Re: [PATCH v1 0/3] Add required-opps support to devfreq passive gov

[Saravana Kannan]

On Thu, Jun 27, 2019 at 11:49 PM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>
> On 26-06-19, 11:10, Saravana Kannan wrote:
> > On Tue, Jun 25, 2019 at 11:32 PM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>
> > > So, when a CPU changes frequency, we must change the performance state
> > > of PM domain and change frequency/bw of the cache synchronously.
> >
> > I mean, it's going to be changed when we get the CPUfreq transition
> > notifiers. From a correctness point of view, setting it inside the OPP
> > framework is not any better than doing it when we get the notifiers.
>
> That's what the problem is. All maintainers now a days ask people to
> stay away from notifiers and we are making that base of another new
> thing we are starting.

In that case we can just add direct calls in cpufreq.c to let devfreq
know about the frequency changes. But then again, CPU is just one
example for this use case. I'm just using that because people are more
familiar with that.

> Over that, with many cpufreq drivers we have fast switching enabled
> and notifiers disabled. How will they make these things work ? We
> still want to scale L3 in those cases as well.

Nothing is preventing them from using the xlate OPP API I added to
figure out all the CPU to L3 frequency mapping and then set the L3
frequency directly from the CPUfreq driver.

Also, whether we use OPP framework or devfreq to set the L3 frequency,
it's going to block fast switching because both these frameworks have
APIs that can sleep.

But really, most mobile use cases don't want to permanently tie L3
freq to CPU freq. Having it go through devfreq and being able to
switch governors is a very important need for mobile products.

Keep in mind that nothing in this series does any of the cpufreq stuff
yet. That'll need a few more changes. I was just using CPUfreq as an
example.

> > I see this as "required for good performance". So I don't see it as
> > redefining required-opps. If someone wants good performance/power
> > balance they follow the "required-opps". Technically even the PM
> > pstates are required for good power. Otherwise, the system could leave
> > the voltage at max and stuff would still work.
> >
> > Also, the slave device might need to get input from multiple master
> > devices and aggregate the request before setting the slave device
> > frequency. So I don't think OPP  framework would be the right place to
> > deal with those things. For example, L3 might (will) have different
> > mappings for big vs little cores. So that needs to be aggregated and
> > set properly by the slave device driver. Also, GPU might have a
> > mapping for L3 too. In which case the L3 slave driver needs to take
> > input from even more masters before it decides its frequency. But most
> > importantly, we still need the ability to change governors for L3.
> > Again these are just examples with L3 and it can get more complicated
> > based on the situation.
> >
> > Most importantly, instead of always going by mapping, one might decide
> > to scale the L3 based on some other governor (that looks at some HW
> > counter). Or just set it to performance governor for a use case for
> > which performance is more important. All of this comes for free with
> > devfreq and if we always set it from OPP framework we don't give this
> > required control to userspace.
> >
> > I think going through devfreq is the right approach for this. And we
> > can always rewrite the software if we find problems in the future. But
> > as it stands today, this will help cases like exynos without the need
> > for a lot of changes. Hope I've convinced you.
>
> I understand the aggregation thing and fully support that the
> aggregation can't happen in OPP core and must be done somewhere else.
> But the input can go from OPP core while the frequency is changing,
> isn't it ?

I'm not opposed to OPP sending input to devfreq to let it know that a
master device frequency change is happening. But I think this is kinda
orthogonal to this patch series.

Today the passive governor looks at the master device's devfreq
frequency changes to trigger the frequency change of the slave
devfreq. It neither supports tracking OPP frequency change nor CPUfreq
frequency change. If that's something we want to add, we can look into
that separately as passive governor (or a new governor) changes.

But then not all devices (CPUfreq or otherwise) use OPP to set the
frequencies. So it's beneficial to have all of these frameworks as
inputs for devfreq passive (like) governor. CPUfreq is actually a bit
more tricky because we'll also have to track hotplug, etc. So direct
calls from CPUfreq to devfreq (similar to cpufreq stats tracking)
would be good.

-Saravana