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