Re: [linux-pm] [PATCH 1/8] PM: Opportunistic suspend support.

Arve Hjønnevåg
2010-05-31T10:03:54Z

On Sun, May 30, 2010 at 1:32 PM, mark gross <640e9920@gmail.com> wrote:
> On Sun, May 30, 2010 at 06:08:46PM +1000, Neil Brown wrote:
>> On Fri, 28 May 2010 21:04:53 -0700
>> Arve Hjønnevåg <arve@android.com> wrote:
>>
>> > On Fri, May 28, 2010 at 7:52 PM, mark gross <640e9920@gmail.com> wrote:
>> > > On Thu, May 27, 2010 at 05:23:54PM +1000, Neil Brown wrote:
>> > >> On Wed, 26 May 2010 14:20:51 +0100
>> > >> Matthew Garrett <mjg59@srcf.ucam.org> wrote:
>> > >>
>> > >> > On Wed, May 26, 2010 at 02:57:45PM +0200, Peter Zijlstra wrote:
>> > >> >
>> > >> > > I fail to see why. In both cases the woken userspace will contact a
>> > >> > > central governing task, either the kernel or the userspace suspend
>> > >> > > manager, and inform it there is work to be done, and please don't
>> > >> > > suspend now.
>> > >> >
>> > >> > Thinking about this, you're right - we don't have to wait, but that does
>> > >> > result in another problem. Imagine we get two wakeup events
>> > >> > approximately simultaneously. In the kernel-level universe the kernel
>> > >> > knows when both have been handled. In the user-level universe, we may
>> > >> > have one task schedule, bump the count, handle the event, drop the count
>> > >> > and then we attempt a suspend again because the second event handler
>> > >> > hasn't had an opportunity to run yet. We'll then attempt a suspend and
>> > >> > immediately bounce back up. That's kind of wasteful, although it'd be
>> > >> > somewhat mitigated by checking that right at the top of suspend entry
>> > >> > and returning -EAGAIN or similar.
>> > >> >
>> > >>
>> > >> (I'm coming a little late to this party, so excuse me if I say something that
>> > >> has already been covered however...)
>> > >>
>> > >> The above triggers a sequence of thoughts which (When they settled down) look
>> > >> a bit like this.
>> > >>
>> > >> At the hardware level, there is a thing that we could call a "suspend
>> > >> blocker".  It is an interrupt (presumably level-triggered) that causes the
>> > >> processor to come out of suspend, or not to go into it.
>> > >>
>> > >> Maybe it makes sense to export a similar thing from the kernel to user-space.
>> > >> When any event happens that would wake the device (and drivers need to know
>> > >> about these already), it would present something to user-space to say that
>> > >> the event happened.
>> > >>
>> > >> When user-space processes the event, it clears the event indicator.
>> > >
>> > > we did I proposed making the suspend enabling a oneshot type of thing
>> > > and all sorts of weak arguments came spewing forth.  I honestly couldn't
>> > > tell if I was reading valid input or fanboy BS.
>> > >
>> >
>> > Can you be more specific? If you are talking about only letting
>> > drivers abort suspend, not block it, then the main argument against
>> > that is that you are forcing user-space to poll until the driver stops
>> > aborting suspend (which according to people arguing against us using
>> > suspend would make the power-manager a "bad" process). Or are you
>> > talking about blocking the request from user-space until all other
>> > suspend-blockers have been released and then doing a single suspend
>> > cycle before returning. This would not be as bad, but it would force
>> > the user-space power manager to be multi-threaded since it now would
>> > have way to cancel the request. Either way, what problem are you
>> > trying to solve by making it a one-shot request?
>> >
>
> Sorry about missing Avr's email, I've been fighting with getting my
> email forwarding working right.
>
> The problems I want to solve with the one-shot styled interface are:
>
> 1) of having to sprinkle suspend blocking sections from isr up to
> usermode and get them right.

Making the interface one-shot does not change where you need to block suspend.

>
> 2) provide a platform / architecture independent framework supporting
> other low power modes.
>

I assume you did not mean that a one-shot interface has any impact on
this. What other low power modes are you trying to support. Suspend
blocker block suspend regardless of the mode selected (standby/mem).
Are there cases where you need to block one but not the other?

--
Arve Hjønnevåg