On Fri, 18 Mar 2016 16:49:04 +0100
Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> wrote:
> Hi Archit,
>
> On Wed, 3 Feb 2016 14:29:50 +0530
> Archit Taneja <architt@xxxxxxxxxxxxxx> wrote:
>
> > +/*
> > + * NAND controller page layout info
> > + *
> > + * Layout with ECC enabled:
> > + *
> > + * |----------------------| |---------------------------------|
> > + * | xx.......yy| | *********xx.......yy|
> > + * | DATA xx..ECC..yy| | DATA **SPARE**xx..ECC..yy|
> > + * | (516) xx.......yy| | (516-n*4) **(n*4)**xx.......yy|
> > + * | xx.......yy| | *********xx.......yy|
> > + * |----------------------| |---------------------------------|
> > + * codeword 1,2..n-1 codeword n
> > + * <---(528/532 Bytes)--> <-------(528/532 Bytes)--------->
> > + *
> > + * n = Number of codewords in the page
> > + * . = ECC bytes
> > + * * = Spare/free bytes
> > + * x = Unused byte(s)
> > + * y = Reserved byte(s)
> > + *
> > + * 2K page: n = 4, spare = 16 bytes
> > + * 4K page: n = 8, spare = 32 bytes
> > + * 8K page: n = 16, spare = 64 bytes
> > + *
> > + * the qcom nand controller operates at a sub page/codeword level. each
> > + * codeword is 528 and 532 bytes for 4 bit and 8 bit ECC modes respectively.
> > + * the number of ECC bytes vary based on the ECC strength and the bus width.
> > + *
> > + * the first n - 1 codewords contains 516 bytes of user data, the remaining
> > + * 12/16 bytes consist of ECC and reserved data. The nth codeword contains
> > + * both user data and spare(oobavail) bytes that sum up to 516 bytes.
> > + *
> > + * When we access a page with ECC enabled, the reserved bytes(s) are not
> > + * accessible at all. When reading, we fill up these unreadable positions
> > + * with 0xffs. When writing, the controller skips writing the inaccessible
> > + * bytes.
> > + *
> > + * Layout with ECC disabled:
> > + *
> > + * |------------------------------| |---------------------------------------|
> > + * | yy xx.......| | bb *********xx.......|
> > + * | DATA1 yy DATA2 xx..ECC..| | DATA1 bb DATA2 **SPARE**xx..ECC..|
> > + * | (size1) yy (size2) xx.......| | (size1) bb (size2) **(n*4)**xx.......|
> > + * | yy xx.......| | bb *********xx.......|
> > + * |------------------------------| |---------------------------------------|
> > + * codeword 1,2..n-1 codeword n
> > + * <-------(528/532 Bytes)------> <-----------(528/532 Bytes)----------->
> > + *
> > + * n = Number of codewords in the page
> > + * . = ECC bytes
> > + * * = Spare/free bytes
> > + * x = Unused byte(s)
> > + * y = Dummy Bad Bock byte(s)
> > + * b = Real Bad Block byte(s)
> > + * size1/size2 = function of codeword size and 'n'
> > + *
> > + * when the ECC block is disabled, one reserved byte (or two for 16 bit bus
> > + * width) is now accessible. For the first n - 1 codewords, these are dummy Bad
> > + * Block Markers. In the last codeword, this position contains the real BBM
> > + *
> > + * In order to have a consistent layout between RAW and ECC modes, we assume
> > + * the following OOB layout arrangement:
> > + *
> > + * |-----------| |--------------------|
> > + * |yyxx.......| |bb*********xx.......|
> > + * |yyxx..ECC..| |bb*FREEOOB*xx..ECC..|
> > + * |yyxx.......| |bb*********xx.......|
> > + * |yyxx.......| |bb*********xx.......|
> > + * |-----------| |--------------------|
> > + * first n - 1 nth OOB region
> > + * OOB regions
> > + *
> > + * n = Number of codewords in the page
> > + * . = ECC bytes
> > + * * = FREE OOB bytes
> > + * y = Dummy bad block byte(s) (inaccessible when ECC enabled)
> > + * x = Unused byte(s)
> > + * b = Real bad block byte(s) (inaccessible when ECC enabled)
> > + *
> > + * This layout is read as is when ECC is disabled. When ECC is enabled, the
> > + * inaccessible Bad Block byte(s) are ignored when we write to a page/oob,
> > + * and assumed as 0xffs when we read a page/oob. The ECC, unused and
> > + * dummy/real bad block bytes are grouped as ecc bytes in nand_ecclayout (i.e,
> > + * ecc->bytes is the sum of the three).
> > + */
> > +
> > +static struct nand_ecclayout *
> > +qcom_nand_create_layout(struct qcom_nand_host *host)
> > +{
> > + struct nand_chip *chip = &host->chip;
> > + struct mtd_info *mtd = nand_to_mtd(chip);
> > + struct qcom_nand_controller *nandc = get_qcom_nand_controller(chip);
> > + struct nand_ecc_ctrl *ecc = &chip->ecc;
> > + struct nand_ecclayout *layout;
> > + int i, j, steps, pos = 0, shift = 0;
> > +
> > + layout = devm_kzalloc(nandc->dev, sizeof(*layout), GFP_KERNEL);
> > + if (!layout)
> > + return NULL;
> > +
> > + steps = mtd->writesize / ecc->size;
> > + layout->eccbytes = steps * ecc->bytes;
> > +
> > + layout->oobfree[0].offset = (steps - 1) * ecc->bytes + host->bbm_size;
> > + layout->oobfree[0].length = steps << 2;
> > + layout->oobavail = steps << 2;
> > +
> > + /*
> > + * the oob bytes in the first n - 1 codewords are all grouped together
> > + * in the format:
> > + * DUMMY_BBM + UNUSED + ECC
> > + */
> > + for (i = 0; i < steps - 1; i++) {
> > + for (j = 0; j < ecc->bytes; j++)
> > + layout->eccpos[pos++] = i * ecc->bytes + j;
> > + }
> > +
> > + /*
> > + * the oob bytes in the last codeword are grouped in the format:
> > + * BBM + FREE OOB + UNUSED + ECC
> > + */
> > +
> > + /* fill up the bbm positions */
> > + for (j = 0; j < host->bbm_size; j++)
> > + layout->eccpos[pos++] = i * ecc->bytes + j;
> > +
> > + /*
> > + * fill up the ecc and reserved positions, their indices are offseted
> > + * by the free oob region
> > + */
> > + shift = layout->oobfree[0].length + host->bbm_size;
> > +
> > + for (j = 0; j < (host->ecc_bytes_hw + host->spare_bytes); j++)
> > + layout->eccpos[pos++] = i * ecc->bytes + shift + j;
> > +
> > + return layout;
> > +}
>
> I'm trying to move this layout definition to the mtd_ooblayout_ops
> approach, and I wonder why you decided to take such a complicated
> representation.
> AFAIU, in each ECC step you have 512 bytes of data, X ECC+reserved
> bytes (you decided to consider all of them as ECC bytes, which is fine
> by me) and 4 usable/free bytes. Am I correct?
>
> If that's the case, then why not exposing the following layout.
>
> eccregion[i] = {
> .offset = i * (ecc->bytes + 4);
> .length = ecc->bytes;
> }
>
> oobfreeregion[i] = {
> .offset = (i * (ecc->bytes + 4)) + ecc->bytes;
> .length = 4;
> }
>
> Are there any userspace tools relying on the ooblayout you're currently
> exposing (remember that the exposed OOB layout is not necessarily
> what you see on the media)?
Okay, I think we already had this discussion :).
I'm still not happy with the exposed layout (it would be much easier to
reserve 4 free bytes per chunk, and declare each chunk as containing 512
data bytes + 4 oob bytes + X ECC/reserved bytes), but IIRC, your ROM
code (and/or bootloader) is already using this layout :-(.
--
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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