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

On Sat, Feb 08, 2020 at 11:25:18AM +0100, Ulf Hansson wrote:

On Fri, 7 Feb 2020 at 17:15, Sudeep Holla [off-list ref] wrote:

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On Fri, Feb 07, 2020 at 04:52:52PM +0100, Ulf Hansson wrote:

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On Fri, 7 Feb 2020 at 15:48, Lorenzo Pieralisi
[off-list ref] wrote:

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On Fri, Feb 07, 2020 at 01:32:28PM +0100, Ulf Hansson wrote:

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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.

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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.

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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)

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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.

Sorry but I need to re-iterate, use OSI if you need all those fancy
caching and other optimizations.

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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.

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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.

I keep saying that but still responding to the discussions. I must stop ;-)

--
Regards,
Sudeep