linux-kernel - A MMC card transfer issue

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Date:	Tue, 09 Nov 2010 14:39:40 +0900
From:	Tomoya MORINAGA <tomoya-linux@....okisemi.com>
To:	linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org
CC:	"Andrew\"" <andrew.chih.howe.khor@...el.com>,
	kok.howg.ewe@...el.com
Subject: A MMC card transfer issue

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 ?

-- 
Thanks,
Tomoya MORINAGA(OKI SEMICONDUCTOR CO., LTD.)
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