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    SubjectRe: Purpose of MMC_DATA_MULTI?
    Russell King wrote:
    > On Wed, Feb 01, 2006 at 10:37:12AM +0100, Pierre Ossman wrote:
    >
    >> Russell King wrote:
    >>
    >>> On Wed, Feb 01, 2006 at 07:40:26AM +0100, Pierre Ossman wrote:
    >>>
    >>>> I noticed that a new transfer flag was recently added to the MMC layer
    >>>> without any immediate users, the MMC_DATA_MULTI flag. I'm guessing the
    >>>> purpose of the flag is to indicate the difference between
    >>>> MMC_READ_SINGLE_BLOCK and MMC_READ_MULTIPLE_BLOCKS with just one block.
    >>>> If so, then that should probably be mentioned in a comment somewhere.
    >>>>
    >>>>
    >>> There are hosts out there (Atmel AT91-based) which need to know if the
    >>> transfer is going to be multiple block. Rather than have them test
    >>> the op-code (which is what they're already doing), we provide a flag
    >>> instead.
    >>>
    >> As far as the hardware is concerned there are two "multi-block" transfers:
    >>
    >> * Multiple, back-to-back blocks.
    >> * One or more blocks that need to be terminated by some form of stop
    >> command.
    >>
    >> The first can be identified by checking the number of blocks in the
    >> request, the latter is harder to identify since it's a protocol semantic
    >> (it could be just one block, but still need a stop). Does MMC_DATA_MULTI
    >> indicate the latter, former or both?
    >>
    >
    > In short, it's defined to be whatever AT91_MCI_TRTYP_MULTIPLE means in
    > the AT91RM9200 MMC host driver, which appears to be set for any of the
    > multiple block commands. They currently are doing:
    >
    >

    That's a bit fuzzy and bound to give problems. Why not settle for the
    first case and change their code to check the block count in the
    request? Less flags should be a good thing. And if that proves to be
    insufficient we should do a more thorough investigation once we have an
    actual failure case.

    > and using that as a lookup table by command for the value to put into
    > the command register. I want to eliminate that, and not passing the
    > MULTI flag prevents elimination of this table.
    >
    >

    Seems to be a common theme in the more recent drivers. Don't they teach
    people proper layering in the schools anymore? :)

    Rgds
    Pierre


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