linux-kernel - Re: MMC: Make the configuration memory resource optional

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Message-ID: <Pine.LNX.4.64.0908060045320.9219@axis700.grange>
Date:	Thu, 6 Aug 2009 00:53:07 +0200 (CEST)
From:	Guennadi Liakhovetski <g.liakhovetski@....de>
To:	Ian Molton <ian@...menth.co.uk>
cc:	Magnus Damm <magnus.damm@...il.com>,
	pHilipp Zabel <philipp.zabel@...il.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	linux-kernel@...r.kernel.org, Pierre Ossman <drzeus@...eus.cx>
Subject: Re: MMC: Make the configuration memory resource optional

On Wed, 5 Aug 2009, Ian Molton wrote:

> Guennadi Liakhovetski wrote:
> 
> > How do we verify this? Do I need some "very fast" card, so that sd would try
> > to drive the clock beyond pdata->hclk / 2, which should then fail? 
> 
> One way would be a 'scope on the card pins, but since you havent said I am
> assuming you have not altered the clock-speed setting routine.
> 
> Here you can see why I didnt like the silently-drop-conf-accesses version of
> the patch.  If you had not made conf area accesses silently succeed, you'd
> have found one lurking in the set_clock function.
> 
> Since your controller has no CNF area, it was discarding the write to the 1:1
> clock bit (that isnt  in its none-existant conf area) and thus you got the
> next lowest clock as a result (/2)
> 
> > Currently I'm using 24MHz (copied from the original driver), and the only
> > thing I know about the controller is its "Maximum operating frequency: 25
> > MHz."
> 
> Thats the HCLK frequency, not the card clock.

Sure, and the card clock is derived from the HCLK, as you describe below.

> the TMIO MFD devices can divide this HCLK by anything from 512 to 2, and  MFD
> devices have a facility to disable the divider, yielding full HCLK speed as
> the card clock.
> 
> If you havent already found a 1:1 clock enable bit on your device, you might
> try looking for it - it'd yield a near linear factor-of-two speed increase.

I think, even getting the driver work with half the maximum speed (without 
the 1:1 speed) would be a good progress for SH. And I personally have no 
idea how I could "find" that divider-disable bit. Please notice, that I'm 
away for a week starting tomorrow, hopefully, Magnus will be able to 
further work with you on this during this time.

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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