linux-kernel - Re: Bugs in MMC [was: [Bug 10030] Suspend doesn't work when SD card is inserted]

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Date:	Mon, 25 Feb 2008 18:41:02 +0100
From:	Pierre Ossman <drzeus-mmc@...eus.cx>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Zdenek Kabelac <zdenek.kabelac@...il.com>,
	David Brownell <david-b@...bell.net>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Bugs in MMC [was: [Bug 10030] Suspend doesn't work when SD card
 is inserted]

On Sun, 24 Feb 2008 10:33:34 -0500 (EST)
Alan Stern <stern@...land.harvard.edu> wrote:

> 
> Well, the patch itself isn't really adequate; it was just a first pass 
> intended to illustrate the nature of the problem.
> 
> The problems in the MMC subsystem go deeper.
> 
> The first is the way it uses flush_workqueue().  This is almost always 
> a bad function to call, because it introduces unnecessary dependencies.  
> cancel_delayed_work_sync() is much better.
> 
> But even changing that won't solve the second issue, which is a genuine
> bug.  There is a race between detect events and suspend events.  The
> mmc_suspend_host() routine starts out by flushing the kmmcd workqueue
> before calling the host's suspend routine.  So what happens if another
> detect event occurs in between?
> 

The idea is that host drivers shouldn't do that. Once they've called
mmc_suspend_host(), then they shouldn't be poking the MMC core in any
other way. None of this is of course properly documented. :/

> This whole area of synchronization between card insertion, card
> removal, suspend, and resume needs to be thought out better.  
> Especially keeping in mind that device registration or unregistration
> during a system sleep is likely to block until the sleep is over.

Trying to keep up with the PM changes is a full time job. For now I've
mostly ignored it as I don't even have time for the other stuff.
Patches welcome.

> 
> Lastly, there are some other questions about confusing aspects of the
> subsystem.  What's the story with __mmc_claim_host()?  Is it really
> nothing more than an abortable mutex_lock()?  Why does it ever need to
> be aborted?  Why is its second argument an atomic_t instead of a normal
> int?
> 

It got that way when SDIO got in. It was needed to make it abortable to
solve a rather nasty deadlock situation. I don't remember the details
right now, but it should be in the LKML archives.

> Why are mmc_detect() and related routines described in comments as
> "Card detection callback from host"?  They don't handle card
> _detection_ -- they handle card _removal_.

They handle both.

> 
> What's the purpose of the card-counting loop in 
> host/omap.c:mmc_omap_switch_handler()?  It looks like dead code.
> 

I'm not too familiar with that driver, but they've been playing around
with multiplexing several cards into one controller. Might be bits and
pieces of that.

Rgds
-- 
     -- Pierre Ossman

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