Re: [PATCH V2 5/5] PM / Domains: Propagate performance state updates

[Stephen Boyd]

Quoting Ulf Hansson (2018-12-03 05:38:35)
> + Stephen, Mike, Graham
> 
> On Fri, 30 Nov 2018 at 12:06, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> >
> > On 30-11-18, 11:18, Ulf Hansson wrote:
> There is one a big difference while comparing with clocks, which make
> this more difficult.
> 
> That is, in dev_pm_genpd_set_performance_state(), we are *not* calling
> ->the set_performance_state() callback of the genpd, unless the genpd
> is already powered on. Instead, for that case, we are only aggregating
> the performance states votes, to defer to invoke
> ->set_performance_state() until the genpd becomes powered on. In some
> way this makes sense, but for clock_set_rate(), the clock's rate can
> be changed, no matter if the clock has been prepared/enabled or not.
> 
> I recall we discussed this behavior of genpd, while introducing the
> performance states support to it. Reaching this point, introducing the
> master-domain propagation of performance states votes, we may need to
> re-consider the behavior, as there is evidently an overhead that grows
> along with the hierarchy.
> 
> As a matter of fact, what I think this boils to, is to consider if we
> want to temporary drop the performance state vote for a device from
> genpd's ->runtime_suspend() callback. Thus, also restore the vote from
> genpd's ->runtime_resume() callback. That's because, this is directly
> related to whether genpd should care about whether it's powered on or
> off, when calling the ->set_performance_state(). We have had
> discussions at LKML already around this topic. It seems like we need
> to pick them up to reach a consensus, before we can move forward with
> this.

What are we trying to gain consensus on exactly?

I've been thinking that we would want there to be a performance state
'target' for the 'devices are runtime suspended' state that we apply
when the genpd is powered down from runtime PM suspend of all devices in
the domain. This would then be overwritten with whatever the aggregated
performance state is when the genpd is powered back on due to some
device runtime resuming. Don't we already sort of have support for this
right now in genpd with commit fc5cbf0c94b6 ("PM / Domains: Support for
multiple states")? So I would think that we either let that state_idx
member be 0 or some implementation defined 'off' state index of the
genpd that can be achieved.

I was also thinking that the genpd_power_state was more than just a
bunch of idle states of a device so that we could combine
on/off/idle/active(performance states?) into one genpd_power_state
space that is affected by idle and on/off runtime PM status.

In the Qualcomm use-case, the state of the clk, either on or off, is
factored into the decision to consider the voltage constraint that the
clk has on the CX domain. So when the clk is disabled, the voltage
constraint on CX can be removed/ignored in the aggregation equation
because the hardware isn't clocking. This needs to work properly so that
devices can disable their clks and save power.

I hope that we can move clk on/off/rate control to be implemented with
clk domains based upon genpds so that we can generically reason about
genpds being on/off and at some performance state (i.e. a frequency)
instead of making something clk specific in the genpd code. If we can do
that, then this can be stated as a slave genpd (the clk domain) that's
linked to a master genpd (the voltage/corner domain) can only affect the
performance state requirement of the master genpd when the slave genpd
is on. When the slave genpd is off, the performance state requirement is
dropped and the master domain settles to a lower requirement based on
the other domains linked to it that are on. The clk domain would turn on
and off when the set of devices it is attached to are all suspended and
that would "do the right thing" by bubbling up the requirements to the
master domain this way.