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

[Saravana Kannan <saravanak@google.com>]

Bump. I saw your email in Sibi's patch series. His patch series is
just one use case/user of this feature. This patch series is useful in
general. Do plan to Ack this? Or thoughts on my earlier response?

Thanks,
Saravana

On Fri, Jun 28, 2019 at 1:26 PM Saravana Kannan <saravanak@google.com> wrote:
>
> On Thu, Jun 27, 2019 at 11:49 PM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> >
> > On 26-06-19, 11:10, Saravana Kannan wrote:
> > > On Tue, Jun 25, 2019 at 11:32 PM Viresh Kumar <viresh.kumar@linaro.org> 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