Re: [PATCH v2] dt: psci: Update DT bindings to support hierarchical PSCI states

[Sudeep Holla]

Hi Ulf,

I will suggest some wording changes not of which are not compulsory and
left to you to pick up or drop.

On 28/12/17 14:40, Ulf Hansson wrote:
> From: Lina Iyer <lina.iyer@xxxxxxxxxx>
> 
> Update DT bindings to represent hierarchical CPU and CPU domain idle states
> for PSCI. Also update the PSCI examples to clearly show how flattened and
> hierarchical idle states can be represented in DT.
> 
> Signed-off-by: Lina Iyer <lina.iyer@xxxxxxxxxx>
> Signed-off-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> ---
> 
> Changes in v2:
> 	- Addressed comments from Rob.
> 	- Updated some labels in the examples to get more consistency.
> 
> For your information, I have picked up the work from Lina Iyer around the so
> called CPU cluster idling series [1,2] and I working on new versions. However,
> I decided to post the updates to the PSCI DT bindings first, as they will be
> needed to be agreed upon before further changes can be done to the PSCI firmware
> driver.
> 
> Note, these bindings have been discussed over and over again, at LKML, but
> especially also at various Linux conferences, like LPC and Linaro Connect. We
> finally came to a conclusion and the changes we agreed upon, should be reflected
> in this update.
> 
> Of course, it's a while ago since the latest discussions, but hopefully people
> don't have too hard time to remember.
> 
> Kind regards
> Uffe
> 
> [1]
> https://www.spinics.net/lists/arm-kernel/msg566200.html
> 
> [2]
> https://lwn.net/Articles/716300/
> 
> ---
>  Documentation/devicetree/bindings/arm/psci.txt | 152 +++++++++++++++++++++++++
>  1 file changed, 152 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/arm/psci.txt b/Documentation/devicetree/bindings/arm/psci.txt
> index a2c4f1d..8a09bd2 100644
> --- a/Documentation/devicetree/bindings/arm/psci.txt
> +++ b/Documentation/devicetree/bindings/arm/psci.txt
> @@ -105,7 +105,159 @@ Case 3: PSCI v0.2 and PSCI v0.1.
>  		...
>  	};
>  
> +PSCI v1.0 onwards, supports OS-Initiated mode for powering off CPUs and CPU
> +clusters from the firmware.

Since we are trying to avoid usage of "clusters"(as it's not architecturally
defined, but I know it's too late as it widely used everywhere). Also this
binding is not just OSI specific, it can be used for Platform Co-ordinated
also so let's not specify them at all.

How about:
"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 such topologies the PSCI firmware driver acts

PSCI firmware can be represented as a pseudo power controller ?

> +as pseudo-controller, which may be specified in the psci DT node. The
> +definitions of the CPU and the CPU cluster topology, must conform to the domain
> +idle state specification [3].

I assume it should be  "..definitions of the idle states for CPU and the CPU
topology" above, otherwise they should conform to topology binding :) rather
than domain idle state bindings.

> 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 CPU and CPU cluster 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. This is the general practice
> +followed in PSCI firmwares that support Platform Coordinated mode.

I would rather drop the above statement or specify in Example 2 that it can be
used for both OSI and PC.

> Note that
> +the idle states are all compatible with "arm,idle-state".
> +
> +Example 2 represents the hierarchical model of CPU and domain idle states.
> +CPUs define their domain provider in their DT node. The domain controls the
> +power to the CPU and possibly other h/w blocks that would be powered off when
> +the CPU is powered off. 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 be embedded within another domain that represents common h/w blocks between
> +these CPUs viz. the cluster. The idle states of the cluster would be
> +represented as the domain's idle states. In order to use OS-Initiated mode of
> +PSCI in the firmware, the hierarchical representation must be used.
> +

Can we avoid using poweroff as it's one of the idle states and not the only
one ?

Other than that, the examples look good to me.

--
Regards,
Sudeep