linux-kernel - Re: [PATCH 3/4] atmel-mci: support multiple mmc slots

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Message-ID: <20080923211222.551389ab@mjolnir.drzeus.cx>
Date:	Tue, 23 Sep 2008 21:12:22 +0200
From:	Pierre Ossman <drzeus-list@...eus.cx>
To:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
Cc:	kernel@...32linux.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] atmel-mci: support multiple mmc slots

On Tue, 23 Sep 2008 20:16:50 +0200
Haavard Skinnemoen <haavard.skinnemoen@...el.com> wrote:

> Pierre Ossman <drzeus-list@...eus.cx> wrote:
> > 
> > You forgot the most important part; how to handle the clock given two
> > different requests.
> 
> I don't think I did. The set_ios() function merely calculates the value
> of MR and stashes it away. It isn't written to the controller until
> after the host has been claimed for a request.
> 

I see... It still doesn't take into account conflicting clocks. Or does
the multiplexer disable the clock for the inactive slot?

> > (This will also get a bit more painful when/if the core starts
> > disabling the clock when a card is idle)
> 
> The atmel-mci driver already stops the clock between requests.

Then I expect a patch to remove that. Fiddling with the clock is
dangerous and I've seen everything from hung cards to silently broken
writes when you disable the clock too early after a request.

> Though I don't understand why the core would care about clocks.
> 

Power management. But it needs to be handled centrally. Primarily
because of the reason above, but you might also have such things as
SDIO functions requiring the clock to be on.

> > 
> > Fall back to polling?
> 
> Probably, yes. Can I fall back to polling after the host has been
> registered?
> 

Hmm... no. Not sure how to handle this...

Rgds
-- 
     -- Pierre Ossman

  Linux kernel, MMC maintainer        http://www.kernel.org
  rdesktop, core developer          http://www.rdesktop.org

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