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    SubjectRe: A MMC card transfer issue
    From
    2010/11/9 Tomoya MORINAGA <tomoya-linux@dsn.okisemi.com>:
    > I am facing the issue about some MMC cards on Intel EG20T PCH (Platform Controller Hub).
    > The linux version is 2.6.36.
    > I can not transfer data the MMC cards (e.g. Transcned MMC card).
    > The card is 1 bit bus width. And the card is according to MMC specification V3.x.
    > And the EG20T has a 4 bit bus width capability
    > Linux MMC standard driver decides the card bus width as 4 bit,
    > if the MMC specification version is larger than or equal to V4.0 like below.
    >
    > linux/drivers/mmc/core/mmc.c
    >
    >  505        /*
    >  506         * Activate wide bus (if supported).
    >  507         */
    >  508        if ((card->csd.mmca_vsn >= CSD_SPEC_VER_4) &&
    >  509            (host->caps & (MMC_CAP_4_BIT_DATA | MMC_CAP_8_BIT_DATA))) {
    >  510                unsigned ext_csd_bit, bus_width;
    >  511
    >  512                if (host->caps & MMC_CAP_8_BIT_DATA) {
    >  513                        ext_csd_bit = EXT_CSD_BUS_WIDTH_8;
    >  514                        bus_width = MMC_BUS_WIDTH_8;
    >  515                } else {
    >  516                        ext_csd_bit = EXT_CSD_BUS_WIDTH_4;
    >  517                        bus_width = MMC_BUS_WIDTH_4;
    >  518                }
    >  519
    >  520                err = mmc_switch(card, EXT_CSD_CMD_SET_NORMAL,
    >  521                                 EXT_CSD_BUS_WIDTH, ext_csd_bit);
    >  522
    >  523                if (err && err != -EBADMSG)
    >  524                        goto free_card;
    >  525
    >  526                if (err) {
    >  527                        printk(KERN_WARNING "%s: switch to bus width %d "
    >  528                               "failed\n", mmc_hostname(card->host),
    >  529                               1 << bus_width);
    >  530                        err = 0;
    >  531                } else {
    >  532                        mmc_set_bus_width(card->host, bus_width);
    >  533                }
    >  534        }
    >  535
    >
    > However the MMC specification version id is the same as V3.x and V4.0.
    > Therefore the driver uses the 4 bit bus width for the MMC card
    > even if the card has only 1 bit bus width.
    > I modified the driver to use 1 bit bus width only tentatively and confirmed that
    > we can mount the card and R/W correctly.
    >
    > I think we can not support MMC_CAP_4_BIT_DATA or MMC_CAP_8_BIT_DATA in MMC V4.0.
    >
    > How do you think ?

    You can find a patch for 4-bit support. the problem is there's some
    bug related so it set the 4 & 8 bit support both.
    So line 512 is always true. you can just remove the one line like this.

    It's quirk-and-dirty patch. now we try to find a generic solution to
    solve this issue.
    --- a/drivers/mmc/host/sdhci.c
    +++ b/drivers/mmc/host/sdhci.c
    @@ -1849,7 +1849,7 @@ int sdhci_add_host(struct sdhci_host *host)
    mmc->caps |= MMC_CAP_SDIO_IRQ;

    if (!(host->quirks & SDHCI_QUIRK_FORCE_1_BIT_DATA))
    - mmc->caps |= MMC_CAP_4_BIT_DATA | MMC_CAP_8_BIT_DATA;
    + mmc->caps |= MMC_CAP_4_BIT_DATA;

    if (caps & SDHCI_CAN_DO_HISPD)
    mmc->caps |= MMC_CAP_SD_HIGHSPEED | MMC_CAP_MMC_HIGHSPEED;
    Thank you,
    Kyungmin Park
    >
    > --
    > Thanks,
    > Tomoya MORINAGA(OKI SEMICONDUCTOR CO., LTD.)
    > --
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