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

["Rafael J. Wysocki" <rjw@xxxxxxx>]

On Thursday, February 10, 2011, Ian Campbell wrote:
> On Thu, 2011-02-10 at 11:00 -0500, 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...  :-)
> 
> Heh.
> 
> > > So PMSG_FREEZE is balanced by either PMSG_RECOVER or PMSG_THAW depending
> > > on whether the suspend was cancelled or not?
> > 
> > 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.
> 
> The documentation of the individual events in pm.h is good. Is there a
> reference for the sequence of events for the different types of
> suspend/hibernate/etc?
> 
> > >  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).
> > 
> > >                   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.
> 
> Both of those call ->thaw ->complete. Did I mean "cancelled ?
> PMSG_THAW : PMSG_RESTORE"? (or s/THAW/RECOVER?)
> 
> If the suspend was cancelled then we want the devices to simply pickup
> where they were before the freeze, wereas if we really did suspend (or
> migrate or whatever) then they need to do a more complete reset and
> reconnect operation so we want some sort of indication to the driver
> which happened.

In that case you should probably use PMSG_THAW (or PMSG_RECOVER) for the
"cancel" case and PMSG_RESTORE for the "success" case (pretty much what
hibernation does).

And please don't forget to update the comments in pm.h to cover your usage
case. :-)

> > > (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.
> 
> The drivers don't actually see the PMSG_* though right? They only see a
> differing sequence of hooks from dev_pm_ops called.

That's correct.

Thanks,
Rafael

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