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    SubjectRe: [PATCH 1/1] mmc: add MODALIAS linkage for MMC/SD devices
    On Tue,  6 Jan 2009 18:56:38 +0000
    Andy Whitcroft <> wrote:

    > Currently we are using an explicit udev rule to trigger loading of the
    > mmc-block module when an MMC or SD card is detected:
    > SUBSYSTEM=="mmc", RUN+="/sbin/modprobe -Qba mmc-block"
    > It makes much more sense for the mmc bus driver and the mmc-block module to
    > share MODALIAS information so that they are linked automatically.

    First, I'd just like to state that I agree that the current situation
    is far from optimal. Things should "just work".

    That said, the MMC/SD stuff is a bit of a mess (the entire ecosystem,
    not just the Linux implementation). The classical method of matching
    devices to drivers isn't a perfect fit here. There is simply no nice
    and proper layering.

    So right now I think that udev (or equivalent system) is the cleanest
    solution. If nothing else, it means we don't have to create a kernel
    ABI that might be problematic to maintain in the future. Feel free to
    convince me I'm wrong though. :)

    > Add MODALIAS attributes to the uevents as generated, and add the
    > corresponding MODULE_ALIAS marks to the mmc-block module so that it is
    > automatically loaded by udev. I have followed the example set by the
    > SCSI subsystem in pushing out numeric typed aliases:
    > mmc:t-0xNN
    > Where NN is the MMC type as defined in include/linux/mmc/card.h.

    Currently the only type that isn't matched is MMC_TYPE_SDIO. But that
    will probably change once we support combo cards. At that point, all
    types will result in mmc_block being loaded and we're basically back to
    always loading mmc_block on card insertion.

    There is also the fact that type isn't the primary thing that mmc_block
    uses to determine if it can use the card. The more interesting thing is
    the command classes. But since those aren't defined for pure SDIO
    cards, it's not something we can sensibly use for MODALIAS.

    In essence, the crappy design of MMC and SD means there is no decent
    identifier for the card at this level.

    Your patch leaves room for adding things in the future, but it
    doesn't seem right adding an ABI with fields that we already know are
    pretty useless at identifying devices.

    Is the udev method causing practical problems, or is this a matter of
    trying to do things the proper way just for the sake of it?

    -- Pierre Ossman

    Linux kernel, MMC maintainer
    rdesktop, core developer

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