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    SubjectRe: [PATCH] MMC Core: Drop initialization frequency floor to 50kHz
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    On Thu, 2009-07-02 at 09:18 +0200, Pierre Ossman wrote:

    >
    > 50 kHz seems very low. I'd be more comfortable if we deviate from
    > specified behaviour as little as possible, say 300 kHz. Has anyone
    > checked what USB readers use?

    Before this patch my controller used ~135kHz, I was just basing the
    floor off that. If the spec says 400 and this is just to work around
    broken cards then sure, 300kHz sounds a good plan.

    >
    > And have you confirmed that the card is actually getting 350 kHz when
    > it fails? Perhaps there is a bug that is causing it to actually run at
    > a higher frequency.

    'scope confirmed that when 400kHz is requested my controller is giving
    the card ~397kHz.

    >
    > > diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
    > > index d84c880..6ee1931 100644
    > > --- a/drivers/mmc/core/core.c
    > > +++ b/drivers/mmc/core/core.c
    > > @@ -708,12 +708,13 @@ static void mmc_power_up(struct mmc_host *host)
    > > */
    > > mmc_delay(10);
    > >
    > > - if (host->f_min > 400000) {
    > > - pr_warning("%s: Minimum clock frequency too high for "
    > > - "identification mode\n", mmc_hostname(host));
    > > - host->ios.clock = host->f_min;
    > > - } else
    > > - host->ios.clock = 400000;
    > > + /*
    > > + * Card discovery is typically done at the controller's minimum
    > > + * allowable frequency but for some controllers this is minimum
    > > + * is unreasonably slow. In that case we limit slow clock rate
    > > + * to 50KHz.
    > > + */
    > > + host->ios.clock = max(host->f_min, 50000);
    > >
    >
    > I like getting that warning for too fast controllers. It makes it
    > easier to spot possible problems.

    For sure, it's just that if the floor is 50kHz then pretty much /every/
    controller is a 'fast' controller and the warning's redundant. If you
    just drop the 400k to 300k, I agree the warning's a good thing to have.

    Cheers,
    --Ben.

    >
    > Rgds



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