On Tue, May 25, 2010 at 03:23:23PM -0700, Arve Hjønnevåg wrote: > On Tue, May 25, 2010 at 11:47 AM, Dmitry Torokhov > <dmitry.torokhov@xxxxxxxxx> wrote: > > On Tue, May 25, 2010 at 02:35:17PM -0400, Alan Stern wrote: > >> On Tue, 25 May 2010, Dmitry Torokhov wrote: > >> > >> > > Here's the scenario: > >> > > > >> > > The system is awake, and the user presses a key. The keyboard driver > >> > > processes the keystroke and puts it in an input queue. A user process > >> > > reads it from the event queue, thereby emptying the queue. > >> > > > >> > > At that moment, the system decides to go into opportunistic suspend. > >> > > Since the input queue is empty, there's nothing to stop it. As the > >> > > first step, userspace is frozen -- before the process has a chance to > >> > > do anything with the keystroke it just read. As a result, the system > >> > > stays asleep until something else wakes it up, even though the > >> > > keystroke was important and should have prevented it from sleeping. > >> > > > >> > > Suspend blockers protect against this scenario. Here's how: > >> > > > >> > > The user process doesn't read the input queue directly; instead it > >> > > does a select or poll. When it sees there is data in the queue, it > >> > > first acquires a suspend blocker and then reads the data. > >> > > > >> > > Now the system _can't_ go into opportunistic suspend, because a suspend > >> > > blocker is active. The user process can do whatever it wants with the > >> > > keystroke. When it is finished, it releases the suspend blocker and > >> > > loops back to the select/poll call. > >> > > > >> > > >> > What you describe can be done in userspace though, via a "suspend manager" > >> > process. Tasks reading input events will post "busy" events to stop the > >> > manager process from sending system into suspend. But this can be confined to > >> > Android userspace, leaving the kernel as is (well, kernel needs to be modified > >> > to not go into suspend with full queues, but that is using existing kernel > >> > APIs). > >> > >> I think that could be made to work. And it might remove the need for > >> the userspace suspend-blocker API, which would be an advantage. It > >> could even remove the need for the opportunistic-suspend workqueue -- > >> opportunistic suspends would be initiated by the "suspend manager" > >> process instead of by the kernel. > >> > >> However you still have the issue of modifying the kernel drivers to > >> disallow opportunistic suspend if their queues are non-empty. Doing > >> that is more or less equivalent to implementing kernel-level suspend > >> blockers. (The suspend blocker approach is slightly more efficient, > >> because it will prevent a suspend from starting if a queue is > >> non-empty, instead of allowing the suspend to start and then aborting > >> it partway through.) > >> > >> Maybe I'm missing something here... No doubt someone will point it out > >> if I am. > >> > > > > Well, from my perspective that would limit changes to the evdev driver > > (well, limited input core plumbing will be needed) but that is using the > > current PM infrastructure. The HW driver changes will be limited to what > > you described "type 2" in your other e-mail. > > > > Also, not suspending while events are in progress) is probably > > beneficial for platforms other than Android as well. So unless I am > > missing something this sounds like a win. > > > > How would this limit the changes you need in the evdev driver? It need > to block suspend when there are unprocessed events in some queues. > Suspend blockers gives you an api to do this, without it, you check > the queues in your suspend hook and abort suspend if they are not > empty. Without suspend blockers you have no api to signal that it is > OK to suspend again, so you are forcing the thread that tried to > suspend to poll until you stop aborting suspend. No, you do not need to poll. You just set a timeout (short or long, depending on your needs) and if no userspace task blocked suspend durng that time you attempt to initiate suspend from your manager process. If it succeeds - good, if not that means that more events came your way and you have to do it later. -- Dmitry _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
