linux-kernel - Re: [PATCH 1/2] OPP: Use _set_opp

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Date:   Wed, 25 Oct 2023 12:40:26 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
        Stephen Boyd <sboyd@...nel.org>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Stephan Gerhold <stephan.gerhold@...nkonzept.com>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] OPP: Use _set_opp_level() for single genpd case

On Wed, 25 Oct 2023 at 08:55, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 19-10-23, 13:16, Ulf Hansson wrote:
> > On Thu, 19 Oct 2023 at 12:22, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > > +static int _link_required_opps(struct dev_pm_opp *opp, struct opp_table *opp_table,
> > >                                struct opp_table *required_table, int index)
> > >  {
> > >         struct device_node *np;
> > > @@ -314,6 +314,25 @@ static int _link_required_opps(struct dev_pm_opp *opp,
> > >                 return -ENODEV;
> > >         }
> > >
> > > +       /*
> > > +        * There are two genpd (as required-opp) cases that we need to handle,
> > > +        * devices with a single genpd and ones with multiple genpds.
> > > +        *
> > > +        * The single genpd case requires special handling as we need to use the
> > > +        * same `dev` structure (instead of a virtual one provided by genpd
> > > +        * core) for setting the performance state. Lets treat this as a case
> > > +        * where the OPP's level is directly available without required genpd
> > > +        * link in the DT.
> > > +        *
> > > +        * Just update the `level` with the right value, which
> > > +        * dev_pm_opp_set_opp() will take care of in the normal path itself.
> > > +        */
> > > +       if (required_table->is_genpd && opp_table->required_opp_count == 1 &&
> > > +           !opp_table->genpd_virt_devs) {
> > > +               if (!WARN_ON(opp->level))
> >
> > Hmm. Doesn't this introduce an unnecessary limitation?
> >
> > An opp node that has a required-opps phande, may have "opp-hz",
> > "opp-microvolt", etc. Why would we not allow the "opp-level" to be
> > used too?
>
> Coming back to this, why would we ever want a device to have "opp-level" and
> "required-opp" (set to genpd's table) ? That would mean we will call:
>
> dev_pm_domain_set_performance_state() twice to set different level values.

Yes - and that would be weird, especially since the PM domain (genpd)
is already managing the aggregation and propagation to parent domains.

I guess I got a bit confused by the commit message for patch2/2, where
it sounded like you were striving towards introducing recursive calls
to set OPPs. Having a closer look, I realize that isn't the case,
which I think makes sense.

>
> And so it should be safe to force that if required-opp table is set to a genpd,
> then opp-level shouldn't be set. Maybe we should fail in that case, which isn't
> happening currently.

Yes, it seems better to fail earlier during the OF parsing of the
required-opps or when adding an OPP dynamically. In that way, the
WARN_ON above could be removed.

That said, sorry for the noise and either way, feel free to add (for
$subject patch):

Reviewed-by: Ulf Hansson <ulf.hansson@...aro.org>

Kind regards
Uffe