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

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Message-ID: <20201209054502.ajomw6glcxx5hue2@vireshk-i7>
Date:   Wed, 9 Dec 2020 11:15:02 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     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 08-12-20, 11:20, Sudeep Holla wrote:
> 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).

Okay and this wasn't happening before this series because the firmware
was only returning the current CPU from scmi_get_sharing_cpus() ?

Is this driver also used for the cases where we have multiple CPUs in
a policy ? Otherwise we won't be required to call
dev_pm_opp_set_sharing_cpus().

So I assume that we want to support both the cases here ?

> 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.

The common stuff (for all the CPUs) is better moved to probe() in this
case, instead of the ->init() callback. Otherwise it will always be
messy. You can initialize the OPP and cpufreq tables in probe()
itself, save the pointer somewhere and then just use it here in
->init().

Also do EM registration from there.

> > > 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 ?
> 
> The whole drama of per-CPU vs perf domain is to have energy model and
> if feeding it opp_shared_cpus once is not sufficient, then something is
> wrong or simply duplicated or just not necessary IMO.
> 
> > What if the count is still 0 ? What about deferred probe we were doing earlier ?
> 
> OK, you made me think with that question. I think the check was original
> added for deferred probe but then scmi core was changed to add the cpufreq
> device only after everything needed is ready. So the condition must never
> occur now.

The deferred probe shall be handled in a different patch in that case.

Nicola, please break the patch into multiple patches, with one patch
dealing only with one task.

-- 
viresh