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SubjectRe: [GIT PULL] PM updates for 2.6.33


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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