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

[Ulf Hansson]

On Tue, 4 May 2021 at 09:18, Rajendra Nayak <rnayak@xxxxxxxxxxxxxx> wrote:
>
>
> []...
> >>>>
> >>>> 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.
>
> That's true, the 2 threads are different in the sense that one talks
> about having OPP core managing power on/off along with setting perf state,
> while the other talks about genpd core managing a default perf state
> along with power on/off, but they are similar in the sense that both
> are related to the discussion whether we should treat powering on and off
> a domain related to setting its performance state or if it should be
> considered completely orthogonal.
>
> I think the clock framework treats setting clock rates and turning
> on/off a clock orthogonal because there is an inherent assumption that
> once the clock is turned off, what rate it was set to should not matter,
> and it can be running at the same rate when we turn the clock back on.
>
> I guess we can have the same assumption here that a perf state of a
> power domain should not matter if the power domain is turned off
> and hence the perf state need not be dropped explicitly during power off,
> atleast that should be true for the qcom power domains supporting perf
> state upstream.
>
> Should that be the approach taken here? I guess that would mean the patch
> I had proposed earlier [1] to manage this in the genpd core would have to set the default
> perf state at attach and remove it only during a detach of the device to
> the pm_domain, and not manage it during the runtime_suspend/resume of the device.

Right, I think this would be a step in the right direction, but it's
not sufficient to solve the complete problem. As you also point out
below.

>
> >> 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?
>
> The above approach should take care of this but the down side of it would be,
> unlike in the case of clocks where the devices assigning a default clock rate
> might be doing so on a device specific clock (rarely shared with other devices)
> in case of power domain, and especially in the qcom implementation of these
> power domains which support perf state, these can be large domains with lots of devices,
> and any device being active (not necessarily wanting any default perf state) will keep
> the domain at the default perf state, requested by a device which isn't really active.

Yep, this certainly sounds suboptimal. To me, this isn't good enough.

>
> > What about doing this just for the assigned-performance-state case as
> > the clients don't want to play with it at all.
>
> well, thats possible too, but you obviously can't reuse the same bindings
> in such cases

Not sure I understand the issue with the DT binding? Let me elaborate
on how I think we could move forward.

It looks like we have two problems to solve:

*) We need a new DT binding.
If that becomes a generic property along the lines of the
"assigned-performance-state" as suggested - or if we decide to add a
SoC specific binding via using an additional cell in "power-domains"
(suggested by Rob), doesn't really matter much to me. The arguments
for the new DT property are very much similar to why we added
"assigned-clock-rates" for clocks.

**) We want to avoid boiler-plate code in drivers to manage
"assigned-performance-state" for their devices.
No matter what DT property we decide on (generic or SoC specific), we
should be able to manage this from the PM domain (genpd) layer. No
changes in the drivers should be needed.
If a generic binding is used, we could consider to let genpd
internally manage the whole thing (DT parsing and updating performance
state votes for assigned-performance-state only).
If we go for an SoC specific binding, the genpd provider needs to be
updated. It can manage DT parsing from the ->attach|detach_dev()
callbacks and update performance votes from the ->start|stop()
callbacks.
We could also consider a hybrid of these two solutions.

>
> [1] https://lore.kernel.org/patchwork/patch/1284042/

Kind regards
Uffe