Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

Rafael J. Wysocki
2010-06-01T22:24:14Z

On Tuesday 01 June 2010, James Bottomley wrote:
> On Tue, 2010-06-01 at 14:51 +0100, Matthew Garrett wrote:
> > On Mon, May 31, 2010 at 04:21:09PM -0500, James Bottomley wrote:
> >
> > > You're the one mentioning x86, not me. I already explained that some
> > > MSM hardware (the G1 for example) has lower power consumption in S3
> > > (which I'm using as an ACPI shorthand for suspend to ram) than any
> > > suspend from idle C state. The fact that current x86 hardware has the
> > > same problem may be true, but it's not entirely relevant.
> >
> > As long as you can set a wakeup timer, an S state is just a C state with
> > side effects. The significant one is that entering an S state stops the
> > process scheduler and any in-kernel timers. I don't think Google care at
> > all about whether suspend is entered through an explicit transition or
> > something hooked into cpuidle - the relevant issue is that they want to
> > be able to express a set of constraints that lets them control whether
> > or not the scheduler keeps on scheduling, and which doesn't let them
> > lose wakeup events in the process.
>
> Exactly, so my understanding of where we currently are is:

Thanks for the recap.

> 1. pm_qos will be updated to be able to express the android suspend
> blockers as interactivity constraints (exact name TBD, but
> probably /dev/cpu_interactivity)

I think that's not been decided yet precisely enough. I saw a few ideas
here and there in the thread, but which of them are we going to follow?

> 2. pm_qos will be updated to be callable from atomic context
> 3. pm_qos will be updated to export statistics initially closely
> matching what suspend blockers provides (simple update of the rw
> interface?)
>
> After this is done, the current android suspend block patch becomes a
> re-expression in kernel space in terms of pm_qos, with the current
> userspace wakelocks being adapted by the android framework into pm_qos
> requirements expressed to /dev/cpu_interactivity (or whatever name is
> chosen). Then opportunistic suspend is either a small add-on kernel
> patch they have in their tree to suspend when the interactivity
> constraint goes to NONE, or it could be done entirely by a userspace
> process. Long term this could migrate to the freezer and suspend from
> idle approach as the various problem timers get fixed.
>
> I think the big unresolved issue is the stats extension. For android,
> we need just a name written along with the value, so we have something
> to hang the stats off ... current pm_qos userspace users just write a
> value, so the name would be optional. From the kernel, we probably just
> need an additional API that takes a stats name or NULL if none
> (pm_qos_add_request_named()?). Then reading the stats could be done by
> implementing a fops read routine on the misc device.

Is the original idea of having that information in debugfs objectionable?

Rafael