linux-kernel - Re: [PATCH v2 2/2] [RFC] CPUFreq: Add support for cpu-perf-dependencies

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Date:   Mon, 19 Oct 2020 15:16:33 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Nicola Mazzucato <nicola.mazzucato@....com>
Cc:     Lukasz Luba <lukasz.luba@....com>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        devicetree@...r.kernel.org, linux-pm@...r.kernel.org,
        vireshk@...nel.org, daniel.lezcano@...aro.org, rjw@...ysocki.net,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org,
        chris.redpath@....com, morten.rasmussen@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 2/2] [RFC] CPUFreq: Add support for
 cpu-perf-dependencies

On 19-10-20, 09:50, Nicola Mazzucato wrote:
> Hi Viresh,
> 
> thank you for your suggestion on using 'opp-shared'.
> I think it could work for most of the cases we explained earlier.
> 
> Summarising, there are two parts of this entire proposal:
> 1) where/how to get the information: now we are focusing on taking advantage of
> 'opp-shared' within an empty opp table
> 2) and how/where this information will be consumed
> 
> Further details below:
> 
> 1) a CPUFreq driver that takes the OPPs from firmware, can call
> dev_pm_opp_of_get_sharing_cpus like you suggested. When doing so, a provided
> cpumaksk will be populated with the corresponding cpus that share the same
> (empty) table opp in DT.
> All good so far.

Great.

> The current opp core is not expecting an empty table and therefore some errors
> are thrown when this happens.
> Since we are now allowing this corner-case, I am presenting below where I think
> some minor corrections may be needed:
> 
> --- a/drivers/opp/of.c
> +++ b/drivers/opp/of.c
> @@ static void _opp_table_alloc_required_tables(struct opp_table *opp_table,
>         struct device_node *required_np, *np;
>         int count, i;
> 
>         /* Traversing the first OPP node is all we need */
>         np = of_get_next_available_child(opp_np, NULL);
>         if (!np) {
> -               dev_err(dev, "Empty OPP table\n");
> +               dev_warn(dev, "Empty OPP table\n");
> +
> +               /*
> +                * With empty table we remove shared_opp. This is to leave the
> +                * responsibility to decide which opp are shared to the opp users
> +                */
> +               opp_table->shared_opp = OPP_TABLE_ACCESS_EXCLUSIVE;
> +
>                 return;
>         }
> 
> @@ int dev_pm_opp_of_find_icc_paths(struct device *dev,
>         int ret, i, count, num_paths;
>         struct icc_path **paths;
> 
>         ret = _bandwidth_supported(dev, opp_table);
> -       if (ret <= 0)
> +       if (ret == -EINVAL)
> +               return 0; /* Empty OPP table is a valid corner-case, let's not
> fail */
> +       else if (ret <= 0)
>                 return ret;
> 
> The above are not 'strictly' necessary to achieve the intended goal, but they
> make clearer that an empty table is now allowed and not an error anymore.
> What it is your point of view on this?

Why is this stuff getting called in your case ? We shouldn't be trying
to create an OPP table here and it should still be an error in the
code if we are asked to parse an empty OPP table.

> In addition, I think it would also be appropriate to update the documentation
> (Documentation/devicetree/bindings/opp/opp.txt) to reflect this new case
> (required properties etc).
> Any different thoughts?

Yes, this needs a small update in the required-opps section.

> 2) Once the driver gets the 'performance dependencies' by
> dev_pm_opp_of_get_sharing_cpus(), this information will have to be shared with
> EAS, thermal, etc.. The natural way to do so would be to add a new cpumask like
> I proposed in this RFC.
> I see this as an improvement for the whole subsystem and a scalable choice since
> we can unambiguously provide the correct information to whoever needs it, given
> that we don't enforce "hw dependencies" for related_cpus.
> The changes would be trivial (it's in the original RFC).
> On the other hand, we can't unload this h/w detail into related_cpus IMO as we
> are dealing with per-cpu systems in this context.
> Hope it makes sense?

I will have another look at this stuff, honestly I haven't looked at
this in detail yet. But I do understand that we can't really use
related-cpu here without changing its earlier meaning.

-- 
viresh