linux-kernel - Re: [PATCH] arm64: topology: Don't support AMU without cpufreq

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Date:   Fri, 10 Jul 2020 08:58:12 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: topology: Don't support AMU without cpufreq

On 09-07-20, 13:46, Ionela Voinescu wrote:
> I saw this case during FVP testing, although I acknowledge the 'virtual'
> part of that platform [1]. But allowing this does enable AMU testing on
> an AEM FVP.

In kernel, we only support things that are in mainline, else we don't
care about them. That's the general rule. And yeah I understand that
this is early support for a new hardware, and so it is better to add
code for things we are sure about.

> While I completely understand the reasoning behind avoiding to introduce
> large changes for small corner-case gains,

I think even that is fine, if there is a problem to be solved it needs
to be solved, big or small doesn't really matter. Just that it needs
to be there in mainline.

> the arguments for this
> support was:
>  - (1) AMUs are a new feature and it will take some time until we see the
>    real usecases. That's always the case with early support for a
>    feature - we want to add it early to enable its use and testing, but
>    it will take some time to establish the true usecases.

Exactly, and so people normally prefer to keep things simple until the
time the needs arises for the same. A patch can be added later, its no
big deal. But it should be added when we need it.

>  - (2) It literally needed 2 lines of code + the weak cpufreq function
>    to support this.

Yeah, small or big doesn't really matter.

> Given that I can't guarantee what hardware will or won't do, and given
> that AMUs are an optional feature, I controlled the only thing I could:
> the software :). By not making assumptions about the hardware, I ensured
> that the code does not break the interaction between cpufreq use or AMU
> use for frequency invariance.
> 
> This will be nicer in the new code as the control will be at CPU level,
> rather than policy level.

I won't try to force you to remove this piece and will leave it for
you to decide.

But, I don't see a future system in mainline which uses AMU but
doesn't have cpufreq for all its CPUs. And so I won't have kept code
for that, even if it is just 2 lines. We can always add it back when
required.

Thanks for the review again Ionela.

-- 
viresh