From: Matthew Garrett on
On Wed, May 05, 2010 at 04:47:55PM -0700, Tony Lindgren wrote:

> How about instead the modem driver fails to suspend until it's done?

Because every attempted suspend requires freezing userspace, suspending
devices until you hit one that refuses to suspend, resuming the devies
that did suspend and then unfreezing userspace. That's not an attractive
option.

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From: Tony Lindgren on
* Matthew Garrett <mjg(a)redhat.com> [100506 06:35]:
> On Wed, May 05, 2010 at 04:47:55PM -0700, Tony Lindgren wrote:
>
> > How about instead the modem driver fails to suspend until it's done?
>
> Because every attempted suspend requires freezing userspace, suspending
> devices until you hit one that refuses to suspend, resuming the devies
> that did suspend and then unfreezing userspace. That's not an attractive
> option.

But how many times per day are you really suspending? Maybe few tens
of times at most if it's based on some user activity?

Or are you suspending constantly, tens of times per minute even if
there's no user activity?

Regards,

Tony
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From: Tony Lindgren on
* Arve Hjønnevåg <arve(a)android.com> [100505 21:11]:
> On Wed, May 5, 2010 at 5:05 PM, Tony Lindgren <tony(a)atomide.com> wrote:
> > * Brian Swetland <swetland(a)google.com> [100505 16:51]:
> >> On Wed, May 5, 2010 at 4:47 PM, Tony Lindgren <tony(a)atomide.com> wrote:
> >> > * Brian Swetland <swetland(a)google.com> [100505 14:34]:
> >> >> On Wed, May 5, 2010 at 2:12 PM, Alan Stern <stern(a)rowland.harvard.edu> wrote:
> >> >> >>
> >> >> >> Oh, like tell the modem that user mode has handled the ring event and
> >> >> >> its ok to un-block?
> >> >> >
> >> >> > No, that's not how it works.  It would go like this:
> >> >> >
> >> >> >        The modem IRQ handler queues its event to the input subsystem.
> >> >> >        As it does so the input subsystem enables a suspend blocker,
> >> >> >        causing the system to stay awake after the IRQ is done.
> >> >
> >> > How about instead the modem driver fails to suspend until it's done?
> >> >
> >> > Each driver could have a suspend_policy sysfs entry with options such
> >> > as [ forced | safe ]. The default would be forced. Forced would
> >> > be the current behaviour, while safe would refuse suspend until the
> >> > driver is done processing.
> >> >
> >> >> >        The user program enables its own suspend blocker before reading
> >> >> >        the input queue.  When the queue is empty, the input subsystem
> >> >> >        releases its suspend blocker.
> >> >
> >> > And also the input layer could refuse to suspend until it's done.
> >> >
> >> >> >        When the user program finishes processing the event, it
> >> >> >        releases its suspend blocker.  Now the system can go back to
> >> >> >        sleep.
> >> >
> >> > And here the user space just tries to suspend again when it's done?
> >> > It's not like you're trying to suspend all the time, so it should be
> >> > OK to retry a few times.
> >>
> >> We actually are trying to suspend all the time -- that's our basic
> >> model -- suspend whenever we can when something doesn't prevent it.
> >
> > Maybe that state could be kept in some userspace suspend policy manager?
> >
> >> >> > At no point does the user program have to communicate anything to the
> >> >> > modem driver, and at no point does it have to do anything out of the
> >> >> > ordinary except to enable and disable a suspend blocker.
> >> >>
> >> >> Exactly -- and you can use the same style of overlapping suspend
> >> >> blockers with other drivers than input, if the input interface is not
> >> >> suitable for the particular interaction.
> >> >
> >> > Would the suspend blockers still be needed somewhere in the example
> >> > above?
> >>
> >> How often would we retry suspending?
> >
> > Well based on some timer, the same way the screen blanks? Or five
> > seconds of no audio play? So if the suspend fails, then reset whatever
> > userspace suspend policy timers.
> >
> >> If we fail to suspend, don't we have to resume all the drivers that
> >> suspended before the one that failed?  (Maybe I'm mistaken here)
> >
> > Sure, but I guess that should be a rare event that only happens when
> > you try to suspend and something interrupts the suspend.
> >
>
> This is not a rare event. For example, the matrix keypad driver blocks
> suspend when a key is down so it can scan the matrix.

Sure, but how many times per day are you suspending?

> >> With the suspend block model we know the moment we're capable of
> >> suspending and then can suspend at that moment.  Continually trying to
> >> suspend seems like it'd be inefficient power-wise (we're going to be
> >> doing a lot more work as we try to suspend over and over, or we're
> >> going to retry after a timeout and spend extra time not suspended).
> >>
> >> We can often spend minutes (possibly many) at a time preventing
> >> suspend when the system is doing work that would be interrupted by a
> >> full suspend.
> >
> > Maybe you a userspace suspend policy manager would do the trick if
> > it knows when the screen is blanked and no audio has been played for
> > five seconds etc?
> >
>
> If user space has to initiate every suspend attempt, then you are
> forcing it to poll whenever a driver needs to block suspend.

Hmm I don't follow you. If the userspace policy daemon timer times
out, the device suspends. If the device does not suspend because of
a blocking driver, then the timers get reset and you try again based
on some event such as when the screen blanks.

Regards,

Tony
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From: Matthew Garrett on
On Thu, May 06, 2010 at 10:01:51AM -0700, Tony Lindgren wrote:

> Or are you suspending constantly, tens of times per minute even if
> there's no user activity?

In this case you'd be repeatedly trying to suspend until the modem
driver stopped blocking it. It's pretty much a waste.

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Matthew Garrett | mjg59(a)srcf.ucam.org
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From: Tony Lindgren on
* Matthew Garrett <mjg(a)redhat.com> [100506 10:05]:
> On Thu, May 06, 2010 at 10:01:51AM -0700, Tony Lindgren wrote:
>
> > Or are you suspending constantly, tens of times per minute even if
> > there's no user activity?
>
> In this case you'd be repeatedly trying to suspend until the modem
> driver stopped blocking it. It's pretty much a waste.

But then the userspace knows you're getting data from the modem, and
it can kick some inactivity timer that determines when to try to
suspend next.

Why would you need to constantly try to suspend in that case?

Regards,

Tony
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