linux-kernel - Re: [PATCH] PM: Prevent dpm_prepare() from returning errors unnecessarily

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Message-Id: <201011302327.24222.rjw@sisk.pl>
Date:	Tue, 30 Nov 2010 23:27:24 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Linux-pm mailing list" <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ming Lei <tom.leiming@...il.com>
Subject: Re: [PATCH] PM: Prevent dpm_prepare() from returning errors unnecessarily

On Tuesday, November 30, 2010, Alan Stern wrote:
> On Mon, 29 Nov 2010, Rafael J. Wysocki wrote:
> 
> > > Oh, I see.  This is a tricky issue.  Every driver for a device that can
> > > have wakeup-enabled children needs to worry about the race between
> > > suspending the device and receiving a wakeup request from a child.  
> > > For example, in drivers/usb/core/hcd-pci.c, the suspend_common()
> > > routine goes out of its way to return -EBUSY if device_may_wakeup() is
> > > true and the controller's root hub has a pending wakeup request.
> > > 
> > > How should drivers handle this in general?  Should we make an effort to
> > > convert them to use the wakeup framework so they they can let the PM
> > > core take care of these races?
> > 
> > I think so.
> > 
> > We also need to put a pm_check_wakeup_events() check into dpm_suspend() IMO,
> > so that we abort the suspending of devices as soon as a wakeup event is
> > reported.
> 
> You might as well add that into this patch.

I'll do that in a separate patch.

> > > Do we have to consider similar races during runtime suspend?
> > 
> > Ideally, yes, but I'm not sure if that's generally possible.  IMO, it won't be
> > a big deal if a parent device is suspended and immediately resumed occasionally
> > due to a pending wakeup signal from one of its children.  It may be a problem
> > if that happens too often, though.
> 
> Okay.
> 
> > Does it mean you're fine with the patch?
> 
> Provided you repair the error that Lei Ming pointed out.  That's the 
> problem with functions that return Boolean values -- you have to name 
> them very carefully.  Ideally the name should be a predicate or a
> question.

I already have fixed it.

The name is unfortunate indeed, perhaps it's better to call that function
pm_new_wakeup_events() or something like this.

Thanks,
Rafael
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