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

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Date:   Fri, 2 Jun 2017 10:20:09 +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 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.

-- 
Regards,
Sudeep