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

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Date:   Fri, 18 Jan 2019 11:32:42 +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 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. 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 ?

> Apart from this, this gives control to f/w to reduce power consumption
> by take appropriate decesion on power rails for plugging-out core.
>

May be, but ipi_cpu_stop is used is machine reboot/poweroff/halt which
may restart or poweroff the system, powering down individual CPUs is not
necessary and may consume lot of time in systems with large number of CPUs.
It would be good to know the use-case in case I am missing to consider
that.

--
Regards,
Sudeep