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    SubjectRe: [PATCH 00/39] mtd: nand: denali: 2nd round of Denali NAND IP patch bomb
    Hi Boris,

    I am almost getting v2 done,
    and now I am testing it.

    I am having one problem. Please teach me.


    2016-11-30 17:17 GMT+09:00 Boris Brezillon <boris.brezillon@free-electrons.com>:
    >> [2]
    >> Remove driver-internal bounce buffer.
    >> The current Denali driver allocate DMA_BIDIRECTIONAL buffer
    >> to use it as a driver-internal bounce buffer.
    >>
    >> The hardware transfer page data into the bounce buffer,
    >> then CPU copies from the bounce buffer to a given buf (and oob_poi).
    >> This is not efficient.
    >>
    >> So, I want to set NAND_USE_BOUNCE_BUFFER flag
    >> and do dma_map_single directly for a given buffer.
    >
    > Sounds good. Be careful though, when you use the generic bounce buffer
    > interface you might have to clear the page cache info (->pagebuf = -1).


    Instead of memcpy() of the whole page,
    I am trying to use dma_map_single() in ecc->read_page() / ecc->write_page().
    This will allow direct transfer between the buffer and the device by DMA.

    But, this does not work for Denali if use_bufpoi is set in nand_do_read_ops().


    In the following code in nand_scan_tail(),

    if (!(chip->options & NAND_OWN_BUFFERS)) {
    nbuf = kzalloc(sizeof(*nbuf) + mtd->writesize
    + mtd->oobsize * 3, GFP_KERNEL);
    if (!nbuf)
    return -ENOMEM;
    nbuf->ecccalc = (uint8_t *)(nbuf + 1);
    nbuf->ecccode = nbuf->ecccalc + mtd->oobsize;
    nbuf->databuf = nbuf->ecccode + mtd->oobsize;

    chip->buffers = nbuf;


    chip->buffers->databuf has no guarantee for DMA'able alignment.
    (actually it has unwanted offset 0xc because sizeof(*nbuf) == 0xc on
    32bit systems)

    If we could change the code as follows,

    nbuf->ecccalc = kmalloc(mtd->oobsize, GFP_KERNEL);
    nbuf->ecccode = kmalloc(mtd->oobsize, GFP_KERNEL);
    nbuf->databuf = kmalloc(mtd->writesize + mtd->oobsize,
    GFP_KERNEL);

    chip->buffers->databuf would have DMA'able alignment in most cases
    without NAND_OWN_BUFFERS. (but, I am not sure if this is a good idea)


    So, the idea of NAND_OWN_BUFFERS is that
    drivers should allocate own buffers if they need to perform DMA-mapping
    in read_page(), write_page(), right?


    However, "git grep NAND_OWN_BUFFERS" shows
    cafe_nand.c is the only driver that does so.

    On the other hand, "git grep dma_map_single" has more hits,
    i.e. some drivers perform dma_map_single() for read/write without
    NAND_OWN_BUFFERS.

    I have no idea how they are working.



    --
    Best Regards
    Masahiro Yamada

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