[linux-pm] [PATCH 7/10 v6] PM / Domains: Don't stop wakeup devices during system sleep transitions

[Kevin Hilman]

"Rafael J. Wysocki" <rjw at sisk.pl> writes:

> On Thursday, June 30, 2011, Kevin Hilman wrote:
>> "Rafael J. Wysocki" <rjw at sisk.pl> writes:
>> 
>> > From: Rafael J. Wysocki <rjw at sisk.pl>
>> >
>> > 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 at sisk.pl>
> 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.

Kevin