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? Rafael _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
