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 -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
