linux-kernel - Re: [PATCH] mmc: Reduce fOD to 200 kHz if possible

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Date:	Wed, 15 Sep 2010 15:08:40 +0100
From:	Chris Ball <cjb@...top.org>
To:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
Cc:	Ben Nizette <bn@...sdigital.com>, linux-mmc@...r.kernel.org,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Pierre Ossman <pierre@...man.eu>
Subject: Re: [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@...top.org>   <http://printf.net/>
One Laptop Per Child
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