linux-kernel - Re: [RFC][PATCH -mm 3/3] PM: Disable _request

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Date:	Mon, 28 May 2007 16:03:30 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Matthew Garrett <mjg59@...f.ucam.org>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Pavel Machek <pavel@....cz>, Oliver Neukum <oliver@...kum.org>
Subject: Re: [RFC][PATCH -mm 3/3] PM: Disable _request_firmware before
 hibernation/suspend

On Mon, 28 May 2007, Matthew Garrett wrote:

> On Mon, May 28, 2007 at 12:43:36PM -0400, Alan Stern wrote:
> > On Mon, 28 May 2007, Matthew Garrett wrote:
> > > On Mon, May 28, 2007 at 12:09:30PM -0400, Alan Stern wrote:
> > > 
> > > > I can't speak for the second example, but there's a good reason the
> > > > first example works this way.  It's not a matter of races; the problem
> > > > is that the kernel thread's job is to selectively suspend and resume
> > > > devices.  We don't want it doing this while a system sleep is in 
> > > > progress; it would (and in fact has, before the thread was made 
> > > > freezable) cause the sleep transition to abort.
> > > 
> > > How does this work on PPC or APM systems?
> > 
> > For hibernation it behaves the same as on other types of systems.
> > 
> > For STR it generally works okay.  There was one report of suspends
> > aborting, and it looked like this was caused by selective resumes
> > originating from userspace.  This seemed to be unrelated to the kernel
> > threads; apparently some program was running while the STR was in
> > progress, and causing the problem.  For example, the lsusb program will
> > do a selective resume on every USB device as it scans through them all.
> > However that's just a guess, we haven't fully resolved that bug report.
> > 
> > The theoretical answer is that it behaves the way we want.  The kernel 
> > thread does selective resumes in response to device requests.  If such 
> > a request comes in while the system is asleep it will awaken the 
> > system; so it's only logical that a request coming in while the system 
> > is in the process of going to sleep should abort the suspend.
> 
> Ok, I guess I'm still not clear on this :) If it doesn't cause major 
> problems on Powermac or APM systems, why is freezing the thread 
> beneficial on ACPI systems?

Isn't it true, even on PPC and APM systems, that tasks are frozen for
hibernation?  The difference is that they aren't frozen for STR.

Hence the answer to your question: Not freezing the thread would work 
okay on ACPI systems for STR.  (In fact, not freezing anything would 
probably work okay for STR.)  But it doesn't work with hibernation.

Alan Stern

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