[PATCH 8/9] mtd: nand: qcom: helper function for raw read

[Abhishek Sahu]

On 2018-04-23 12:28, Miquel Raynal wrote:
> Hi Abhishek,
> 
> On Mon, 23 Apr 2018 11:58:42 +0530, Abhishek Sahu
> <absahu at codeaurora.org> wrote:
> 
>> On 2018-04-22 21:49, Miquel Raynal wrote:
>> > Hi Abhishek,
>> > > On Thu, 12 Apr 2018 12:36:42 +0530, Abhishek Sahu
>> > <absahu at codeaurora.org> wrote:
>> > >> On 2018-04-10 15:14, Miquel Raynal wrote:
>> >> > Hi Abhishek,
>> >> > > On Wed,  4 Apr 2018 18:12:24 +0530, Abhishek Sahu
>> >> > <absahu at codeaurora.org> wrote:
>> >> > >> This patch does minor code reorganization for raw reads.
>> >> >> Currently the raw read is required for complete page but for
>> >> >> subsequent patches related with erased codeword bit flips
>> >> >> detection, only few CW should be read. So, this patch adds
>> >> >> helper function and introduces the read CW bitmask which
>> >> >> specifies which CW reads are required in complete page.
>> >> > > I am not sure this is the right approach for subpage reads. If the
>> >> > controller supports it, you should just enable it in chip->options.
>> >> >
>> >>   Thanks Miquel.
>> >> >>   It is internal to this file only. This patch makes one static helper
>> >>   function which provides the support to read subpages.
>> > > Drivers should expose raw helpers, why keeping this helper static then?
>> > >> >> >> >> Signed-off-by: Abhishek Sahu <absahu at codeaurora.org>
>> >> >> ---
>> >> >>  drivers/mtd/nand/qcom_nandc.c | 186 >> +++++++++++++++++++++++++-----------------
>> >> >>  1 file changed, 110 insertions(+), 76 deletions(-)
>> >> >> >> diff --git a/drivers/mtd/nand/qcom_nandc.c >> b/drivers/mtd/nand/qcom_nandc.c
>> >> >> index 40c790e..f5d1fa4 100644
>> >> >> --- a/drivers/mtd/nand/qcom_nandc.c
>> >> >> +++ b/drivers/mtd/nand/qcom_nandc.c
>> >> >> @@ -1590,6 +1590,114 @@ static int check_flash_errors(struct >> qcom_nand_host *host, int cw_cnt)
>> >> >>  }
>> >> >> >>  /*
>> >> >> + * Helper to perform the page raw read operation. The read_cw_mask >> will be
>> >> >> + * used to specify the codewords for which the data should be read. >> The
>> >> >> + * single page contains multiple CW. Sometime, only few CW data is >> required
>> >> >> + * in complete page. Also, start address will be determined with
>> >> >> + * this CW mask to skip unnecessary data copy from NAND flash device. >> Then,
>> >> >> + * actual data copy from NAND controller to data buffer will be done >> only
>> >> >> + * for the CWs which have the mask set.
>> >> >> + */
>> >> >> +static int
>> >> >> +nandc_read_page_raw(struct mtd_info *mtd, struct nand_chip *chip,
>> >> >> +		    u8 *data_buf, u8 *oob_buf,
>> >> >> +		    int page, unsigned long read_cw_mask)
>> >> >> +{

  <snip>

>> >> >> -	free_descs(nandc);
>> >> >> -
>> >> >> -	if (!ret)
>> >> >> -		ret = check_flash_errors(host, ecc->steps);
>> >> >> -
>> >> >> -	return 0;
>> >> >> +	return nandc_read_page_raw(mtd, chip, buf, chip->oob_poi, page,
>> >> >> +				   BIT(chip->ecc.steps) - 1);
>> >> > > I don't understand this. chip->ecc.steps is constant, right? So you
>> >> > always ask for the same subpage?
>> >> >>   We need to do raw read for subpages in which we got uncorrectable
>> >>   error in next patch for erased page bitflip detection. This patch >> does
>> >>   reorganization of raw read and moves common code in helper function
>> >>   nandc_read_page_raw.
>> >> >>   For nomral raw read, all the subpages will be read so
>> >>   BIT(chip->ecc.steps) - 1 is used for qcom_nandc_read_page_raw.
>> >> >>   While for erased page raw read, only those sub pages will be
>> >>   read for which the controller gives the uncorrectable error.
>> > > Still not okay: the driver should expose a way to do raw reads no
>> > matter the length and the start and you should use that in a generic
>> > way.
>> >
>>   Thanks Miquel.
>>   I will explore regarding that.
>>   Looks like, we can some helper like read_subpage.
> 
> Of course, when you implement raw accessors you can have static helpers
> to clarify the code.
> 

  Hi Miquel,

  I checked regarding generic function.

  Normally the other NAND controller stores the data in main area
  and ecc bytes in oob area. So if page size is 2048+64 then 2048
  data bytes will go in main area followed by ECC bytes

  Main                 |        OOB
  ------------------------------------
  D1 D2...........D2048|E1..........E64

  The QCOM nand contoller follows different layout in which this
  2048+64 is internally divided in 528 bytes codeword so it will
  come to 4 codewords. Each codeword contains 516 bytes followed
  by ECC parity bytes. Also in at 0x1f0 offset in each CW, the bad
  block marker is stored. so each CW contains badblock marker also.
  The avilable OOB bytes are 16 which is also protected by ECC.

  516 * 4 = 2048 data bytes + 16 available oob bytes.

  So for QCOM layout, it will be something lile


  0       D1.........D495 B1 D496....D516 E1...E7  U1..U4
  0x210   D517......D1012 B2 D1016..D1032 E8...E14 U5..U8
  0x420   D1033.....D1528 B3 D1529..D1548 E15..E21 U9..U12
  0x630   D1549.....D2044 B4 D2045..D2048 O1...O16 E22 E28 U13..U16

  Where D - Data bytes
        B - BBM bytes
        E - ECC parity bytes
        U - Unused bytes
        O - Avilable OOB bytes

  Now, the raw read expect data to be returned in buffer in the
  form of

  D1.....D2048 B1 E1...E7 U1..U4 B2 E8...E14 U5..U8 B3 E15..E21
  U9..U12 B4 O1...O16 E22 E28 U13..U16

  Now, the puporose of that helper function is read selected
  codewords. This codeword is QCOM specific and other controller
  normally doesn't follow this kind of layout.

  Now If we make generic fuction for subpgage raw read with any
  start and length then that function we can't use for erased page
  bitflip detection.

  If we give the start as 0 with length 528 then our requirement is
  to get 516 data bytes in data buffer and 12 bytes in OOB buffer
  but according to standard operations, it will be interpreted as
  all data bytes from D1..D528

  so that helper function is to read selected Codewords from
  QCOM NAND controller. This Codeword is different from subpages

  Regarding more info for layout, you can refer comment in
  
https://elixir.bootlin.com/linux/latest/source/drivers/mtd/nand/qcom_nandc.c#L2191

  Thanks,
  Abhishek