Re: [PATCH 1/9] mtd: nand: qcom: use the ecc strength from device parameter

[Abhishek Sahu <absahu@codeaurora.org>]

On 2018-04-06 18:01, Miquel Raynal wrote:
> Hi Abhishek,
> 
> On Wed,  4 Apr 2018 18:12:17 +0530, Abhishek Sahu
> <absahu@codeaurora.org> wrote:
> 
>> Currently the driver uses the ECC strength specified in
>> device tree. The ONFI or JEDEC device parameter page
>> contains the ‘ECC correctability’ field which indicates the
>> number of bits that the host should be able to correct per
>> 512 bytes of data.
> 
> This is misleading. This field is not about the controller but rather
> the chip requirements in terms of minimal strength for nominal use.
> 

  Thanks Miquel.

  Yes. Its NAND chip requirement. I have used the description for
  NAND ONFI param page

  5.6.1.24. Byte 112: Number of bits ECC correctability
  This field indicates the number of bits that the host should be
  able to correct per 512 bytes of data.

>> The ecc correctability is assigned in
>> chip parameter during device probe time. QPIC/EBI2 NAND
>> supports 4/8-bit ecc correction. The Same kind of board
>> can have different NAND parts so use the ecc strength
>> from device parameter (if its non zero) instead of
>> device tree.
> 
> That is not what you do.
> 
> What you do is forcing the strength to be 8-bit per ECC chunk if the
> NAND chip requires at least 8-bit/chunk strength.
> 
> The DT property is here to force a strength. Otherwise, Linux will
> propose to the NAND controller to use the minimum strength required by
> the chip (from either the ONFI/JEDEC parameter page or from a static
> table).
> 

  The main problem is that the same kind of boards can have different
  NAND parts.

  Lets assume, we have following 2 cases.

  1. Non ONFI/JEDEC device for which chip->ecc_strength_ds
     will be zero. In this case, the ecc->strength from DT
     will be used
  2. ONFI/JEDEC device for which chip->ecc_strength_ds > 8.
     Since QCOM nand controller can not support
     ECC correction greater than 8 bits so we can use 8 bit ECC
     itself instead of failing NAND boot completely.

> IMHO, you have two solutions:
> 1/ Remove these properties from the board DT (breaks DT backward
> compatibility though);

  - nand-ecc-strength: This is optional property in nand.txt and
    Required property in qcom_nandc.txt. We can't remove since
    if the device is Non ONFI/JEDEC, then ecc strength will come
    from DT only.

> 2/ Create another DT for the board.
> 

  Its not about board but about part. We have IPQ8074 HK01 board
  with 4 bit ECC chip/8 bit ECC chip/non ONFI/JEDEC chip.

> However, there is something to do in this driver: the strength chosen
> should be limited to the controller capabilities (8-bit/512B if I
> understand correctly). In this case you have two options: either you
> limit the strength like the size [1] if (ecc->strength > 8);

  Limiting the strength will make all the boards with ecc strength > 8
  to fail completely

> just limit the maximum strength to 8 like this [2] and the core will
> spawn a warning in the dmesg telling that the ECC strength used is
> below the NAND chip requirements.

  Yes its good idea. I can update the patch with dmesg warning.

  Thanks,
  Abhishek

> 
> Thanks,
> Miquèl
> 
> [1]
> https://elixir.bootlin.com/linux/latest/source/drivers/mtd/nand/qcom_nandc.c#L2332
> [2] http://code.bulix.org/nyf63w-315268
> 
> 
>> 
>> Signed-off-by: Abhishek Sahu <absahu@codeaurora.org>
>> ---
>>  drivers/mtd/nand/qcom_nandc.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>> 
>> diff --git a/drivers/mtd/nand/qcom_nandc.c 
>> b/drivers/mtd/nand/qcom_nandc.c
>> index 563b759..8dd40de 100644
>> --- a/drivers/mtd/nand/qcom_nandc.c
>> +++ b/drivers/mtd/nand/qcom_nandc.c
>> @@ -2334,6 +2334,14 @@ static int qcom_nand_host_setup(struct 
>> qcom_nand_host *host)
>>  		return -EINVAL;
>>  	}
>> 
>> +	/*
>> +	 * Read the required ecc strength from NAND device and overwrite
>> +	 * the device tree ecc strength for devices which require
>> +	 * ecc correctability bits >= 8
>> +	 */
>> +	if (chip->ecc_strength_ds >= 8)
>> +		ecc->strength = 8;
>> +
>>  	wide_bus = chip->options & NAND_BUSWIDTH_16 ? true : false;
>> 
>>  	if (ecc->strength >= 8) {