On Sat, Feb 7, 2009 at 10:56 AM, Rafael J. Wysocki <rjw@xxxxxxx> wrote: > If my understanding is correct, a wakelock is a mechanism that, if held, will > prevent the system from (automatically) entering a sleep state, but why do we > need a number of such wakelocks instead of just one reference counter with the > rule that (automatic) suspend can only happen if the counter is zero? Using wakelocks instead of a global reference count ensures that your request cannot be cleared by someone else. It means that you can, without knowing the current state of the lock, safely call wake_lock when you know that you need the wakelock, and wake_unlock when you are done. It allows wakelocks with timeouts and detailed stats. > Then, code paths wanting to prevent the suspend from happening would only need > to increase the counter and it shouldn't be difficult to provide a user space > interface for that. No, but you would also need to provide way to decrement it, which would allow user-space to override a request to stay awake in the kernel or from another client. >> +This works whether the wakelock is already held or not. It is useful if the >> +driver woke up other parts of the system that do not use wakelocks but >> +still need to run. Avoid this when possible, since it will waste power >> +if the timeout is long or may fail to finish needed work if the timeout is >> +short. > > And what's this call needed for? It is needed to give code that do not use wakelocks a chance to run. For instance we have not added wakelocks to the network stack. We also use it in some places when passing data to untrusted userspace code. > Is there a mechanism allowing us to see what wakelocks have been created by > the user land? Something like this would be useful for debugging. /proc/wakelocks shows all wakelocks. It does not currently indicate if a wakelock is from userspace, but if you are looking for a specific lock it is easy to find. I can add a prefix to the name of all user space wakelocks of you want. >> +Do not use randomly generated wakelock names as there is no api to free >> +a user-space wakelock. > > This appears to be very fragile. Please rethink it. I'm adding a /dev interface which removes this limitation. >> +/* wake_lock_active returns a non-zero value if the wake_lock is currently >> + * locked. If the wake_lock has a timeout, it does not check the timeout, >> + * but if the timeout had already expired when it was checked elsewhere >> + * this function will return 0. >> + */ >> +int wake_lock_active(struct wake_lock *lock); > > What's the purpose of this function? It is used by our alarm driver to abort suspend. >> +/* has_wake_lock returns 0 if no wake locks of the specified type are active, >> + * and non-zero if one or more wake locks are held. Specifically it returns >> + * -1 if one or more wake locks with no timeout are active or the >> + * number of jiffies until all active wake locks time out. >> + */ >> +long has_wake_lock(int type); > > And this? It is used to abort suspend. Currently when freezing processes, but it could also be called between each driver suspend call to improve wakeup latency when a wakelock is locked while suspending. We also use it (with WAKE_LOCK_IDLE) to select the idle sleep mode. -- Arve Hjønnevåg _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
