On Sun, 20 Jun 2010 22:23:38 -0400 (EDT) Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > On Sun, 20 Jun 2010, Rafael J. Wysocki wrote: > > > In what way is this better than suspend blockers? > > > > It doesn't add any new framework and it doesn't require the users of > > pm_wakeup_event() to "unblock" suspend, so it is simpler. It also doesn't add > > the user space interface that caused so much opposition to appear. > > Okay. A quick comparison shows that in your proposal: > > There's no need to register and unregister suspend blockers. > But instead you create the equivalent of a suspend blocker > inside every struct device. > > Drivers (or subsystems) don't have to activate suspend > blockers. But instead they have to call pm_wakeup_event(). > > Drivers don't have to deactivate suspend blockers. You don't > have anything equivalent, and as a result your scheme is > subject to the race described below. > > There are no userspace suspend blockers and no opportunistic > suspend. Instead a power-manager process takes care of > initiating or preventing suspends as needed. > > In short, you have eliminated the userspace part of the suspend blocker > approach just as in some of the proposals posted earlier, and you have > replaced the in-kernel suspend blockers with new data in struct device > and a new PM API. On the whole, it doesn't seem very different from > the in-kernel part of suspend blockers. The most notable difference is > the name: pm_wake_event() vs. suspend_blocker_activate(), or whatever > it ended up being called. > > This is the race I was talking about: > > > > What happens if an event arrives just before you read > > > /sys/power/wakeup_count, but the userspace consumer doesn't realize > > > there is a new unprocessed event until after the power manager checks > > > it? > > > I think this is not the kernel's problem. In this approach the kernel makes it > > possible for the user space to avoid the race. Whether or not the user space > > will use this opportunity is a different matter. > > It is _not_ possible for userspace to avoid this race. Help from the > kernel is needed. It is possible if every (relevant) userspace program implements a callback for the powermanager to check if one of it's wakeup-sources got activated. That way the powermanager would read /sys/power/wakeup_count, then do the roundtrip to all it's registered users and only then suspend. This turns the suspend_blockers concept around. Instead of actively signaling the suspend_blockers, the userspace programs only answer "yes/no" when asked. (i.e. polling?) You _can not_ implement userspace suspend blockers with this approach, as it is vital for every userspace program to get scheduled and check it's wakeup-source (if even possible) before you know that the right parties have won the race. Cheers, Flo _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
