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 11:36:14 +0530
From:   Pramod Kumar <pramod.kumar@...adcom.com>
To:     Sudeep Holla <sudeep.holla@....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>,
        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 11:30 AM Pramod Kumar <pramod.kumar@...adcom.com> wrote:
>
>
>
> On Mon, Jan 21, 2019 at 11:28 AM Pramod Kumar <pramod.kumar@...adcom.com> wrote:
>>
>> Hi Sudeep,
>>
>> Thanks for having a glance on the patch. Please see my reply in line.
>>
>> Regards,
>> Pramod
>>
>> 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.
>>
>>>
>>> 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.
>>
>> 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 which ultimately switch to ATF to flush out all the CPUs caches and comes out of coherency domain so that its power rails could be switched-off.
>>
>>>
>>> > 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.
>>
>>
>> Use case as explained above.
>>
>>>
>>> --
>>> Regards,
>>> Sudeep