Re: [RFC/PATCH 0/5] DVFS in the OPP core

[Viresh Kumar <viresh.kumar@linaro.org>]

On 06-02-19, 23:58, Stephen Boyd wrote:
> Quoting Viresh Kumar (2019-01-31 01:23:49)
> > FWIW, I suggested exactly this solution sometime back [1]
> > 
> > - Drivers need to use two API sets to change clock rate (OPP helpers)
> >   and enable/disable them (CLK framework helpers) and this leads us to
> >   exactly that combination. Is that acceptable ? It doesn't look great
> >   to me as well..
> 
> Agreed. I don't think anyone thinks this looks great, but I'll argue
> that it's improving OPP for devices that already use it so that we can
> remove voltage requirements when their clk is off. Think about CPUs that
> are in their own clk domain where we want to remove the voltage
> requirement when those CPUs are offline, or a GPU that wants to remove
> its voltage requirement when it turns off clks. The combination is
> already out there, just OPP hasn't solved this problem.
> 
> The only other plan I had was to implement another API like
> dev_pm_set_state() or something like that and have that do something
> like what the OPP rate API does right now. The main difference being
> that the argument to the function would be some opaque u64 that's
> converted by the bus/class/genpd attached to the device into whatever
> frequency/voltage/performance state is desired (and sequenced in the
> right order too). And then I was thinking that runtime PM or explicit
> dev_pm_set_state() calls would be used to tell this new layer that the
> device was going to a lower power mode with some other number (sub-kHz
> integer?) and have that be translated again into some
> frequency/voltage/performance state.
> 
> Either way, each driver will have to change from using the clk APIs to
> changes rates to something else like one of these APIs, so I don't see a
> huge difference. Drivers will have to change.

I agree, that's why I wrote the dev_pm_opp_set_rate() API initially.

> > 
> > - Do we expect the callers will disable clk before calling
> >   opp-set-rate with 0 ? We should remove the regulator requirements as
> >   well along with perf-state.
> 
> Yes, that's the plan. Problems come along with this though, like shared
> resource constraints and actually knowing the clk on/off state,
> frequency, voltage, etc. at boot time and making sure to keep those
> constraints satisfied during normal operation.

But that isn't any different from drivers doing clk_disable directly,
right ? So that shouldn't worry us.

> > - What about enabling/disabling clock as well from OPP framework. We
> >   can enable it on the very first call to opp-set-rate and disable
> >   when freq is 0. That will simplify the drivers as well.
> 
> It works when those drivers aren't calling clk_disable() directly from
> some irq handler. I don't think that's very common, but in those cases
> we would probably want to allow drivers to quickly gate and ungate their
> clks but then defer the sleeping stuff (voltages and off chip clks) to
> the schedulable contexts. We'll still be left with a small number of
> drivers that want to only call clk_prepare() and clk_unprepare() from
> within OPP and keep calling clk_enable() and clk_disable() from their
> driver. So introduce different APIs for those drivers to indicate this
> to OPP? And only do that when it becomes a requirement?

I am not sure I understood this well. The reference counting within
clk/regulator should let both the layers (driver and opp core) work
just fine. Why would a driver don't want OPP core to call
clk_prepare_enable() all the time ?

> Otherwise I don't really see a problem with the OPP call handling the
> enable state of the clk as well.

Right, so I would like that to be part of this series when this gets
implemented.

> > > One nice feature of this approach is that we don't need to change the
> > > OPP binding to support this. We can specify only the max frequencies and
> > > the voltage requirements in DT with the existing binding and slightly
> > > tweak the OPP code to achieve these results. 
> > > 
> > > This series includes a conversion of the uart and spi drivers on
> > > qcom sdm845 where these patches are being developed. It shows how a
> > > driver is converted from the clk APIs to the OPP APIs and how
> > > enable/disable state of the clk is communicated to the OPP layer.
> > > 
> > > Some open topics and initial points for discussion are:
> > > 
> > > 1) The dev_pm_opp_set_rate() API changes may break something that's 
> > > relying on the rate rounding that OPP provides. If those exist,
> > > we may need to implement another API that is more explicit about using
> > > the clk API instead of the OPP tables.

I don't remember any such cases, I may have forgotten about those
though.

> > > 2) dev_pm_opp_set_rate(0) is an interesting solution to the problem of
> > > dropping the rate requirement. Is there anything better than this?

I am okay with it. I don't want to invent another set of APIs to
enable / disable the resources.

> > > 3) How do we handle devices that already have power-domains specified in
> > > DT? The opp binding for required-opps doesn't let us specify the power
> > > domain to target, instead it assumes that whatever power domain is
> > > attached to a device is the one that OPP needs to use to change the
> > > genpd performance state. Do we need a
> > > dev_pm_opp_set_required_opps_name() or something to be explicit about

Yeah, we may need to come up with something like this eventually. I
had written something like that earlier, but then it wasn't required.

> > > this? Can we have some way for the power domain that required-opps
> > > correspond to be expressed in the OPP tables themselves?

Not sure I understood it. Can you explain with some example please ?

> > > 4) How do we achieve the "full constraint" state? i.e. what do we do
> > > about some devices being active and others being inactive during boot
> > > and making sure that the voltage for the shared power domains doesn't
> > > drop until all devices requiring it have informed OPP about their
> > > power requirements?

We need the boot-constraint framework for that. I think this is a
problem which we have currently as well. I am waiting for the
bus-scaling framework to get in, after that we will have lot of cases
where boot-constraints would be required and it won't be limited to
just clcd then.

-- 
viresh