linux-kernel - Re: [GIT PULL] PM updates for 2.6.33

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Date:	Sat, 5 Dec 2009 18:05:14 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	LKML <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [GIT PULL] PM updates for 2.6.33

On Sun, 6 Dec 2009, Rafael J. Wysocki wrote:
> 
> While the current settings are probably unsafe (like enabling PCI devices
> to be suspended asynchronously by default if there are not any direct
> dependences between them), there are provisions to make eveything safe, if
> we have enough information (which also is needed to put the required logic into
> the drivers).

I disagree.

Think of a situation that we already handle pretty poorly: USB mass 
storage devices over a suspend/resume.

> The device tree represents a good deal of the dependences
> between devices and the other dependences may be represented as PM links
> enforcing specific ordering of the PM callbacks.

The device tree means nothing at all, because it may need to be entirely 
rebuilt at resume time. 

Optimally, what we _should_ be doing (and aren't) for suspend/resume of 
USB is to just tear down the whole topology and rebuild it and re-connect 
the things like mass storage devices. IOW, there would be no device tree 
to describe the topology, because we're finding it anew. And it's one of 
the things we _would_ want to do asynchronously with other things.

We don't want to build up some irrelevant PM links and callbacks. We don't 
want to have some completely made-up new infrastructure for something that 
we _already_ want to handle totally differently for init time.

IOW, I argue very strongly against making up something PM-specific, when 
there really doesn't seem to be much of an advantage. We're much better 
off trying to share the init code than making up something new.

> I'd say if there's a worry that the same register may be accessed concurrently
> from two different code paths, there should be some locking in place.

Yeah. And I wish ACPI didn't exist at all. We don't know.

And we want to _limit_ our exposure to these things.

			Linus
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