linux-kernel - Re: [PATCH v2 1/5] arch_topology: Introduce thermal pressure update function

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Date:   Thu, 28 Oct 2021 16:12:27 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, sudeep.holla@....com,
        will@...nel.org, catalin.marinas@....com, linux@...linux.org.uk,
        gregkh@...uxfoundation.org, rafael@...nel.org,
        viresh.kumar@...aro.org, amitk@...nel.org,
        daniel.lezcano@...aro.org, amit.kachhap@...il.com,
        thara.gopinath@...aro.org, agross@...nel.org
Subject: Re: [PATCH v2 1/5] arch_topology: Introduce thermal pressure update
 function

On Thu 28 Oct 00:16 PDT 2021, Lukasz Luba wrote:

> 
> 
> On 10/27/21 7:43 PM, Bjorn Andersson wrote:
> > On Fri 15 Oct 07:45 PDT 2021, Lukasz Luba wrote:
> > > diff --git a/arch/arm64/include/asm/topology.h b/arch/arm64/include/asm/topology.h
> > [..]
> > > +/**
> > > + * topology_thermal_pressure_update() - Update thermal pressure for CPUs
> > > + * @cpus	: The related CPUs for which capacity has been reduced
> > > + * @capped_freq	: The maximum allowed frequency that CPUs can run at
> > 
> > I know this matches what I see in e.g. the Qualcomm cpufreq hw driver,
> > but in what cases will @capped_freq differ from
> > cpufreq_get_hw_max_freq(cpumask_first(cpus))?
> 
> The @capped_freq is the maximum allowed frequency value due to
> thermal reasons, which will always be lower or equal to the value
> returned by cpufreq_get_hw_max_freq()
> (effectively: 'policy->cpuinfo.max_freq').
> 

Read patch 3 and 4 again and now this makes sense to me.

Thanks,
Bjorn

> We limit the frequency (and voltage) of CPU to reduce power (and heat)
> in the passive cooling system. That information is important to us,
> because scheduler needs to know how fast the CPU can go. It cannot
> assume that the speed is always 'policy->cpuinfo.max_freq'. Often
> it's less then that at heavy load or GPU heavy load (the same SoC).
> 
> Regards,
> Lukasz