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

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Message-ID: <20100506000552.GJ29604@atomide.com>
Date:	Wed, 5 May 2010 17:05:53 -0700
From:	Tony Lindgren <tony@...mide.com>
To:	Brian Swetland <swetland@...gle.com>
Cc:	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.

* 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.
 
> 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?

Regards,

Tony
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