Date:	Thu, 21 Nov 2013 02:07:35 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Patch Tracking <patches@...aro.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lan Tianyu <tianyu.lan@...el.com>, Nishanth Menon <nm@...com>,
	jinchoi@...adcom.com,
	Sebastian Capella <sebastian.capella@...aro.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: Re: [PATCH] cpufreq: suspend/resume governors with PM notifiers

On Wednesday, November 20, 2013 11:04:28 AM Viresh Kumar wrote:
> On 18 November 2013 11:09, viresh kumar <viresh.kumar@...aro.org> wrote:
> > On 18 November 2013 03:07, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >> On Sunday, November 17, 2013 10:27:43 PM Viresh Kumar wrote:
> >
> >>> Okay.. Even these notifiers would be fine for me. To make things more clear
> >>> before I start implementing them:
> >>> - What about implementing syscore's suspend_prepare and post_suspend?
> >>
> >> I'm not sure how useful that would be.  When would you like to execute those
> >> things?
> >
> > Maybe after freezing userspace. So I was actually looking to move the existing
> > code I wrote in PM notifiers to those..
> >
> > Because in our usecase, we just want to know when suspend has started or
> > resume has finished. And so we really don't need a per cpu callback.
> >
> > And so I felt probably it would be better to implement those instead of
> > cpu_subsys callbacks.
> >
> >>> - Or you want to extend only CPU subsystems notifiers? What notifiers exactly?
> >>> And at which places we want to issue them from?
> >>
> >> Why do we need to use notifiers?  What about PM callbacks?
> >
> > Yeah, we don't need notifiers but callbacks.
> >
> >>> Okay, so you were asking about extending following list: CPU_ONLINE,
> >>> CPU_UP_PREPARE, CPU_UP_CANCELED, CPU_DOWN_PREPARE, etc.. to
> >>> include suspend/resume ones as well?
> >>
> >> No.  Bus types (among other things) may provide suspend/resume callbacks for
> >> handling devices.  We have a bus type for CPUs, which is called cpu_subsys
> >> and currently doesn't define any PM callbacks, although it could do that in
> >> principle.  Have you investigated that possibility?
> >
> > I did it now and got really confused. :)
> >
> > This is what my understanding is:
> > - bus can register PM hooks, like suspend/resume/prepare, etc..
> > - devices under that bus would register themselves to that bus and eventually
> > can get their _driver's_ callbacks called via bus hooks.. For example and I2C
> > controller driver's callbacks will get called via i2c core bus..
> >
> > - In case of cpu subsystem, even if cpu_subsys adds those hooks in
> > drivers/base/cpu.c, then those hooks will get called for each cpu. CPU's don't
> > have a driver and so the only callbacks called are the ones of cpu_subsys.
> > - How will we bind/use them with cpufreq?
> >
> > Our sole requirement here is to get notify cpufreq core that system
> > suspend/resume/hibernation/restore has started/finished. How will that get
> > fulfilled with cpu_subsys callbacks?
> >
> >>> Logically speaking, all existing ones does look correct as they are more or
> >>> less cpu related. But suspend/resume doesn't look any similar, Atleast to me.
> >>>
> >>> Suspend/resume are system's state rather than CPU's.. We aren't suspending
> >>> or resuming CPUs, we are shutting them off.. So I thought maybe syscore ops
> >>> is a better place (which is already used by cpufreq)..
> >>
> >> cpufreq uses syscore_ops for the boot CPU only and that admittedly is a hack.
> >
> > Why do you call it a hack?
> >
> >> syscore_ops is specifically for things that have to be suspended with only one
> >> CPU online and with interrupts off.  I'm not sure how that applies to cpufreq.
> >
> > Currently syscore_ops only implements suspend/resume/shutdown callbacks and
> > those precisely happen the way you mentioned. i.e. after removing all non-boot
> > CPUs and disabling interrupts (And before bringing back all CPUs and enabling
> > interrupts on resume side).. So, yes we have limitation currently..
> >
> > Honestly speaking I have looked at syscore ops for the first time now, when we
> > got to this problem.. I couldn't find much information about it anywhere,
> > leaving the commit itself: 40dc16
> >
> > And by that, this is the definition of this framework: "PM / Core: Introduce
> > struct syscore_ops for core subsystems PM"
> >
> > I can see that you mentioned the limitations like single cpu and disabled
> > interrupts even in the log, but I think we can enhance this framework a little bit.
> >
> > Also I can see that there are many users of this framework which aren't core
> > frameworks but simply drivers. I don't think that was the intention behind this
> > framework, but that's how others went to use it.
> >
> > So, this framework exists to service core frameworks for their requirements
> > about PM stages. Currently that is only limited to late suspend and early resume
> > but I feel there is space for more..
> >
> > For example, our current problem.. A core framework wants to prepare before
> > suspend starts and after everything has resumed. Obviously that would violate
> > one of the basic rules with which this was designed, but still this feature lies
> > in scope of syscore. And so we can keep the limitations as is for
> > suspend/resume/shutdown but not for prepare and resume_late.
> >
> > And I really feel even if we would be able to use cpu callbacks for
> > suspend/resume, that would be a real *Hack*, because our framework doesn't want
> > to get a callback for each of its devices (i.e. cpu) but a single callback at
> > certain instances.. And syscore suits very well to this scenario..
> 
> Hi Rafael,
> 
> I need few more suggestions from you on this :)

And I need some more time to think about that.

Thanks!

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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