Re: [RFD] Automatic suspend

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Wednesday 18 February 2009, Arve Hjønnevåg wrote:
> On Tue, Feb 17, 2009 at 3:21 PM, Rafael J. Wysocki <rjw@sisk.pl> wrote:
> > On Tuesday 17 February 2009, Alan Stern wrote:
> >> On Tue, 17 Feb 2009, Rafael J. Wysocki wrote:
> >>
> >> > Phase 1: I agree that system-auto-suspend-on, system-auto-suspend-off would be
> >> > useful, but I don't like the wakelocks interface.  Do you think there is an
> >> > alternative way/mechanism of doing this?
> >>
> >> I rather like the suggestions Matthew Garrett has been making.  They
> >> show how to improve the wakelock interface without losing any function.
> >>
> >> Really, the idea behind wakelocks comes down to the question of how to
> >> determine when the system is sufficiently idle to go into auto-suspend.
> >> There may be occasions when no task is runnable but userspace knows
> >> that the system should not go to sleep because some work will be done
> >> in the near future.  (Arve's example of a non-empty input buffer is
> >> such a case.)  How should userspace let the kernel know whether it's
> >> okay to suspend at these times?  That is the problem userspace
> >> wakelocks are meant to solve.
> >
> > Still, do we really need multiple user space wakelocks (I'd prefer to call them
> > sleeplocks)?  It seems that one such lock and a user space manager controlling
> > it should be sufficient.
> 
> Yes, we could have a user space manager that all userspace wakelocks
> go through, but it would have to start before any other processes that
> need wakelocks and it would need a blocking ipc mechanism. The
> wakelock api that is provided to android applications does all this,
> but it is only available to java code. Supporting multiple userspace
> wakelocks in the kernel is simpler than adding another userspace
> wakelock layer.
> 
> >> Kernel wakelocks are a separate matter.  They are more like a form of
> >> optimization, preventing the kernel from starting an auto-suspend when
> >> some driver knows beforehand that it will return -EBUSY.
> >
> > I think kernel-side autosuspend (or rather autosleep) should only happen
> > after certain subset of devices have been suspended using a per-device
> > run-time autosuspend mechanism.
> 
> When the last wakelock is released the task that we woke up to perform
> has finished. Why wait to re-enter suspend.

I don't really understand this comment.  Could you please explain a bit?

> >> > Phase 3: Probably explicit control left to open/close.
> >>
> >> While that's generally a good idea, it's important to recognize that
> >> some devices should be runtime-suspended even while they are open.
> >
> > From the kernel side, yes (and that should be transparent to the user space
> > having them open).  By the user space, no.
> 
> Allowing user space to suspend input devices while they are still open
> is useful. The user-space code that reads from the input devices does
> not need to know if the device is suspended or not, and the kernel
> cannot auto suspend input devices based on inactivity.

Hmm.  Why can't it?

Rafael