linux-kernel - Re: [linux-pm] [PATCH 1/8] PM: Add suspend block api.

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Date:	Thu, 6 May 2010 10:04:21 -0700
From:	Tony Lindgren <tony@...mide.com>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	Brian Swetland <swetland@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	mark gross <mgross@...ux.intel.com>, markgross@...gnar.org,
	Len Brown <len.brown@...el.com>, linux-doc@...r.kernel.org,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [linux-pm] [PATCH 1/8] PM: Add suspend block api.

* Arve Hjønnevåg <arve@...roid.com> [100505 21:11]:
> On Wed, May 5, 2010 at 5:05 PM, Tony Lindgren <tony@...mide.com> wrote:
> > * Brian Swetland <swetland@...gle.com> [100505 16:51]:
> >> On Wed, May 5, 2010 at 4:47 PM, Tony Lindgren <tony@...mide.com> wrote:
> >> > * Brian Swetland <swetland@...gle.com> [100505 14:34]:
> >> >> On Wed, May 5, 2010 at 2:12 PM, Alan Stern <stern@...land.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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