linux-kernel - Re: [PATCH V2 4/5] cpufreq: Register notifiers with the PM QoS framework

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Date:   Mon, 25 Feb 2019 10:01:49 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>, mka@...omium.org,
        juri.lelli@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 4/5] cpufreq: Register notifiers with the PM QoS
 framework

On 22-02-19, 11:44, Qais Yousef wrote:
> Hi Verish
> 
> On 02/21/19 16:59, Viresh Kumar wrote:
> 
> [...]
> 
> > @@ -2239,6 +2314,8 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
> >  			      struct cpufreq_policy *new_policy)
> >  {
> >  	struct cpufreq_governor *old_gov;
> > +	struct device *cpu_dev = get_cpu_device(policy->cpu);
> > +	unsigned long min, max;
> >  	int ret;
> >  
> >  	pr_debug("setting new policy for CPU %u: %u - %u kHz\n",
> > @@ -2253,11 +2330,23 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
> >  	if (new_policy->min > new_policy->max)
> >  		return -EINVAL;
> >  
> > +	min = dev_pm_qos_read_value(cpu_dev, DEV_PM_QOS_MIN_FREQUENCY);
> > +	max = dev_pm_qos_read_value(cpu_dev, DEV_PM_QOS_MAX_FREQUENCY);
> > +
> > +	if (min > new_policy->min)
> > +		new_policy->min = min;
> > +	if (max < new_policy->max)
> > +		new_policy->max = max;
> > +
> 
> Assuming for example min and max range from 1-10, and thermal throttles max to
> 5 using pm_qos to deal with temporary thermal pressure. But shortly after
> a driver thinks that max shouldn't be greater than 7 for one reason or another.
> 
> What will happen after thermal pressure removes its constraint? Will we still
> remember the driver's request and apply it so max is set to 7 instead of 10?

Once everything comes via PM QoS, it will remember all the presently available
requests and choose a target min/max frequency based on that.

But even with this patchset, with half stuff done with PM QoS and half done with
cpufreq notifiers, it should still work that way only.

-- 
viresh