Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

Linus Torvalds
2009-12-08T22:32:39Z

On Tue, 8 Dec 2009, Alan Stern wrote:
>
> Suppose A and B are unrelated devices and we need to impose the
> off-tree constraint that A suspends after B.

Ah. Ok, I can imagine the off-tree constraints, but part of my "keep it
simple" was to simply not do them. If there are constraints that aren't
in the topology of the tree, then I simply don't think that async is worth
it in the first place.

> You misunderstand. The suspend algorithm will look like this:
>
> dpm_suspend()
> {
> list_for_each_entry_reverse(dpm_list, dev) {
> down_write(dev->lock);
> async_schedule(device_suspend, dev);
> }
> }
>
> device_suspend(dev)
> {
> device_for_each_child(dev, child) {
> down_read(child->lock);
> up_read(child->lock);
> }
> dev->suspend(dev); /* May do off-tree down+up pairs */
> up_write(dev->lock);
> }

Ok, so the above I think work (and see my previous email: I think
completions would be workable there too).

It's just that I think the "looping over children" is ugly, when I think
that by doing it the other way around you can make the code simpler and
only depend on the PM device list and a simple parent pointer access.

I also think that you are wrong that the above somehow protects against
non-topological dependencies. If the device you want to keep delay
yourself suspending for is after you in the list, the down_read() on that
may succeed simply because it hasn't even done its down_write() yet and
you got scheduled early.

But I guess you could do that by walking the list twice (first to lock
them all, then to actually call the suspend function). That whole
two-phase thing, except the first phase _only_ locks, and doesn't do any
callbacks.

Linus