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

[Viresh Kumar]

Sorry Roja for dragging this too long, unfortunately I didn't have a
lot to add on. Lemme try start this thread again.

On 19-01-21, 12:02, Ulf Hansson wrote:
> On Mon, 18 Jan 2021 at 06:36, Rajendra Nayak <rnayak@xxxxxxxxxxxxxx> 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?

So I looked again at that thread in detail today. The basic idea was
to enable/disable the genpd from within the OPP core and there were
doubts on how to do that efficiently as there are cases where domains
may be enabled for an OPP, but not for others.. etc. etc.

I am not sure if I consider that thread as part of the discussion we
are having here, they may be related, but that thread doesn't block
anything to be done in the genpd core.

> 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?

What about doing this just for the assigned-performance-state case as
the clients don't want to play with it at all.

-- 
viresh