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

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Message-ID: <20081002103137.57f5a9ad@mjolnir.drzeus.cx>
Date:	Thu, 2 Oct 2008 10:31:37 +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 Thu, 25 Sep 2008 15:51:49 +0200
Haavard Skinnemoen <haavard.skinnemoen@...el.com> wrote:

> Pierre Ossman <drzeus-list@...eus.cx> wrote:
> > On Wed, 24 Sep 2008 16:35:27 +0200
> > Haavard Skinnemoen <haavard.skinnemoen@...el.com> wrote:
> > 
> > Remember to deal with the case of a powered but unclocked card. This
> > will primarily happen during card init, but it could should up later as
> > well if we start implementing aggressive power management.
> 
> Ok, but how _do_ I deal with that? If the clock is turned off due to
> power management, it should be safe to leave it running at the same
> rate. But how about initialization? Should I stall the queue until the
> newly-powered card gets a clock rate?
> 

Yes, I don't see any other safe way of handling it.

> Btw, I just noticed that other thread about pxamci clock management.
> Will it cause problems if the clock stops briefly from time to time?
> Currently, the controller stops the clock if the FIFO gets full/empty
> during transfer -- this is configurable, but I'd much rather let the
> clock stop than risk FIFO overruns/underruns...

Hopefully not. :)

The spec gives no other means of flow control than messing with the
clock. But as has been noted, some cards get a bit freaky if we leave
them without a clock for too long, so care should be taken to try to
minimise the time it is stopped.

Rgds
-- 
     -- Pierre Ossman

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

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