[PATCH] PM: Make it possible to avoid wakeup events from being lost
in [Kernel]
|
Prev: i2c/mips: Fix error return codes from Sibyte i2c bus driver
Next: i915 in kernel 2.6.35-rc3, high number of wakeups
From: Alan Stern on 26 Jun 2010 13:20 On Sat, 26 Jun 2010, David Brownell wrote: > This is a repeat of an issue I posted before, but > which for some reason I never saw email back ... I replied to one of your emails on this topic. Maybe you didn't see the reply. > basically, I think the notion of counting wakeup > events seems dubious on common hardware, so the > focus might perhaps better be placed on ensuring > userspace just receives events rather than > trying to track events which in one context ended > up being wakeup events.� (That's simpler, and the > system is by definition awake if it can handle any > events at all.) I disagree. There's no question that userspace will eventually receive these events. The question is whether they will be received in a timely manner. The "counting" isn't meant as a way of keeping track of the absolute number of these events. It's more like a technique for seeing how many remain outstanding at any time. > Thing is, "wakeup" is, for e.g. most ARMs, just a hardware attribute of what's otherwise a routine > event, which happens in other contexts and needs > to be handled consistently .... nothing special > about having woken the system too, the result ought to > be the same regardless (from the user P.O.V.) ...� (Common Examples include SoC peripheral IRQs that can wake the system (including GPIO and other types > of external IRQ signal.) There's nothing wrong with that. All these routine events may qualify as "wakeup" events. The real deciding factor is whether the event should cause tasks to be unfrozen (if they have already been frozen by the PM core). If yes then it is a "wakeup" event; otherwise it isn't. > BRIEFLY: if that event doesn't arrive reliably, > it's an issue regardless of wakeup: either TX from > kernel, or RX in userspace. Such bugs would need to > be fixed. Having them fixed will help the wakeup scenarios too of course. That is irrelevant to the topic at hand. We know that these events arrive; what we don't know is that they will properly block tasks from being frozen (or will unfreeze them if they are already frozen). > (The raciness issues might boil down to something as simple as not letting userspace know about transition events to/from suspend states, but that issue > ought to be cleanly separable; ISTR other messages on the suspend blocker threads have shown how to work with such clean factoring.) > > > > So trying to track whether a given event is what > woke the system will often be implausible, since > several such events might each have fired (one > or more concurrent wakeup sources, even ... it > could be indeterminate which one[s] happened.) > And the event could fire without being a wakeup. It doesn't matter which event(s) woke the system, and Rafael's patch doesn't try to track this. > Yes, there are a few cases (like USB remote wakeup > signaling and some PCI mechanisms, plus a few BIOS > assisted situations) where certain events may be > identifiable as wakeup sources, perhaps runtime not > system-wide).� But the common case just includes > an event, not the ability to know that event had > the "woke whole system from low power state" > side effect too. > > +��� ��� The > > /sys/power/wakeup_count file allows user space to avoid > > +��� ��� losing wakeup events > > when transitioning the system into a sleep > > +��� ��� state.� Reading This could have been phrased better: The /sys/power/wakeup_count file allows user space to put the system into a sleep state conditionally subject to the arrival of concurrent wakeup events, which will either block the sleep transition or cause it to fail. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Alan Stern on 26 Jun 2010 16:00 On Sat, 26 Jun 2010, Rafael J. Wysocki wrote: > On Saturday, June 26, 2010, Alan Stern wrote: > > On Sat, 26 Jun 2010, David Brownell wrote: > ... > > > > + The > > > > /sys/power/wakeup_count file allows user space to avoid > > > > + losing wakeup events > > > > when transitioning the system into a sleep > > > > + state. Reading > > > > This could have been phrased better: The /sys/power/wakeup_count file > > allows user space to put the system into a sleep state conditionally > > subject to the arrival of concurrent wakeup events, which will either > > block the sleep transition or cause it to fail. > > I'll chage this in the final version. Thanks! Even this should be simplified, if possible. Maybe something like: ... put the system into a sleep state while taking into account the concurrent arrival of wakeup events ... I tried to put the word "reliably" in there, but it didn't seem to fit. After all, the current /sys/power/state is already very reliable about putting the system to sleep! Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Alan Stern on 26 Jun 2010 23:10 On Sat, 26 Jun 2010, David Brownell wrote: > > > basically, I think the notion of counting wakeup > > > events seems dubious on common hardware, ... > > I disagree.� > > > > The "counting" isn't meant as a way of keeping track of the absolute > > number of these events.� It's more like a technique for seeing how many > > remain outstanding at any time. > > But if you can't count them with any > reliability, you can't know *that* either... so > there must be a a problem with that model. Why do you say we can't count them with any reliability? Or, let's put it another way: You'll grant that we can count _some_ set of events. Given that, you'll probably also grant that we can keep track of their number reliably enough to know when the count has dropped to 0. Then this becomes a question of how closely does the set of events we can count match up with the set of "wakeup" events? In fact, it doesn't have to match up perfectly. There may be a few wakeup events where we don't really care if one of them occurs while the system is going to sleep and the sleep isn't delayed or aborted. (Although by definition this is never _supposed_ to happen, there may be cases where we just don't care.) The other possibility is relatively harmless too: an event that wouldn't wake up a sleeping system nevertheless can delay or abort a suspend-in-progress. So overall I don't see a problem with this. Do you have any especially pernicious failure modes in mind? Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Alan Stern on 29 Jun 2010 16:00
On Mon, 28 Jun 2010, David Brownell wrote: > Did someone post the canonical driver changes > to make use of this? No, not really. The patch itself contains an example (PCI) but it doesn't demonstrate the full range of possible usages. > Something like > > suspend() { /* if wake-enabled, up count */ } > resume() { /* if upcounted, downcount */ } > > is what first comes to mind.. expecting that > the suspend/resume methods in the driver are > already doing the right things for enabling > and later disabling the "system wake" behavior > on the various relevant hardware events... The PCI example looks like this: resume() { ... if (device_may_wakeup(dev)) pm_wakeup_event(dev, TIMEOUT_GUESS); ... } where TIMEOUT_GUESS is an estimate of how long to wait before allowing the system to sleep. For things like keyboards, an example would go more like this: irq_handler() { ... if (key-press event occurred) { ... if (input queue is empty) pm_stay_awake(dev); add event to input queue; ... } ... } read_queue() { ... send queued data to userspace if (input queue is empty) pm_relax(); ... } I left out the device_may_wakeup tests; things become rather complicated if you can have more than one keyboard feeding the same input queue and some of them are wakeup-enabled while others aren't. Clearly the appropriate changes will depend on the subsystem and the kind of event. They may also end up depending on the platform; perhaps this will be used only on relatively small systems like an Android phone. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |