On Tue, 8 Dec 2009, Rafael J. Wysocki wrote: > > Well, one difficulty. It arises only because we are contemplating > > having the PM core fire up the async tasks, rather than having the > > drivers' suspend routines launch them (the way your original proposal > > did -- the difficulty does not arise there). > > > > Suppose A and B are unrelated devices and we need to impose the > > off-tree constraint that A suspends after B. With children taking > > their parent's lock, the way to prevent A from suspending too soon is > > by having B's suspend routine acquire A's lock. > > > > But B's suspend routine runs entirely in an async task, because that > > task is started by the PM core and it does the method call. Hence by > > the time B's suspend routine is called, A may already have begun > > suspending -- it's too late to take A's lock. To make the locking > > work, B would have to acquire A's lock _before_ B's async task starts. > > Since the PM core is unaware of the off-tree dependency, there's no > > simple way to make it work. > > Do not set async_suspend for B and instead start your own async thread > from its suspend callback. The parent-children synchronization is done by the > core anyway (at least I'd do it that way), so the only thing you need to worry > about is the extra dependency. I don't like that because it introduces "artificial" dependencies: It makes B depend on all the preceding synchronous suspends, even totally unrelated ones. But yes, it would work. > I would be slightly more comfortable using completions, but the rwsem-based > approach is fine with me as well. On the principle of making things as easy and foolproof as possible for driver authors, I also favor completions since it makes dealing with non-tree dependencies easier. However either way would be okay. I do have to handle some non-tree dependencies in USB, but oddly enough they affect only resume, not suspend. So this "who starts the async task" issue doesn't apply. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
