Re: [PATCH v3 5/7] drivers: firmware: psci: Add hierarchical domain idle states converter

[Ulf Hansson]

On Fri, 7 Feb 2020 at 17:15, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
>
> On Fri, Feb 07, 2020 at 04:52:52PM +0100, Ulf Hansson wrote:
> > On Fri, 7 Feb 2020 at 15:48, Lorenzo Pieralisi
> > <lorenzo.pieralisi@xxxxxxx> wrote:
> > >
> > > On Fri, Feb 07, 2020 at 01:32:28PM +0100, Ulf Hansson wrote:
> > > > [...]
> > > >
> > > > > > I understand the arguments for using PC vs OSI and agree with it. But
> > > > > > what in PSCI is against Linux knowing when the last core is powering
> > > > > > down when the PSCI is configured to do only Platform Cordinated.
> > > > >
> > > > > Nothing :D. But knowing the evolution and reasons for adding OSI in the
> > > > > PSCI specification and having argued about benefits of OSI over PC for
> > > > > years and finally when we have it in mainline, this argument of using
> > > > > PC for exact reasons why OSI evolved is something I can't understand
> > > > > and I am confused.
> > > > >
> > > > > > There should not be any objection to drivers knowing when all the cores
> > > > > > are powered down, be it reference counting CPU PM notifications or using
> > > > > > a cleaner approach like this where GendPD framwork does everything
> > > > > > cleanly and gives a nice callback. ARM architecture allows for different
> > > > > > aspects of CPU access be handled at different levels. I see this as an
> > > > > > extension of that approach.
> > > > > >
> > > > >
> > > > > One thing that was repeatedly pointed out during OSI patch review was no
> > > > > extra overhead for PC mode where firmware can make decisions. So, just
> > > > > use OSI now and let us be done with this discussion of OSI vs PC. If PC
> > > > > is what you think you need for future, we can revert all OSI changes and
> > > > > start discussing again :-)
> > > >
> > > > Just to make it clear, I fully agree with you in regards to overhead
> > > > for PC-mode. This is especially critical for ARM SoCs with lots of
> > > > cores, I assume.
> > > >
> > > > However, the overhead you refer to, is *only* going to be present in
> > > > case when the DTS has the hierarchical CPU topology description with
> > > > "power-domains". Because, that is *optional* to use, I am expecting
> > > > only those SoC/platforms that needs to manage last-man activities to
> > > > use this layout, the others will remain unaffected.
> > >
> > > In PC mode not only there is no need but it is wrong to manage
> > > any last-man activity in the kernel. I wonder why we are still
> > > talking about this to be honest.
> >
> > I guess the discussion is here because there is a use case to consider now.
> >
>
> If this is what Bjorn presented in his email, I have responded to that.
> If it's any different, please let us know the complete details.
>
> > For sure, we agree on what is the best solution. But this is rather
> > about what can we do to improve the current situation, if we should do
> > anything.
> >
>
> Sure, and I haven't found a reason to do that in OSPM yet(as part of the
> discussion in this thread)
>
> > >
> > > Code to handle PSCI platform coordinated mode has been/is in
> > > the kernel today and that's all is needed according to the PSCI
> > > specifications.
> >
> > PSCI specifies CPU power management, not SoC power management. If
> > these things were completely decoupled, I would agree with you, but
> > that's not the case. Maybe SCMI, etc, helps with this in future.
> >
>
> Why does that not work even if they are not decoupled. The IO/device
> that share with CPU votes from OSPM and the CPU/Cluster from PSCI in
> PC mode. There is no argument there, but why it needs to be done in OSPM
> is the objection here.

That implies the votes from I/O devices needs to reach the FW
immediately when the vote is done. No caching or other optimizations
can be done at OSPM.

In principle, the FW needs to have an always up to date view of the
votes, etc. That sounds highly inefficient, both from energy and
latency point of view, at least in my opinion.

>
> > Anyway, my fear is that not many ARM vendors implements OSI support,
> > but still they have "last-man-activities" to deal with. This is not
> > only QCOM.
> >
>
> I am interested to hear from them. And the same question to same too as
> above.

I have been talking to some of them. But, yes, we need to hear more from them.

>
> > I guess an option would be to add OSI support to the public ARM
> > Trusted Firmware, then we could more easily point to that - rather
> > than trying to mitigate the problem on the kernel side.
> >
>
> I would say go for it. But don't mix responsibility of OSPM in PC vs OSI.
> We have discussed this for years and I hope this discussion ends ASAP.
> I don't see any point in dragging this any further.

Okay.

KInd regards
Uffe