linux-kernel - Re: [PATCH v4 3/4] scmi-cpufreq: get opp_shared

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Date:   Tue, 8 Dec 2020 12:22:22 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Nicola Mazzucato <nicola.mazzucato@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pm@...r.kernel.org, devicetree@...r.kernel.org,
        rjw@...ysocki.net, vireshk@...nel.org, robh+dt@...nel.org,
        sboyd@...nel.org, nm@...com, daniel.lezcano@...aro.org,
        morten.rasmussen@....com, chris.redpath@....com
Subject: Re: [PATCH v4 3/4] scmi-cpufreq: get opp_shared_cpus from opp-v2 for
 EM

On Tue, Dec 08, 2020 at 11:34:36AM +0000, Lukasz Luba wrote:
> 
> 
> On 12/8/20 11:20 AM, Sudeep Holla wrote:
> > On Tue, Dec 08, 2020 at 12:56:11PM +0530, Viresh Kumar wrote:
> > > On 08-12-20, 07:22, Nicola Mazzucato wrote:
> > > > On 12/8/20 5:50 AM, Viresh Kumar wrote:
> > > > > On 02-12-20, 17:23, Nicola Mazzucato wrote:
> > > > > >   	nr_opp = dev_pm_opp_get_opp_count(cpu_dev);
> > > > > >   	if (nr_opp <= 0) {
> > > > > > -		dev_dbg(cpu_dev, "OPP table is not ready, deferring probe\n");
> > > > > > -		ret = -EPROBE_DEFER;
> > > > > > -		goto out_free_opp;
> > > > > > +		ret = handle->perf_ops->device_opps_add(handle, cpu_dev);
> > > > > > +		if (ret) {
> > > > > > +			dev_warn(cpu_dev, "failed to add opps to the device\n");
> > > > > > +			goto out_free_cpumask;
> > > > > > +		}
> > > > > > +
> > > > > > +		ret = dev_pm_opp_set_sharing_cpus(cpu_dev, opp_shared_cpus);
> > > > > > +		if (ret) {
> > > > > > +			dev_err(cpu_dev, "%s: failed to mark OPPs as shared: %d\n",
> > > > > > +				__func__, ret);
> > > > > > +			goto out_free_cpumask;
> > > > > > +		}
> > > > > > +
> > > > > 
> > > > > Why do we need to call above two after calling
> > > > > dev_pm_opp_get_opp_count() ?
> > > > 
> > > > Sorry, I am not sure to understand your question here. If there are no opps for
> > > > a device we want to add them to it
> > > 
> > > Earlier we used to call handle->perf_ops->device_opps_add() and
> > > dev_pm_opp_set_sharing_cpus() before calling dev_pm_opp_get_opp_count(), why is
> > > the order changed now ?
> > > 
> > > 
> > > I am not sure why they would be duplicated in your case. I though
> > > device_opps_add() is responsible for dynamically adding the OPPs here.
> > > 
> > 
> > It is because of per-CPU vs per domain drama here. Imagine a system with
> > 4 CPUs which the firmware puts in individual domains while they all are
> > in the same perf domain and hence OPP is marked shared in DT.
> > 
> > Since this probe gets called for all the cpus, we need to skip adding
> > OPPs for the last 3(add only for 1st one and mark others as shared).
> > If we attempt to add OPPs on second cpu probe, it *will* shout as duplicate
> > OPP as we would have already marked it as shared table with the first cpu.
> > Am I missing anything ? I suggested this as Nicola saw OPP duplicate
> > warnings when he was hacking up this patch.
> > 
> > > > otherwise no need as they would be duplicated.
> > > > > And we don't check the return value of
> > > > > the below call anymore, moreover we have to call it twice now.
> > 
> > Yes, that looks wrong, we need to add the check for non zero values, but ....
> > 
> > > > 
> > > > This second get_opp_count is required such that we register em with the correct
> > > > opp number after having added them. Without this the opp_count would not be correct.
> > > 
> > 
> > ... I have a question here. Why do you need to call
> > 
> > em_dev_register_perf_domain(cpu_dev, nr_opp, &em_cb, opp_shared_cpus..)
> > 
> > on each CPU ? Why can't that be done once for unique opp_shared_cpus ?
> 
> It just have to be called once, for one CPU from the mask. Otherwise for
> the next CPUs you should see error:
> "EM: exists for CPU%d"

OK cool, at least it is designed and expected to be used like I thought.
Ah, I might have seen those, but never thought it was error message 😄 

> It can happen that this print is not seen when the get_cpu_device(cpu)
> failed, but that would lead to investigation why CPU devices are not
> there yet.
>
> Nicola: have you seen that print?
>

I assume you must see that and you need to pull this inside if condition
to do this once for each performance domain.

--
Regards,
Sudeep