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    SubjectRe: [PATCH] mmc: Reduce fOD to 200 kHz if possible
    Hi Haavard,

    On Wed, Sep 15, 2010 at 12:51:38PM +0200, Haavard Skinnemoen wrote:
    > Thanks for the references. IMO Hein's patch is overkill. There is
    > absolutely no reason why 200 kHz should be a problem on any setup, and
    > I haven't found any indication in any discussions that it is.

    Pierre was worried that we'd ping-pong between having an f_min too high
    for some cards and too low for others, and be breaking a new set of
    cards each time we changed the value. He may have been being overly
    pessimistic, but I don't think being cautious comes with any significant
    downsides here.

    There's also a (small) performance concern when f_min gets low, though
    I agree that it wouldn't be a problem at 200 kHz.

    > The reason why fOD was set to 400 kHz in the first place is that some
    > controllers have a very low f_min so running the initialization at that
    > frequency causes problems. Which makes sense because the SD standard
    > clearly says that the clock can't be slower than 100 kHz.
    >
    > But I have never seen any reasons why we absolutely _have_ to run the
    > clock at the maximum frequency allowed by the spec. In fact, Sascha
    > Hauer, who was the one who changed the minimum clock frequency to 400
    > kHz, said he would be fine with any frequency between 50 kHz and 400
    > kHz [1].

    I agree that we aren't trying to run at the maximum possible frequency,
    just trying to avoid having to choose a single perfect value without
    enough information on what it is.

    Thanks,

    --
    Chris Ball <cjb@laptop.org> <http://printf.net/>
    One Laptop Per Child


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