linux-kernel - Re: [PATCH 3/3] i2c: i2c-qcom-geni: Add support for 'assigned-performance-states'

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Date:   Tue, 19 Jan 2021 12:02:01 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Rajendra Nayak <rnayak@...eaurora.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Roja Rani Yarubandi <rojay@...eaurora.org>,
        Rob Herring <robh+dt@...nel.org>,
        Wolfram Sang <wsa@...nel.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Doug Anderson <dianders@...omium.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Matthias Kaehlcke <mka@...omium.org>, akashast@...eaurora.org,
        msavaliy@....qualcomm.com, parashar@...eaurora.org,
        Linux PM <linux-pm@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Andy Gross <agross@...nel.org>, linux-i2c@...r.kernel.org
Subject: Re: [PATCH 3/3] i2c: i2c-qcom-geni: Add support for 'assigned-performance-states'

On Mon, 18 Jan 2021 at 06:36, Rajendra Nayak <rnayak@...eaurora.org> wrote:
>
>
> On 1/15/2021 8:13 PM, Bjorn Andersson wrote:
> > On Thu 24 Dec 05:12 CST 2020, Roja Rani Yarubandi wrote:
> >
> >> @@ -629,6 +658,16 @@ static int __maybe_unused geni_i2c_runtime_suspend(struct device *dev)
> >>      struct geni_i2c_dev *gi2c = dev_get_drvdata(dev);
> >>
> >>      disable_irq(gi2c->irq);
> >> +
> >> +    /* Drop the assigned performance state */
> >> +    if (gi2c->assigned_pstate) {
> >> +            ret = dev_pm_genpd_set_performance_state(dev, 0);
> >> +            if (ret) {
> >> +                    dev_err(dev, "Failed to set performance state\n");
> >> +                    return ret;
> >> +            }
> >> +    }
> >> +
> >
> > Ulf, Viresh, I think we discussed this at the time of introducing the
> > performance states.
> >
> > The client's state does not affect if its performance_state should
> > be included in the calculation of the aggregated performance_state, so
> > each driver that needs to keep some minimum performance state needs to
> > have these two snippets.
> >
> > Would it not make sense to on enable/disable re-evaluate the
> > performance_state and potentially reconfigure the hardware
> > automatically?
>
> I agree, this will be repeated across multiple drivers which would
> need some minimal vote while they are active, handling this during
> genpd enable/disable in genpd core makes sense.

Initially that's what we tried out, but we realized that it was
difficult to deal with this internally in genpd, but more importantly
it also removed some flexibility from consumers and providers. See
commit 68de2fe57a8f ("PM / Domains: Make genpd performance states
orthogonal to the idlestates").

As a matter of fact this was quite recently discussed [1], which also
pointed out some issues when using the "required-opps" in combination,
but perhaps that got resolved? Viresh?

My concern is, if we would make this kind of change to the internals
of genpd, it would lead to the following limitation: A consumer driver
can no longer make its vote for its device to stick around, when the
device becomes runtime suspended - and how do we know that we never
need to support such a case?

>
> >
> > Regards,
> > Bjorn
> >
> >>      ret = geni_se_resources_off(&gi2c->se);
> >>      if (ret) {
> >>              enable_irq(gi2c->irq);
> >> @@ -654,6 +693,16 @@ static int __maybe_unused geni_i2c_runtime_resume(struct device *dev)
> >>      if (ret)
> >>              return ret;
> >>
> >> +    /* Set the assigned performance state */
> >> +    if (gi2c->assigned_pstate) {
> >> +            ret = dev_pm_genpd_set_performance_state(dev,
> >> +                                                     gi2c->assigned_pstate);
> >> +            if (ret) {
> >> +                    dev_err(dev, "Failed to set performance state\n");
> >> +                    return ret;
> >> +            }
> >> +    }
> >> +
> >>      enable_irq(gi2c->irq);
> >>      gi2c->suspended = 0;
> >>      return 0;

Kind regards
Uffe

[1]
https://lkml.org/lkml/2020/9/11/230