On Tue, May 25, 2010 at 04:13:35PM -0700, Arve Hjønnevåg wrote: > 2010/5/25 Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>: > > On Tue, May 25, 2010 at 03:37:48PM -0700, Arve Hjønnevåg wrote: > >> 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... > >> > > > > If your userpsace is that stupid - sure. However, you can: > > > > 1. Notify the suspend manager process that he rest of your userspace is > > busy handling keystrokes so that it does not try to suspend while there > > are events pending. > > You are missing the point. There are no event pending. The kernel > reported the key down event, it was handled, but the keypad driver is > still scanning to see if the user presses another key, Employ reasonable timeout. > or releases the > currently held key. > Userspace consumer should wait for the key release and retract "busy" once event is received and handled. > > > > 2. Wait a tiny bit after last application notified you that it finished > > processing events. > > > > So basically the difference is that with in-kernel suspend blockers, > > there is a tiny window where we haven't started the suspend yet but are > > about to the driver has a chance to prevent entire system from starting > > sleep. > > No, the difference is that if a driver needs to prevent suspend for an > extended period of time, you don't have user space continuously > polling to see if it can suspend. Why would a driver, on its own, prevent suspend for extended periods of time? I think that the decision should originate from userspace, kernel is here just to serve the requests. > > > > > Without the blocker we may start suspending and will stop midcycle. We > > may be even better off in the end since we could leave some devices > > still powered down after aborting system-wide suspend. > > > > That does not sound right. Why doesn't it? If a device implements runtime PM it may chose remain in powered-down mode even if system is awake. -- Dmitry _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
