linux-kernel - Re: [PATCH] MMC Core: Drop initialization frequency floor to 50kHz

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Date:	Thu, 02 Jul 2009 21:10:50 +1000
From:	Ben Nizette <bn@...sdigital.com>
To:	Pierre Ossman <pierre@...man.eu>
Cc:	s.hauer@...gutronix.de,
	linux-kernel <linux-kernel@...r.kernel.org>,
	kernel <kernel@...32linux.org>
Subject: Re: [PATCH] MMC Core: Drop initialization frequency floor to 50kHz

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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