linux-kernel - Re: [Xen-devel] Re: [linux-pm] [PATCH 0/2] Fix hangup after creating checkpoint on Xen.

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Date:	Thu, 10 Feb 2011 17:26:30 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Ian Campbell <ijc@...lion.org.uk>,
	Brendan Cully <brendan@...ubc.ca>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-pm@...ts.linux-foundation.org" 
	<linux-pm@...ts.linux-foundation.org>,
	"SUZUKI, Kazuhiro" <kaz@...fujitsu.com>
Subject: Re: [Xen-devel] Re: [linux-pm] [PATCH 0/2] Fix hangup after creating checkpoint on Xen.

On Thursday, February 10, 2011, Alan Stern wrote:
> On Thu, 10 Feb 2011, Ian Campbell wrote:
> 
> > On Wed, 2011-02-09 at 23:42 +0000, Alan Stern wrote:
> > > In fact there already is a "fast suspend & resume" path in the PM core.  
> > > It's the freeze/thaw procedure used when starting to hibernate.  The
> > > documentation specifically says that drivers' freeze methods are
> > > supposed to quiesce their devices but not change power levels.  In
> > > addition, the thaw method is invoked as part of recovery from a failed
> > > hibernation attempt, so it already has the "cancel" semantics that xen 
> > > seems to want.
> > 
> > Sounds like that would work and I would much prefer to simply make
> > correct use of the core functionality.
> 
> It seems like a reasonable approach.  Whether it will actually _work_ 
> is a harder question...  :-)
> 
> > So PMSG_FREEZE is balanced by either PMSG_RECOVER or PMSG_THAW depending
> > on whether the suspend was cancelled or not?

That's correct, but from drivers' point of view PMSG_THAW is equivalent to
PMSG_RECOVER, because the both of them cause ->thaw() callbacks to be executed.

> Basically yes.  It is also "balanced" by PMSG_RESTORE, which is used
> after a memory image has been restored (although this isn't relevant to
> your snapshotting).  See the comments in include/linux/pm.h.

Yup.

> >  So the sequence of events
> > is something like:
> > 	dpm_suspend_start(PMSG_FREEZE);
> >          
> > 		dpm_suspend_noirq(PMSG_FREEZE);
> >                          
> > 			sysdev_suspend(PMSG_QUIESCE);
> 
> This should say sysdev_suspend(PMSG_FREEZE).

Yes, PMSG_QUIESCE is restore-specific.

> > 			cancelled = suspend_hypercall()
> 
> At this point swsusp_arch_suspend() is called.  If that translates to 
> suspend_hypercall() in your setting, then yes.
> 
> > 			sysdev_resume();
> >                  
> > 		dpm_resume_noirq(cancelled ? PMSG_RECOVER : PMSG_THAW);
> >          
> > 	dpm_resume_end(cancelled ? PMSG_RECOVER : PMSG_THAW);
> > ?
> 
> Yes.

Actually, I think PMSG_THAW can be used in both cases.  The resume-side
routines only use the 'state' argument for diagnostics.

> > (For comparison we currently have:
> > > > >         dpm_suspend_start(PMSG_SUSPEND);
> > > > >         
> > > > >                 dpm_suspend_noirq(PMSG_SUSPEND);
> > > > >                         
> > > > >                         sysdev_suspend(PMSG_SUSPEND);
> > > > >                         /* suspend hypercall */
> > > > >                         sysdev_resume();
> > > > >                 
> > > > >                 dpm_resume_noirq(PMSG_RESUME);
> > > > >         
> > > > >         dpm_resume_end(PMSG_RESUME);
> > )
> 
> Right.  The sequence of calls is the same, but the PMSG_ argument is 
> different so drivers are expected to act differently in response.

That's correct.

Thanks,
Rafael
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