RE: commit mmc: sdhci: add MMC_CAP_8_BIT_DATA in the host

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On Sat, 2010-10-30 at 16:01 +0200, Maxim Levitsky wrote:
> 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.
Ping.
> 
> 


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