linux-kernel - Re: [PATCH v6 1/3] PM: cpu: Add CPU_LAST_PM_ENTER and CPU_FIRST_PM

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Date:   Fri, 25 Feb 2022 14:20:36 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Shawn Guo <shawn.guo@...aro.org>
Cc:     Marc Zyngier <maz@...nel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Maulik Shah <quic_mkshah@...cinc.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.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 v6 1/3] PM: cpu: Add CPU_LAST_PM_ENTER and
 CPU_FIRST_PM_EXIT support

On Fri, Feb 25, 2022 at 12:33:11PM +0800, Shawn Guo wrote:
> On Wed, Feb 23, 2022 at 07:30:50PM +0000, Sudeep Holla wrote:
> > On Wed, Feb 23, 2022 at 08:55:34PM +0800, Shawn Guo wrote:
> > > It becomes a common situation on some platforms that certain hardware
> > > setup needs to be done on the last standing cpu, and rpmh-rsc[1] is such
> > > an existing example.  As figuring out the last standing cpu is really
> > > something generic, it adds CPU_LAST_PM_ENTER (and CPU_FIRST_PM_EXIT)
> > > event support to cpu_pm helper, so that individual driver can be
> > > notified when the last standing cpu is about to enter low power state.
> > 
> > Sorry for not getting back on the previous email thread.
> > When I meant I didn't want to use CPU_CLUSTER_PM_{ENTER,EXIT}, I wasn't
> > thinking new ones to be added as alternative. With this OSI cpuidle, we
> > have introduces the concept of power domains and I was check if we can
> > associate these requirements to them rather than introducing the first
> > and last cpu notion. The power domains already identify them in order
> > to turn on or off. Not sure if there is any notification mechanism in
> > genpd/power domains. I really don't like this addition. It is disintegrating
> > all the solutions for OSI and makes it hard to understand.
> > 
> > One solution I can think of(not sure if others like or if that is feasible)
> > is to create a parent power domain that encloses all the last level CPU
> > power domains, which means when the last one is getting powered off, you
> > will be asked to power off and you can take whatever action you want.
> 
> Thanks Sudeep for the input!  Yes, it works for me (if I understand your
> suggestion correctly).  So the needed changes on top of the current
> version would be:
> 
> 1) Declare MPM as a PD (power domain) provider and have it be the
>    parent PD of cpu cluster (the platform has only one cluster including
>    4 cpus).
> 

[...]

> 
> Let's me know if this is what you are asking for, thanks!

Matches exactly. I don't know if there is anything I am missing to see,
but if this possible, for me it is easier to understand as this is all
linked to power-domains like other things in OSI cpuidle.

So yes, I prefer this, but let us see what others have to say about this.

-- 
Regards,
Sudeep