On Sat, 2010-10-30 at 05:47 -0700, Philip Rakity wrote:
> eMMC unlike SD does not have a field to inside the card data to say the bit
> width of the card.
> In addition some mmc cards (from Transcend) only support 1 bit mode. The
> physical pins to support 4 bit data are not there as well as no card specific
> data saying the bus width of the card.
>
> The only solution is to probe the bus by sending a CMD19 and CMD14
> (BUSTEST_W/BUSTEST_R).
> This procedure is defined in the JEDEC Standard No. 84-A441 spec -- Annex
> A.8.3 and this has been working in our linux 2.6.28/2.6.29 for quite a while.
> I can submit a patch if this makes sense.
> However, it may not work all the time; some controllers do not send out the
> CMD19 sequence. The (BUSTEST_W/BUSTEST_R) procedure is used in BSD. Also,
> in SD v3.0 CMD19 is defined for tuning and its definition is slightly
> different then in the JEDEC standard.
>
> One option for the problem you are seeing would be for my patch
> http://permalink.gmane.org/gmane.linux.kernel.mmc/3966
> or something like it to be accepted. As well as adding the bustest
> procedure.
>
> At least the board specific data can then say 8 bit data lines are supported
> on the physical slot. The controller can say 8 bit works but normally does
> not have knowledge of the lower level board design.
I see two solutions to this problem which you proposed above:
1. Allow the controller to tell host that it doesn't support 8-bit.
However, what about generic sdhci controllers? These that don't have
quirks in sdhci.c. Are there any desktop sdhci controllers that support
8-bit. Note that SDHCI controllers are primary for SD/SDHCI cards not
MMC. I don't know if extra pins have same locations on SD and MMC cards.
2. Test the card for being readable.
In memstick subsystem I recently had a lot of expirence with
(unfortunately its maintainer isn't easy to work with - probably
underestimation...)
It should be possible to set bus width and then just test the card for
being readable. While I don't yet know MMC spec and meaning of the
commands, I thing sending an ordinary command like reading card ID,
or something like that would suffiece to see if it accepts the bus
width. If such command fails, 4-bit bus width should be used.
I now assume that above commit broke all MMC cards in sdhci readers.
This has to be fixed somehow.
Best regards,
Maxim Levitsky
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