Re: [PATCH RFC 1/1] arm64: Use PSCI calls for CPU stop when hotplug is supported

From: Sudeep Holla <sudeep.holla@arm.com>
To: Pramod Kumar <pramod.kumar@broadcom.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Rob Herring <robh@kernel.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Suzuki K Poulose <Suzuki.Poulose@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Steve Capper <steve.capper@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-kernel@vger.kernel.org,
	BCM Kernel Feedback <bcm-kernel-feedback-list@broadcom.com>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Dave Martin <dave.martin@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH RFC 1/1] arm64: Use PSCI calls for CPU stop when hotplug is supported
Date: Wed, 23 Jan 2019 16:27:03 +0000	[thread overview]
Message-ID: <20190123162514.GA22147@e107155-lin> (raw)
In-Reply-To: <CAJ+tv6++eXwu7Ez-GW62Avgaj+h0aPhyL0K-n3mBgwM2RtXXjQ@mail.gmail.com>

On Wed, Jan 23, 2019 at 10:21:05AM +0530, Pramod Kumar wrote:
> Thanks Sudeep for reviewing. Please see my comment inline below.
>
> On Mon, Jan 21, 2019 at 4:52 PM Sudeep Holla <sudeep.holla@arm.com> wrote:

[...]

> >
> > OK, but I was expecting the complete call path for this use-case.
>
> OK. Once power loss happens, Application processor gets interrupt and
> it has to switch to MCU asap so that it can switch-off all irrelevant
> power domain to reduce battery drain.
> flow is like this-
> Power loss interrupt --> interrupt handler -> send IPI to stop all
> secondary CPUs-> switch primary CPU to ATF which ultimately transition
> to MCU where all power off action is taken.
>

So does the entire AP domain get powered off or is that the intention ?
If so, just stopping secondaries through IPI is not sufficient. What
happens to all the running tasks and other operations going on in the
system.

> >
> > > Since battery can provide limited power for a very short time hence need to
> > > transition to lowest power. As per the transition process , CPUs power
> > > domain has to be off but before that it needs to flush out its content to
> > > system memory(L3) so that content could be backed-up by a MCU, a controller
> > > consuming very less power. Since we can not afford plugging-out every
> > > individual CPUs in sequence hence uses  ipi_cpu_stop for all other CPUs
> >
> > So, you are randomly using ipi_cpu_stop for something it's not supposed to
> > be used. What do you exactly want to achieve in that context where you need
> > to save power ?
>
> Need to back-up DDR from MCU which will require L1/L2 content gets
> flushed before CPUs getting shutdown.
>

Understood.

> >Why system off or reset not sufficient ?
> System off or reset is not sufficient as we can not afford that much
> delay in transition.
>

But who will save the system state ? You may corrupt data at the cost
of saving transition delay. So IMO, you don't have a valid use-case or
you are failing to present one properly.

> Flushing L1/L2 contents to System memory and Bringing out CPUs
> Clusters out of coherency domain before shutting down Clusters puts
> rest of system in sane state.  If cluster are not being taken out
> properly from coherency domain before shutting down, it could lead
> system un-reliable for rest parts.
>

Looks like you are concerned only with CPUs in this scenario, but what
about other devices in the domain. So suspend-to-{ram,idle} are only
sane options to make sure you don't loose/corrupt any data/state.

Further point me to the code either part of this series or upstream
handling the "Power loss interrupt" explained above.

--
Regards,
Sudeep

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