linux-kernel - Re: [PATCH v5 1/3] cpuidle: psci: Call cpu_cluster_pm

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Message-ID: <20220217133702.GF31965@dragon>
Date:   Thu, 17 Feb 2022 21:37:03 +0800
From:   Shawn Guo <shawn.guo@...aro.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Maulik Shah <quic_mkshah@...cinc.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/3] cpuidle: psci: Call cpu_cluster_pm_enter() on the
 last CPU

On Thu, Feb 17, 2022 at 08:52:35AM +0000, Marc Zyngier wrote:
> On Thu, 17 Feb 2022 07:31:32 +0000,
> Shawn Guo <shawn.guo@...aro.org> wrote:
> > 
> > On Wed, Feb 16, 2022 at 03:58:41PM +0000, Marc Zyngier wrote:
> > > On 2022-02-16 14:49, Sudeep Holla wrote:
> > > > +Ulf (as you he is the author of cpuidle-psci-domains.c and can help you
> > > > with that if you require)
> > 
> > Thanks, Sudeep!
> > 
> > > > 
> > > > On Wed, Feb 16, 2022 at 09:28:28PM +0800, Shawn Guo wrote:
> > > > > Make a call to cpu_cluster_pm_enter() on the last CPU going to low
> > > > > power
> > > > > state (and cpu_cluster_pm_exit() on the firt CPU coming back), so that
> > > > > platforms can be notified to set up hardware for getting into the
> > > > > cluster
> > > > > low power state.
> > > > > 
> > > > 
> > > > NACK. We are not getting the notion of CPU cluster back to cpuidle
> > > > again.
> > > > That must die. Remember the cluster doesn't map to idle states
> > > > especially
> > > > in the DSU systems where HMP CPUs are in the same cluster but can be in
> > > > different power domains.
> > 
> > The 'cluster' in cpu_cluster_pm_enter() doesn't necessarily means
> > a physical CPU cluster.  I think the documentation of the function has a
> > better description.
> > 
> >  * Notifies listeners that all cpus in a power domain are entering a low power
> >  * state that may cause some blocks in the same power domain to reset.
> > 
> > So cpu_domain_pm_enter() might be a better name?  Anyways ...
> > 
> > > > 
> > > > You need to decide which PSCI CPU_SUSPEND mode you want to use first. If
> > > > it is
> > > > Platform Co-ordinated(PC), then you need not notify anything to the
> > > > platform.
> > > > Just request the desired idle state on each CPU and platform will take
> > > > care
> > > > from there.
> > > > 
> > > > If for whatever reason you have chosen OS initiated mode(OSI), then
> > > > specify
> > > > the PSCI power domains correctly in the DT which will make use of the
> > > > cpuidle-psci-domains and handle the so called "cluster" state correctly.
> > 
> > Yes, I'm running a Qualcomm platform that has OSI supported in PSCI.
> > 
> > > 
> > > My understanding is that what Shawn is after is a way to detect the "last
> > > man standing" on the system to kick off some funky wake-up controller that
> > > really should be handled by the power controller (because only that guy
> > > knows for sure who is the last CPU on the block).
> > > 
> > > There was previously some really funky stuff (copy pasted from the existing
> > > rpmh_rsc_cpu_pm_callback()), which I totally objected to having hidden in
> > > an irqchip driver.
> > > 
> > > My ask was that if we needed such information, and assuming that it is
> > > possible to obtain it in a reliable way, this should come from the core
> > > code, and not be invented by random drivers.
> > 
> > Thanks Marc for explain my problem!
> > 
> > Right, all I need is a notification in MPM irqchip driver when the CPU
> > domain/cluster is about to enter low power state.  As cpu_pm -
> > kernel/cpu_pm.c, already has helper cpu_cluster_pm_enter() sending
> > CPU_CLUSTER_PM_ENTER event, I just need to find a caller to this cpu_pm
> > helper.  
> > 
> > Is .power_off hook of generic_pm_domain a better place for calling the
> > helper?
> 
> I really don't understand why you want a cluster PM event generated by
> the idle driver.  Specially considering that you are not after a
> *cluster* PM event, but after some sort of system-wide event (last man
> standing).

That's primarily because MPM driver is used in the idle context, either
s2idle or cpuidle, and idle driver already has some infrastructure that
could help trigger the event.

> It looks to me that having a predicate that can be called from a PM
> notifier event to find out whether you're the last in line would be
> better suited, and could further be used to remove the crap from the
> rpmh-rsc driver.

I will see if I can come up with a patch for discussion.

Shawn