>>>>>>>> -	for (i = 0, count = 0; i < nand->ecc.steps; i++, count += 2) {
>>>>>>>> +	for (i = 0; i < nand->ecc.steps; i++) {
>>>>>>>>  		info = &meson_chip->info_buf[i];
>>>>>>>> -		oob_buf[count] = *info;
>>>>>>>> -		oob_buf[count + 1] = *info >> 8;
>>>>>>>> +		/* Always ignore user bytes programming. */      
>>>>>>>
>>>>>>> Why?      
>>>>>>
>>>>>> I think comment message is wrong a little bit. Here "user bytes" are
>>>>>> user bytes protected by ECC (e.g. location of these bytes differs from new
>>>>>> OOB layout introduced by this patch). During page write this hardware
>>>>>> always writes these bytes along with data. But, new OOB layout always ignores
>>>>>> these 4 bytes, so set them to 0xFF always.    
>>>>>
>>>>> When performing page reads/writes, you need to take the data as it's
>>>>> been provided. You may move the data around in the buffer provided to
>>>>> the controller, so that it get the ECC data at the right location, and
>>>>> you need of course to reorganize the data when reading as well, so that
>>>>> the user sees XkiB of data + YB of OOB. That's all you need to do in
>>>>> these helpers.
>>>>>     
>>>>
>>>> I think there is some misunderstanding about these "user bytes" above: there are 4
>>>> bytes which this NAND controller always writes to page in ECC mode - it was free OOB
>>>> bytes covered by ECC. Controller grabs values from DMA buffer (second DMA buffer which
>>>> doesn't contains page data) and writes it along with data and ECC codes. Idea of this
>>>> change is to always suppress this write by setting them to 0xFF (may be there is some
>>>> command option to not write it, but I don't have doc), because all of them (4 bytes)
>>>> become unavailable to reader/writer.  
>>>
>>> At the NAND controller level, I would rather avoid doing things like
>>> that.
>>>
>>> I believe you can just update the ooblayout so that protected OOB bytes
>>> are not exposed to the user as free bytes. Then your buffers should
>>> already contain 0xffffff at the problematic location.  
>>
>> So Your idea is to continue fill DMA buffer (for these 4 bytes) from provided OOB buffer,
>> relying on that as these bytes are unused, they will be 0xFF in OOB buffer so we get the same result?
> 
> Yes.
> 
> The problem you face is due to jffs2 using free OOB bytes to store some
> data. If this data is in the protected area -> BOOM.
> 
> If another application wants to use all the bytes and writes them all
> in the same PROGRAM operation it's fine.
> 
> Jffs2 accesses the free area through the OOB layouts only, so just
> tweaking the OOB layouts should work.

I see, I'll try this.

Thanks,
Arseniy

> 
> Thanks,
> Miquèl

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