linux-kernel - Re: [linux-pm] [PATCH] PM: Make it possible to avoid wakeup events from being lost

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Message-ID: <Pine.LNX.4.44L0.1006291533130.1314-100000@iolanthe.rowland.org>
Date:	Tue, 29 Jun 2010 15:57:24 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	David Brownell <david-b@...bell.net>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>, mark gross <640e9920@...il.com>,
	Neil Brown <neilb@...e.de>, <linux-pci@...r.kernel.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	<Arve@...p1.linux-foundation.org>,
	Florian Mickler <florian@...kler.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	<linux-pm@...ts.linux-foundation.org>
Subject: Re: [linux-pm] [PATCH] PM: Make it possible to avoid wakeup events
 from being lost

On Mon, 28 Jun 2010, David Brownell wrote:

> Did someone post the canonical driver changes
> to make use of this?

No, not really.  The patch itself contains an example (PCI) but it 
doesn't demonstrate the full range of possible usages.

>  Something like
> 
>   suspend() { /* if wake-enabled, up count */ }
>   resume() { /* if upcounted, downcount */ }
> 
> is what first comes to mind.. expecting that
> the suspend/resume methods in the driver are
> already doing the right things for enabling
> and later disabling the "system wake" behavior
> on the various relevant hardware events...

The PCI example looks like this:

	resume()
	{
		...
		if (device_may_wakeup(dev))
			pm_wakeup_event(dev, TIMEOUT_GUESS);
		...
	}

where TIMEOUT_GUESS is an estimate of how long to wait before allowing 
the system to sleep.

For things like keyboards, an example would go more like this:

	irq_handler()
	{
		...
		if (key-press event occurred) {
			...
			if (input queue is empty)
				pm_stay_awake(dev);
			add event to input queue;
			...
		}
		...
	}

	read_queue()
	{
		...
		send queued data to userspace
		if (input queue is empty)
			pm_relax();
		...
	}

I left out the device_may_wakeup tests; things become rather 
complicated if you can have more than one keyboard feeding the same 
input queue and some of them are wakeup-enabled while others aren't.

Clearly the appropriate changes will depend on the subsystem and the
kind of event.  They may also end up depending on the platform; perhaps
this will be used only on relatively small systems like an Android
phone.

Alan Stern

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