[PATCH v3 02/15] dt/bindings: Update binding for PM domain idle states

Hi Lina,

Apologies, I sent this reply before and automatically included an "IMPORTANT
NOTICE" footer, please disregard that email, here's the same thing without the
footer.

On Thu, Aug 11, 2016 at 03:10:23PM -0600, Lina Iyer wrote:

On Wed, Aug 10 2016 at 12:09 -0600, Sudeep Holla wrote:

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On 10/08/16 17:40, Lina Iyer wrote:

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Hi Sudeep,

On Wed, Aug 10 2016 at 09:15 -0600, Sudeep Holla wrote:

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Hi Lina,

I have few concerns mainly due to the lack of description and not the
binding per say.

[...]

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It is pretty clear that CPUs cannot not define the domain idle states.
Domains define their own idle states. Just as you mention above. CPU is
just a single component in its domain. There may be other devices like
PMUs, Coresights etc that also may have a say in the idle state the
domain may be put in, when the devices are idle. As such, adding domain
idle states to the CPU's idle state property is not appropriate.

No I am not saying we need to add domain idle states to the CPU's idle
state property. I am saying we need to remove cpu-idle-states or ignore
it when PD is present. And get all the idle state information for PD.

I am objecting the split we are creating across CPU and higher level
power domains. And this binding document is incomplete as it skips all
those details. We just need PD handle in CPU and no idle state
information there. Create PD hierarchy and have all idle state
information at one place.

Let me think about this a bit and see what I can come up with.

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Our kernel has runtime PM for devices and then there is CPUidle, both
are diverging without one knowing about the other. We have to start
unifying them inorder to have better holistic power management in the
SoC. To that regard, we have to start imagining CPUs as just another
device, albeit a special device. But for our purposes in determining
domain idle state, it will just be a device attached to the domain.

Absolutely agree on that. No arguments. I am asking to go a step ahead
to include even cpu/core level power domains not just cluster/higher
level domains.

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We need to have all the idle state information at one place and in this
case PD seems more appropriate instead of splitting them across.

That approach isn't correct. Where will we put the idle states of other
devices that are also part of the domain? We are thinking about a model,
where every device defines its own idle states and we define
relationships between those idle states and their parents' idle states.

Yes I understand. You confused me here. Won't that be one-to-one
relationship ? If not, how is that dealt in the current bindings ?

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Ofcourse, devices don't have idle states today, but that is something we
have been pondering over.

Yes we these binding should be easily extensible, I don't see any issue.

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We can also keep the code clean and not break compatibility. Whenever
both PD and CPU contains idle-states, PD must take precedence.

Why?
The CPU and PD states are orthogonal. While the PD state is dependent on
the CPU state, the latter is not true. Devices determine their own
states. Based on the individual device states, we then determine the
state of the parent and bubble up on the hierarchy.

I may be missing something. Now with your example in the binding, if
another device shares the cluster PD, can it have different idle states?
If so how does it map ?


In general whatever binding we come up must not just address OS
coordinated mode. Also I was thinking to have better coverage in
the description by having a bit more complex system like:

cluster0
CLUSTER_RET(Retention)
CLUSTER_PG(Power Gate)
core0
	CORE_RET
	CORE_PG
core1
	CORE_RET
	CORE_PG

cluster1
CLUSTER_RET
CLUSTER_PG
core0
	CORE_RET
	CORE_PG
core1
	CORE_RET
	CORE_PG

Platform Co-ordinate supports the following states and we should
be able to determine that from the binding:

CORE_RET
CORE_PG
CORE_RET + CLUSTER_RET

The problem that we have to sove here is knowing that CORE_RET +
CLUSTER_PG (hypothetically) an invalid combination. Kevin and
I debated it in the earlier RFC and we dont have a good way to solve
this generically for all devices.

This is interesting. I had been working on the assumption that a parent
power domain cannot enter any idle state until its children were all in
their deepest idle state. I now realise that it's easy to imagine
platforms where this isn't the case.

However, I don't understand how your current bindings solve this issue
and why using domain-power-states for all states (i.e. ignoring
cpu-idle-states and putting CPU idle states in the domain-idle-states of
a per-CPU power domain - I believe this is what Sudeep is suggesting)
makes it any more difficult.

Could you link to this previous discussion you mentioned? I'm having
trouble finding it (R.I.P Gmane).

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CORE_PG + CLUSTER_RET
CORE_PG + CLUSTER_PG

Cheers,
Brendan