From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: NeilBrown <neilb@suse.de>
Subject: Re: [RFC][PATCH 0/8] PM: Implement autosleep and "wake locks"
Date: Fri, 10 Feb 2012 01:44:10 +0100
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Cc: Linux PM list <linux-pm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Magnus Damm <magnus.damm@gmail.com>, markgross@thegnar.org,
        Matthew Garrett <mjg@redhat.com>, Greg KH <greg@linuxfoundation.org>,
        Arve =?utf-8?q?Hj=C3=B8nnev=C3=A5g?= <arve@android.com>,
        John Stultz <john.stultz@linaro.org>,
        Brian Swetland <swetland@google.com>,
        Alan Stern <stern@rowland.harvard.edu>
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Hi,

On Thursday, February 09, 2012, NeilBrown wrote:
> On Tue, 7 Feb 2012 02:00:55 +0100 "Rafael J. Wysocki" <rjw@sisk.pl> wrote:
> 
> 
> > All in all, it's not as much code as I thought it would be and it seems to be
> > relatively simple (which rises the question why the Android people didn't
> > even _try_ to do something like this instead of slapping the "real" wakelocks
> > onto the kernel FWIW).  IMHO it doesn't add anything really new to the kernel,
> > except for the user space interfaces that should be maintainable.  At least I
> > think I should be able to maintain them. :-)
> > 
> > All of the above has been tested very briefly on my test-bed Mackerel board
> > and it quite obviously requires more thorough testing, but first I need to know
> > if it makes sense to spend any more time on it.
> > 
> > IOW, I need to know your opinions!
> 
> I've got opinions!!!

Good! :-)

It seems that no one else has.

> I'll try to avoid the obvious bike-shedding about interface design...
> 
> The key point I want to make is that doing this in the kernel has one very
> import difference to doing it in userspace (which, as you know, I prefer)
> which may not be obvious to everyone at first sight.  So I will try to make it
> apparent.
> 
> In the user-space solution that we have previously discussed, it is only
> necessary for the kernel to hold a wakeup_source active until the event is
> *visible* to user-space.  So a low level driver can queue e.g. an input event
> and then deactivate their wakeup_source.  The event can remain in the input
> queue without any wakeup_source being active and there is no risk of going to
> sleep inappropriately.
> This is because - in the user-space approach - user-space must effectively
> poll every source of interesting wakeup events between the last wakeup_source
> being deactivate and the next attempt to suspend.  This poll will notice the
> event sitting in a queue so that a well-written user-space will not go to
> sleep but will read the event.
> (Note that this 'poll-of-every-device' need not be expensive.  It can be a
> single 'poll' or 'select' or even 'read' on a pollfd).

So I see one little problem with that, which is that you'd need to teach user
space developers what to do an how to do that correctly.

Also, when you say "user space", it isn't exactly clear whether you mean a
power manager (that would carry out the attmepts to suspend) or applications
(that would need to communicate with the power manager to let it know what
they are doing).  This is important, because in general, before deactivating
a wakeup source the kernel subsystem should know that the associated event
has become visible not only to the "polling" application, but also (perhaps
indirectly) to the power manager, so that it doesn't trigger suspend too
early.

> In the kernel based approach that you have presented this is not the case.
> As the kernel will initiate suspend the moment the last wakeup_source is
> released (with no polling of other queues), there must be an unbroken chain of
> wakeup_sources from the initial interrupt all the way up to the user.
> In particular, any subsystem (such as 'input') must hold a wakeup_source
> active as long as any designated 'wakeup event' is in any of its queues.
> This means that the subsystem must be able to differentiate wakeup events
> from non-wakeup events.
> This might be easy (maybe "all events are wakeup events" or "all events on
> this queue are wakeup events") but it is not obvious to me that that is the
> case.
> 
> To summarise: for this solution to be effective it also requires that
>  1/ every subsystem that carries wakeup events must know about wakeup_sources
>     and must activate/deactivate them as events are queued/dequeued.
>  2/ these subsystems must be able to differentiate between wakeup events and
>     non-wakeup events, and this must be a configurable decision.
> 
> Currently, understanding wakeup events is restricted to:
>  - drivers that are capable of configuring wakeup
>  - user-space which cares about wakeup
> The proposed solution adds:
>  - intermediate subsystems which might queue wakeup events
> 
> I think that is a significant addition to make and not one to be made
> lightly.  It might end up adding more code than you thought it would be :-)

I'm aware of that and I expect people to come up with patches adding the
handling of wakeup events to a number of subsystems (this is kind of needed
regardless of autosleep if we want to be sure that user space has actually
consumed events we want it to take from us before suspending).  However,
I'm not expecting that to be a lot of code (I think we both can only speculate
about that at this point) and those subsystems have maintainers and the
decision whether or not to take that code is theirs.

That may be a long process, but at least we can see from Android what's
needed and where.

Still, the point here is to give people something to start with so that they
can take the Android user space, test it against the mainline and see what
doesn't work and why and come up with fixes.  Perhaps they will have better
ideas than we think right now, but surely nothing more is going to happen
without this starting point.

I'd like us and Android to use the same low-level data structures for power
management and the same API eventually, at least for drivers.  This is not
the case at the moment and it's actively hurting us as a project quite a bit.
If Android needs to add patches on top of whatever we have to get the desired
functionality, I'm fine with that, as long as they don't require drivers to use
APIs that are incompatible with the mainline.  Insisting that Android should
use a user-space-based autosleep implementation wouldn't help at all, because
realistically this isn't going to happen.

> Thanks for the opportunity to comment,

No need to thank for that, it's Open Source after all ...

Thanks,
Rafael
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