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: Mon, 21 Jan 2019 11:22:48 +0000	[thread overview]
Message-ID: <20190121112248.GA27784@e107155-lin> (raw)
In-Reply-To: <CAJ+tv6+6yqT1xkmkY3x6SNQ83K+J6zqKZr7PV5A_ffvyEzceqg@mail.gmail.com>

On Mon, Jan 21, 2019 at 11:28:27AM +0530, Pramod Kumar wrote:
> On Fri, Jan 18, 2019 at 5:02 PM Sudeep Holla <sudeep.holla@arm.com> wrote:
> > On Fri, Jan 18, 2019 at 11:16:20AM +0530, Pramod Kumar wrote:
> > > If CPU hotplug is supported, ipi_cpu_stop should use PSCI cpudie
> > > call to stop the CPU. This call ensures L1/L2 cache flush,
> > > CPUs cache-cohenrecy setting w.r.to interconnect.
> >
> > Firstly, this is not specific to PSCI and I don't see any PSCI calls as
> > $subject claims.
>
> I had seen all the other cpu ops methods where only PSCI supports only
> cpu_die features hence used PSCI reference in my subject line to be more
> clear where this die gets converted in last.
>

OK, but since you are not directly dealing with PSCI APIs, it's misleading.

> > Next, you fail to explain why do you have to ensure
> > caches are cleaned and why do you need that in ipi_cpu_stop ?
> > What's the use case ?
> >
> Need comes from a specific use case where one Accelerator card(SoC) is
> plugged in a sever over a PCIe interface.  This Card gets supply from a
> battery, which could provide very less power for a very small time, in case
> of any power loss. Once Card switches to battery, this has to reduce its
> power consumption to its lowest point and back-up the DDR contents asap
> before battery gets fully drained off.
>

OK, but I was expecting the complete call path for this use-case.

> 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 ? Why system off or reset not sufficient ?

--
Regards,
Sudeep

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