Re: [PATCH v9 04/11] dt: psci: Update DT bindings to support hierarchical PSCI states

[Sudeep Holla]

On Thu, Oct 11, 2018 at 04:44:07PM +0200, Ulf Hansson wrote:
> +Raju
>
> On 10 October 2018 at 17:03, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
> > On Wed, Oct 03, 2018 at 04:38:17PM +0200, Ulf Hansson wrote:
> >> From: Lina Iyer <lina.iyer@xxxxxxxxxx>
> >>
> >> Update DT bindings to represent hierarchical CPU and CPU PM domain idle
> >> states for PSCI. Also update the PSCI examples to clearly show how
> >> flattened and hierarchical idle states can be represented in DT.
> >>
> >> Cc: Lina Iyer <ilina@xxxxxxxxxxxxxx>
> >> Signed-off-by: Lina Iyer <lina.iyer@xxxxxxxxxx>
> >> Co-developed-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> >> Signed-off-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> >> Reviewed-by: Rob Herring <robh@xxxxxxxxxx>
> >> Reviewed-by: Sudeep Holla <sudeep.holla@xxxxxxx>
> >> ---
> >>  .../devicetree/bindings/arm/psci.txt          | 156 ++++++++++++++++++
> >>  1 file changed, 156 insertions(+)
> >>
> >> diff --git a/Documentation/devicetree/bindings/arm/psci.txt b/Documentation/devicetree/bindings/arm/psci.txt
> >> index a2c4f1d52492..17aa3d3a1c8e 100644
> >> --- a/Documentation/devicetree/bindings/arm/psci.txt
> >> +++ b/Documentation/devicetree/bindings/arm/psci.txt
> >> @@ -105,7 +105,163 @@ Case 3: PSCI v0.2 and PSCI v0.1.
> >>               ...
> >>       };
> >>
> >> +ARM systems can have multiple cores sometimes in hierarchical arrangement.
> >> +This often, but not always, maps directly to the processor power topology of
> >> +the system. Individual nodes in a topology have their own specific power states
> >> +and can be better represented in DT hierarchically.
> >> +
> >> +For these cases, the definitions of the idle states for the CPUs and the CPU
> >> +topology, must conform to the domain idle state specification [3]. The domain
> >> +idle states themselves, must be compatible with the defined 'domain-idle-state'
> >> +binding [1], and also need to specify the arm,psci-suspend-param property for
> >> +each idle state.
> >> +
> >> +DT allows representing CPUs and CPU idle states in two different ways -
> >> +
> >> +The flattened model as given in Example 1, lists CPU's idle states followed by
> >> +the domain idle state that the CPUs may choose. Note that the idle states are
> >> +all compatible with "arm,idle-state".
> >> +
> >> +Example 2 represents the hierarchical model of CPUs and domain idle states.
> >> +CPUs define their domain provider in their psci DT node. The domain controls
> >> +the power to the CPU and possibly other h/w blocks that would enter an idle
> >> +state along with the CPU. The CPU's idle states may therefore be considered as
> >> +the domain's idle states and have the compatible "arm,idle-state". Such domains
> >> +may also be embedded within another domain that may represent common h/w blocks
> >> +between these CPUs. The idle states of the CPU topology shall be represented as
> >> +the domain's idle states.
> >> +
> >> +In PSCI firmware v1.0, the OS-Initiated mode is introduced. In order to use it,
> >> +the hierarchical representation must be used.
> >> +
> >> +Example 1: Flattened representation of CPU and domain idle states
> >
> > [...]
> >
> >> +Example 2: Hierarchical representation of CPU and domain idle states
> >
> > I understand that this may not be of interest for this series, but do
> > we need to add any suggestions on how to arrive Flattened representation
> > of CPU idle states from its hierarchical representation. If the DT has
> > latter and PSCI call returns as PC mode only for idle. We need to deal
> > with that case.
>
> For sure, I think this is valid comment for this series (or at least
> v8 which contains the hole set).
>

Thanks.

> The approach I have taken, so far, is to closely tie the support for
> PSCI OSI mode to the hierarchical representation in DT of the idle
> states. Simply because of changing as little as possible, in a first
> step, then build on top.
>

That's fine. I just want a note in the bindings to state that we can use
the hierarchical representation and generate flattened list for PC.

> However, in the offlist discussion I had with Lorenzo, he raised a
> concern, very similar to what you are bringing up here. There are
> indeed platform configurations, using PSCI PC mode, which would
> benefit from the using the hierarchical representation. BTW, that is
> kind of what Raju just tried to get working for QCOM SDM845 [1], but
> let's discuss that separately.
>

OK, we can discuss details in that thread, but I don't even see the PSCI
domains there.

> The conclusion I made, is that no matter of we are using the PC mode
> or the OSI mode, the hierarchical representation shall just work.
>

Indeed.

> To me, this means that I have to re-work the series, such that the
> PSCI driver (and cpuidle), dynamically in runtime, can agree on which
> of the idle states that are "shared among a group of CPUs" and which
> are CPU specific. Exactly how, I am still exploring a few different
> approaches on.
>
> Does it make sense?
>

Yes

> So, when it comes to the updated DT bindings in regards to $subject
> patch, I think it's good as is. Or do you think there is something
> that needs to be clarified?
>

Yes, nearly there. Just thought good to add a note that the representation
has no affinity towards any PSCI idle state mechanism(PC or OSI). So
that it's never assumed or misunderstood.

--
Regards,
Sudeep