linux-kernel - Re: [PATCH v3 5/7] cpufreq: add function to get the hardware max frequency

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Message-ID: <20200213125918.GA2397@arm.com>
Date:   Thu, 13 Feb 2020 12:59:18 +0000
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     catalin.marinas@....com, will@...nel.org, mark.rutland@....com,
        maz@...nel.org, suzuki.poulose@....com, sudeep.holla@....com,
        lukasz.luba@....com, rjw@...ysocki.net, peterz@...radead.org,
        mingo@...hat.com, vincent.guittot@...aro.org,
        viresh.kumar@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 5/7] cpufreq: add function to get the hardware max
 frequency

On Thursday 13 Feb 2020 at 11:59:56 (+0000), Valentin Schneider wrote:
> On 2/11/20 6:45 PM, Ionela Voinescu wrote:
> > +/**
> > + * cpufreq_get_hw_max_freq - get the max hardware frequency of the CPU
> > + * @cpu: CPU number
> > + *
> > + * The default return value is the max_freq field of cpuinfo.
> > + */
> > +__weak unsigned int cpufreq_get_hw_max_freq(unsigned int cpu)
> > +{
> > +	struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
> > +	unsigned int ret_freq = 0;
> > +
> > +	if (policy) {
> > +		ret_freq = policy->cpuinfo.max_freq;
> > +		cpufreq_cpu_put(policy);
> 
> What about intel_pstate / turbo stuff? IIRC one of Giovanni's issues was that
> turbo freq is not always reported as the max freq. Dunno if we can do
> anything about it; at the very least maybe document the caveat?
>

Okay, I can add details in the description in regards to potential
reasons to overwrite this function. But basically this is one of the
reasons for making this a weak function. The best information we can
generically get for maximum hardware frequency is cpuinfo.max_freq.
But if platforms have the possibility to obtain this differently from
either hardware or firmware they can overwrite this.

Thanks,
Ionela.

> > +	}
> > +
> > +	return ret_freq;
> > +}
> > +EXPORT_SYMBOL(cpufreq_get_hw_max_freq);
> > +
> >  static unsigned int __cpufreq_get(struct cpufreq_policy *policy)
> >  {
> >  	if (unlikely(policy_is_inactive(policy)))
>