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

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Date:   Mon, 21 Jan 2019 11:22:48 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Pramod Kumar <pramod.kumar@...adcom.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Suzuki K Poulose <Suzuki.Poulose@....com>,
        Dave Martin <dave.martin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Rob Herring <robh@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Steve Capper <steve.capper@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 1/1] arm64: Use PSCI calls for CPU stop when hotplug
 is supported

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@....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