linux-kernel - Re: [PATCH V2] ARM: cpuidle: Support asymmetric idle definition

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Date:   Fri, 2 Jun 2017 10:39:21 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>, rjw@...ysocki.net,
        lorenzo.pieralisi@....com
Cc:     Sudeep Holla <sudeep.holla@....com>, leo.yan@...aro.org,
        "open list:CPUIDLE DRIVERS" <linux-pm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] ARM: cpuidle: Support asymmetric idle definition



On 02/06/17 10:25, Daniel Lezcano wrote:
> On 02/06/2017 11:20, Sudeep Holla wrote:
>>
>>
>> On 01/06/17 12:39, Daniel Lezcano wrote:
>>> Some hardware have clusters with different idle states. The current code does
>>> not support this and fails as it expects all the idle states to be identical.
>>>
>>> Because of this, the Mediatek mtk8173 had to create the same idle state for a
>>> big.Little system and now the Hisilicon 960 is facing the same situation.
>>>
>>> Solve this by simply assuming the multiple driver will be needed for all the
>>> platforms using the ARM generic cpuidle driver which makes sense because of the
>>> different topologies we can support with a single kernel for ARM32 or ARM64.
>>>
>>> Every CPU has its own driver, so every single CPU can specify in the DT the
>>> idle states.
>>>
>>> This simple approach allows to support the future dynamIQ system, current SMP
>>> and HMP.
>>>
>>> It is unoptimal from a memory point of view for a system with a large number of
>>> CPUs but nowadays there is no such system with a cpuidle driver on ARM.
>>>
>>
>> While I agree this may be simple solution, but just not necessary for
>> systems with symmetric idle states especially one with large number of
>> CPUs. I don't like to see 96 CPU Idle driver on say ThunderX. So we
>> *must* have some basic distinction done here.
>>
>> IMO, we can't punish a large SMP systems just because they don't have
>> asymmetric idle states.
> 
> Can you point me in the upstream kernel a DTS with 96 cpus and using the
> cpuidle-arm driver ?
> 

The bindings are upstream right. Not all DTS are upstream, firmware
generate them especially for large systems.

Check arch/arm64/boot/dts/cavium/thunder{,2}-{88,99}xx.dtsi, it has
supports PSCI and firmware can update DTB to add the idle states.
They are systems with 96 and 128 CPUs.

-- 
Regards,
Sudeep