Date:   Sun, 3 Dec 2023 00:49:16 +0100
From:   David Heidelberg <david@...t.cz>
To:     Rob Herring <robh@...nel.org>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] dt-bindings: arm: merge qcom,idle-state with
 idle-state

Thank you for the feedback, I tried to incorporate it and sent V4.

David

On 04/01/2022 21:49, Rob Herring wrote:
> On Fri, Dec 24, 2021 at 03:48:34PM +0100, David Heidelberg wrote:
>> Merge Qualcomm specific idle-state binding with generic one.
>>
>> Signed-off-by: David Heidelberg <david@...t.cz>
>>
>> ---
>> v3:
>>   - integrate into idle-state.yml
>>   - orig. patch name was:
>>     "[v2] dt-bindings: arm/msm/qcom,idle-state convert to YAML"
>>
>> Signed-off-by: David Heidelberg <david@...t.cz>
>> ---
>>   .../devicetree/bindings/arm/idle-states.yaml  | 107 ++++++++++++++++++
>>   .../bindings/arm/msm/qcom,idle-state.txt      |  84 --------------
>>   2 files changed, 107 insertions(+), 84 deletions(-)
>>   delete mode 100644 Documentation/devicetree/bindings/arm/msm/qcom,idle-state.txt
>>
>> diff --git a/Documentation/devicetree/bindings/arm/idle-states.yaml b/Documentation/devicetree/bindings/arm/idle-states.yaml
>> index 52bce5dbb11f..fde1557f2332 100644
>> --- a/Documentation/devicetree/bindings/arm/idle-states.yaml
>> +++ b/Documentation/devicetree/bindings/arm/idle-states.yaml
>> @@ -241,6 +241,64 @@ description: |+
>>     [6] ARM Linux Kernel documentation - Booting AArch64 Linux
>>         Documentation/arm64/booting.rst
>>   
>> +  ===========================================
>> +  5 - Qualcomm specific STATES
>> +  ===========================================
>> +
>> +  cpuidle-qcom is the cpuidle driver for Qualcomm SoCs and uses these idle
> What's cpuidle?
>
> (Linux detail doesn't belong here)
>
>> +  states. Idle states have different enter/exit latency and residency values.
>> +  The idle states supported by the QCOM SoC are defined as -
>> +
>> +    * Standby
>> +    * Retention
>> +    * Standalone Power Collapse (Standalone PC or SPC)
>> +    * Power Collapse (PC)
>> +
>> +  Standby: Standby does a little more in addition to architectural clock gating.
>> +  When the WFI instruction is executed the ARM core would gate its internal
>> +  clocks. In addition to gating the clocks, QCOM cpus use this instruction as a
>> +  trigger to execute the SPM state machine. The SPM state machine waits for the
>> +  interrupt to trigger the core back in to active. This triggers the cache
>> +  hierarchy to enter standby states, when all cpus are idle. An interrupt brings
>> +  the SPM state machine out of its wait, the next step is to ensure that the
>> +  cache hierarchy is also out of standby, and then the cpu is allowed to resume
>> +  execution. This state is defined as a generic ARM WFI state by the ARM cpuidle
>> +  driver and is not defined in the DT. The SPM state machine should be
>> +  configured to execute this state by default and after executing every other
>> +  state below.
>> +
>> +  Retention: Retention is a low power state where the core is clock gated and
>> +  the memory and the registers associated with the core are retained. The
>> +  voltage may be reduced to the minimum value needed to keep the processor
>> +  registers active. The SPM should be configured to execute the retention
>> +  sequence and would wait for interrupt, before restoring the cpu to execution
>> +  state. Retention may have a slightly higher latency than Standby.
>> +
>> +  Standalone PC: A cpu can power down and warmboot if there is a sufficient time
>> +  between the time it enters idle and the next known wake up. SPC mode is used
>> +  to indicate a core entering a power down state without consulting any other
>> +  cpu or the system resources. This helps save power only on that core.  The SPM
>> +  sequence for this idle state is programmed to power down the supply to the
>> +  core, wait for the interrupt, restore power to the core, and ensure the
>> +  system state including cache hierarchy is ready before allowing core to
>> +  resume. Applying power and resetting the core causes the core to warmboot
>> +  back into Elevation Level (EL) which trampolines the control back to the
>> +  kernel. Entering a power down state for the cpu, needs to be done by trapping
>> +  into a EL. Failing to do so, would result in a crash enforced by the warm boot
>> +  code in the EL for the SoC. On SoCs with write-back L1 cache, the cache has to
>> +  be flushed in s/w, before powering down the core.
>> +
>> +  Power Collapse: This state is similar to the SPC mode, but distinguishes
>> +  itself in that the cpu acknowledges and permits the SoC to enter deeper sleep
>> +  modes. In a hierarchical power domain SoC, this means L2 and other caches can
>> +  be flushed, system bus, clocks - lowered, and SoC main XO clock gated and
>> +  voltages reduced, provided all cpus enter this state.  Since the span of low
>> +  power modes possible at this state is vast, the exit latency and the residency
>> +  of this low power mode would be considered high even though at a cpu level,
>> +  this essentially is cpu power down. The SPM in this state also may handshake
>> +  with the Resource power manager (RPM) processor in the SoC to indicate a
>> +  complete application processor subsystem shut down.
> I'm on the fence whether any of this belongs here... But I don't have a
> better suggestion.
>
>> +
>>   properties:
>>     $nodename:
>>       const: idle-states
>> @@ -323,6 +381,44 @@ patternProperties:
>>         - exit-latency-us
>>         - min-residency-us
>>   
>> +  "^(ret|spc|pc)$":
> Either these need to be added to the existing pattern for node names or
> the node names in the dts files be changed to match the existing
> binding. I think it is safe to do the latter as the driver doesn't care
> about node names.
>
> And then you just need to update the 'compatible' schema.
>
>> +    type: object
>> +    description:
>> +      Each state node represents a domain idle state description.
>> +
>> +    properties:
>> +      compatible:
>> +        items:
>> +          - enum:
>> +              - qcom,idle-state-ret
>> +              - qcom,idle-state-spc
>> +              - qcom,idle-state-pc
>> +          - const: arm,idle-state
>> +
>> +      entry-latency-us:
>> +        description:
>> +          The worst case latency in microseconds required to enter the idle
>> +          state. Note that, the exit-latency-us duration may be guaranteed only
>> +          after the entry-latency-us has passed.
>> +
>> +      exit-latency-us:
>> +        description:
>> +          The worst case latency in microseconds required to exit the idle
>> +          state.
>> +
>> +      min-residency-us:
>> +        description:
>> +          The minimum residency duration in microseconds after which the idle
>> +          state will yield power benefits, after overcoming the overhead while
>> +          entering the idle state.
>> +
>> +    required:
>> +      - compatible
>> +      - entry-latency-us
>> +      - exit-latency-us
>> +      - min-residency-us
>> +
>> +
>>   additionalProperties: false
>>   
>>   examples:
>> @@ -658,4 +754,15 @@ examples:
>>           };
>>       };
>>   
>> +  - |
>> +    // Example 3 - QCOM SPC
>> +    idle-states {
>> +      cpu_spc: spc {
>> +        compatible = "qcom,idle-state-spc", "arm,idle-state";
>> +        entry-latency-us = <150>;
>> +        exit-latency-us = <200>;
>> +        min-residency-us = <2000>;
>> +      };
>> +    };
>> +
>>   ...

-- 
David Heidelberg

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