"Rafael J. Wysocki" <rjw@xxxxxxx> writes: > On Friday, July 01, 2011, Kevin Hilman wrote: >> "Rafael J. Wysocki" <rjw@xxxxxxx> writes: >> >> > On Thursday, June 30, 2011, Kevin Hilman wrote: >> >> "Rafael J. Wysocki" <rjw@xxxxxxx> writes: >> >> >> >> > From: Rafael J. Wysocki <rjw@xxxxxxx> >> >> > >> >> > Devices that are set up to wake up the system from sleep states >> >> > should not be stopped and power should not be removed from them >> >> > when the system goes into a sleep state. >> >> >> >> I don't think this belongs in the generic layer since the two >> >> assumptions above are not generally true on embedded systems, and would >> >> result in rather significant power consumption unnecessarily. >> > >> > As to whether or not this belongs to the generic layer, I don't quite agree >> > (see below), but the changelog seems to be a bit inaccurate. >> > >> >> First, whether the device should be stopped on device_may_wakeup(): >> >> b >> >> Some IP blocks (at least on OMAP) have "asynchronous" wakeups. Meaning >> >> that they can generate wakeups even when they're not clocked (a.k.a >> >> stopped). So in this case, even after a ->stop_device (which clock >> >> gates the IP), it can still generate wakeups. >> >> >> >> Second, whether the device should be powered off if device_may_wakeup(): >> >> >> >> Embedded SoCs have other ways to wakeup than device-level wakeups. >> >> >> >> For example, on OMAP, every pad on the SoC can be configured as a wakeup >> >> source So, for example, you could completely power down the UART IP >> >> blocks (and the enclosing power domain), configure the UART RX pad as a >> >> wakeup source, and still wakeup the system on UART activity. The OMAP >> >> docs call these IO pad wakeups. >> >> >> >> On OMAP in fact, this is the common, default behavior when we enable >> >> "off-mode" in idle and/or suspend, since most of the IPs are powered off >> >> but can still wake up the system. >> >> >> >> So in summary, even if device_may_wakeup() is true, many devices (with >> >> additional SoC magic) can still generate wakeups even when stopped and >> >> powered off. >> > >> > Well, on the other hand, on some SoCs there are devices that can't be >> > powered off (or "declocked") if they are supposed to generate wakeups. >> >> Correct. >> >> > Also, I'm sure there are cases in which wakeups can be generated for devices >> > with their clocks off, but only if power is present. >> >> Yes. >> >> > So there are multiple >> > cases, but not so many overall. So, IMO, it makes sense to handle that at >> > the generic level, although not necessarily in such a simplistic way. >> > >> > Now, at this point, I want to do something very simple, which I think is >> > done by this patch. >> > >> > Is this optimal power comsumption-wise for every potential >> > user of the framework? >> >> Well, sub-optimal would be an understatement. I would consider this a >> major regression since if we were to use this for OMAP, we would never >> hit the full-chip low-power states if *any* device had wakeups enabled, >> whereas today we can. >> >> > No, but certainly for some it's sufficient. Is it >> > going to work in general? I think it is. >> > >> > Of course, there's the question how to handle that more accurately and I have >> > some ideas. If you have any, please let me know. >> > >> > In the meantime, I'm going to modify the changelog so that it's clear that >> > it's a "first approximation" thing, like in the patch below. >> > >> > Thanks, >> > Rafael >> > >> > --- >> > From: Rafael J. Wysocki <rjw@xxxxxxx> >> > Subject: PM / Domains: Don't stop wakeup devices during system sleep transitions >> > >> > There is the problem how to handle devices set up to wake up the >> > system from sleep states during system-wide power transitions. >> > In some cases, those devices can be turned off entirely, because the >> > wakeup signals will be generated on their behalf anyway. In some >> > other cases, they will generate wakeup signals if their clocks are >> > stopped, but only if power is not removed from them. Finally, in >> > some cases, they can only generate wakeup signals if power is not >> > removed from them and their clocks are enabled. >> >> That's a good summary. >> >> > In the future, it will be necessary to take all of the above >> > situations into account, but for starters it is possible to use >> > the observation that if all wakeup devices are treated like the >> > last group (i.e. their clocks are enabled and power in not removed >> > from them during system suspend transitions), they all will be able >> > to generate wakeups, although power consumption in the resulting >> > system sleep state may not be optimal in some cases. >> >> I'm not opposed to this kind of check happening. I'm only opposed to it >> happening in this "generic" layer because..., well, it's not generic. >> >> Not only is it not generic, it would be a major regression in power >> consumption for anyone moving to this layer that has the various >> different wakeup capabilities already described. >> >> The decision of whether or not to clock gate and/or power gate based on >> wakeup capabilies has to be made somewhere (and in fact is already made >> by existing code.) But IMO, that decision should only be made where >> wakeup capabilies are known, so that sensible decisions (for power >> management) can be made. >> >> Until there is a way in the generic code to distinguish between the >> various ways a device can wakeup, this decision should be left up to the >> code that knows how. > > OK, so I suppose your suggestion is to drop the patch and let the > .stop_device() and .power_off() PM domain callbacks to hand that, is this > correct? Correct. Initially I was thinking only about .power_off(), but you'd probably want this at .stop_device() too. In order to do that, probably want .stop_device() to be able to return an error code such that an error would prevent .power_off(). Kevin _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
