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

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Date:	Wed, 15 Sep 2010 17:10:45 +0200
From:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
To:	Hein_Tibosch <hein_tibosch@...oo.es>
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>,
	Chris Ball <cjb@...top.org>
Subject: Re: [PATCH] mmc: Reduce fOD to 200 kHz if possible

Hein_Tibosch <hein_tibosch@...oo.es> wrote:
> >> We've now submitted one fix each, mine was similar to yours [1] but there's been movement on Hein's more comprehensive patch recently [2].  I don't know who's supposed to be merging MMC patches atm, Chris Ball added on CC on a hunch.
> > 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.
> >
> 
> I have also seen situations in which the SD will only start up at 180 Khz
> or lower.

Ok, so my patch would obviously not fix those. Could you tell me more
about those situations? What kind of board are you running at, etc?

> IMO my patch is indeed a bit of an overkill (the amount of code changed),
> I'd rather see a patch like yours or Ben's, but settings fOD at 100 or 50 Khz.

I guess 100 kHz would make sense. 50 kHz is not allowed according to
the SD card spec.

Another solution might be to introduce a new field, f_max_od, which the
host controller driver could use to limit the bus frequency in open
drain mode.

> It was Pierre who suggested to try several frequencies[1]: most systems will start
> using 400 Khz (so they won't suffer any loss of performance) and in some rare
> cases, the SD will only be identified at 100 Khz (which does happen, sometimes)

I think any performance hit in open drain mode is not likely to be
noticeable, as it is only used for a couple of commands. The only thing
that really matters for performance is push-pull mode, where too strong
pull-ups might cause problems at high speed.

But if there really are cards that have problems at 100 kHz, I guess
your patch might be the best bet.

Haavard
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