Re: [RFD] Automatic suspend

Rafael J. Wysocki
2009-02-17T23:21:58Z

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.

> 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.

> > 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.

> Basically, any device that is "always open" falls in this category.
> Some examples are the screen, the keyboard, the mouse, and disk drives.
> And of course, some of these things already have runtime power
> management.

Thanks,
Rafael