2010/5/25 Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>: > 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. > How is that not polling? If the user is holding down a key, the keypad driver has to block suspend, and user space will try to suspend again and again and again... -- Arve Hjønnevåg _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
