Date: Mon, 7 Dec 2009 08:55:51 -0800 (PST)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Alan Stern <stern@rowland.harvard.edu>
cc: Zhang Rui <rui.zhang@intel.com>, "Rafael J. Wysocki" <rjw@sisk.pl>,
       LKML <linux-kernel@vger.kernel.org>,
       ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
       pm list <linux-pm@lists.linux-foundation.org>
Subject: Re: [GIT PULL] PM updates for 2.6.33
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On Mon, 7 Dec 2009, Linus Torvalds wrote:
> 
> And during phase #1, C and Q won't do anything at all. We _could_ do them 
> during this phase, and it would actually all work out fine, but we 
> wouldn't want to do that for a simple reason: we _want_ the pre_suspend 
> and post_resume phases to be total mirror images, because if we end up 
> doing error handling for the pre-suspend case, then the post-resume phase 
> would be the "fixup" for it, so we actually want leaf things to happen 
> during phase #2 - not because it would screw up locking or ordering, but 
> because of other issues.

Ho humm.

This part made me think. Since I started mulling over the fact that we 
could do the resume thing in a single phase (and really only wanted the 
second phase in order to be a mirror image to the suspend), I started 
thinking that we could perhaps do even the suspend with a single phase, 
and avoid introducing that pre-suspend/post-resume phase at all.

And now that I think about it, we can do that by simply changing the 
locking just a tiny bit.

I originally envisioned that two-pase suspend because I was thinking that 
the first phase would start off the suspend, and the second phase would 
finish it, but we can actually do it all with a single phase that does 
both. So starting with just the regular depth-first post-ordering that is 
a suspend:

        suspend(root)
        {
                for_each_child(root)
			suspend(child);
		suspend_one_node(root)
        }

the rule would be that for something like USB that wants to do the suspend 
asynchronously, the node suspend routine would do

	usb_node_suspend(node)
	{
		// Make sure parent doesn't suspend: this will not block, 
		// because we'll call the 'suspend' function for all nodes
		// before we call it for the parent.
		down_read(node->parent->lock);

		// Do the part that may block asynchronously
		async_schedule(do_usb_node_suspend, node);
	}

	do_usb_node_suspend(node)
	{
		// Start out suspend. This will block if we have any
		// children that are still busy suspending (they will
		// have done a down_read() in their suspend).
		down_write(node->lock);
		node->suspend(node);
		up_write(node->lock);

		// This lets our parent continue
		up_read(node->parent->lock);
	}

and it looks like we don't even need a second phase at all.

IOW, I think USB could do this on its own right now, with no extra 
infrastructure from the device layer AT ALL, except for one small thing: 
that new "rwsem" lock in the device data structure, and then we'd need the 
"wait for everybody to have completed" loop, ie

	for_each_dev(dev) {
		down_write(dev->lock);
		up_write(dev->lock);
	}

thing at the end of the suspend loop (same thing as I mentioned about 
resuming).

So I think even that whole two-phase thing was unnecessarily complicated.

		Linus
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