Re: [PATCH v5 1/3] cpuidle: psci: Call cpu_cluster_pm_enter() on the last CPU

[Marc Zyngier <maz@kernel.org>]

On Thu, 17 Feb 2022 07:31:32 +0000,
Shawn Guo <shawn.guo@linaro.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).

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.

	M.

-- 
Without deviation from the norm, progress is not possible.