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From:   Pramod Kumar <pramod.kumar@broadcom.com>
Date:   Mon, 21 Jan 2019 11:36:14 +0530
Message-ID: <CAJ+tv6+q+GzBXyv3DH=vX+bc+62YMUCfH7vYLNssyRzBzx6zFA@mail.gmail.com>
Subject: Re: [PATCH RFC 1/1] arm64: Use PSCI calls for CPU stop when hotplug
 is supported
To:     Sudeep Holla <sudeep.holla@arm.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Suzuki K Poulose <Suzuki.Poulose@arm.com>,
        Dave Martin <dave.martin@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Rob Herring <robh@kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Steve Capper <steve.capper@arm.com>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@broadcom.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
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On Mon, Jan 21, 2019 at 11:30 AM Pramod Kumar <pramod.kumar@broadcom.com> w=
rote:
>
>
>
> On Mon, Jan 21, 2019 at 11:28 AM Pramod Kumar <pramod.kumar@broadcom.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@arm.com> wrot=
e:
>>>
>>> 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 c=
pu_die features hence
>> used PSCI reference in my subject line to be more clear where this die g=
ets 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 p=
lugged in a sever over a PCIe interface.  This Card gets supply from a batt=
ery, 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 b=
attery 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 s=
ystem 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 indiv=
idual CPUs in sequence hence uses  ipi_cpu_stop for all other CPUs which ul=
timately switch to ATF to flush out all the CPUs caches and comes out of co=
herency domain so that its power rails could be switched-off.
>>
>>>
>>> > Apart from this, this gives control to f/w to reduce power consumptio=
n
>>> > 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 no=
t
>>> necessary and may consume lot of time in systems with large number of C=
PUs.
>>> It would be good to know the use-case in case I am missing to consider
>>> that.
>>
>>
>> Use case as explained above.
>>
>>>
>>> --
>>> Regards,
>>> Sudeep