linux-kernel - Re: [PATCH 1/3] dt-bindings: arm,psci: Allow S2RAM power

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Message-ID: <349bac70-87e0-4870-a3f0-9f6a3b3e6824@oss.qualcomm.com>
Date: Fri, 20 Dec 2024 13:54:45 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Sudeep Holla <sudeep.holla@....com>,
        Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: Konrad Dybcio <konradybcio@...nel.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley
 <conor+dt@...nel.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Bjorn Andersson <bjorn.andersson@....qualcomm.com>
Subject: Re: [PATCH 1/3] dt-bindings: arm,psci: Allow S2RAM power_state
 parameter description

On 20.12.2024 12:27 PM, Sudeep Holla wrote:
> On Thu, Dec 19, 2024 at 08:43:27PM +0100, Konrad Dybcio wrote:
>> On 6.12.2024 11:21 AM, Sudeep Holla wrote:
>>> On Mon, Oct 28, 2024 at 03:22:57PM +0100, Konrad Dybcio wrote:
>>>> From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
>>>>
>>>> Certain firmware implementations (such as the ones found on Qualcomm
>>>> SoCs between roughly 2015 and 2023) expose an S3-like S2RAM state
>>>> through the CPU_SUSPEND call, as opposed to exposing PSCIv1.0's
>>>> optional PSCI_SYSTEM_SUSPEND.
>>>>
>>>
>>> If so, can you elaborate why s2idle doesn't work as an alternative to what
>>> you are hacking up here.
>>
>> Please see other branches of this thread
>>
>>>
>>>> This really doesn't work well with the model where we associate all
>>>> calls to CPU_SUSPEND with cpuidle. Allow specifying a single special
>>>> CPU_SUSPEND suspend parameter value that is to be treated just like
>>>> SYSTEM_SUSPEND from the OS's point of view.
>>>>
>>>> Signed-off-by: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
>>>> ---
>>>>  Documentation/devicetree/bindings/arm/psci.yaml | 6 ++++++
>>>>  1 file changed, 6 insertions(+)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/arm/psci.yaml b/Documentation/devicetree/bindings/arm/psci.yaml
>>>> index cbb012e217ab80c1ca88e611e7acc06c6d56fad0..a6901878697c8e1ec1cbfed62298ae3bc58f2501 100644
>>>> --- a/Documentation/devicetree/bindings/arm/psci.yaml
>>>> +++ b/Documentation/devicetree/bindings/arm/psci.yaml
>>>> @@ -98,6 +98,12 @@ properties:
>>>>        [1] Kernel documentation - ARM idle states bindings
>>>>          Documentation/devicetree/bindings/cpu/idle-states.yaml
>>>>  
>>>> +  arm,psci-s2ram-param:
>>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>>> +    description:
>>>> +      power_state parameter denoting the S2RAM/S3-like system suspend state
>>>
>>> Yet another NACK as this corresponds to PSCI SYSTEM_SUSPEND and as per
>>> specification it takes no such parameter. This is just misleading.
>>>
>>
>> Yeah PSCI_SYSTEM_SUSPEND takes care of this on platforms that expose it.
>>
> 
> And those that don't advertise/expose don't get to use, simple.

The spec says:

"The call is equivalent to using the CPU_SUSPEND call for the
deepest possible platform powerdown state."

so by that logic, I'd rather call implementing PSCI_SYSTEM_SUSPEND in
Linux unnecessary bloat..

>> DEN0022F.b Section 6.5. recommends that CPU_SUSPEND StateID includes
>> a field for system-level power down states. This binding change only
>> adds a way for DT-based platforms to associate such state with S2RAM
>> suspend.
>>
> 
> Sure, just use the CPU_SUSPEND bindings that already exist. No need to
> define this as some special case if it is exposed as CPU_SUSPEND idle
> state. Not sure why you want to do it differently. I understand the
> need to handle it different in the kernel, but I don't understand to
> define the new bindings for that. Just use the existing bindings for the
> idle states. Again I see no exception for this case.

The bindings exist for core/cluster idle states. This whole series tries
to include a system-wide suspend state that has no business being
described as a cpuidle one and depends on more than just the CPUs being
powered down.

>> That may be a bit Linux-specific whereas bindings are supposed to be
>> OS-agnostic, but since we effectively want one PSCI state for deep
>> suspend regardless of the OS, I would think this kind of hint is fine.
>>
> 
> Exactly, that's the reason for not changing this into special case and
> special binding for that special case created.

I simply don't think it's fitting to lie about system suspend states being
just CPU idle states, see above.

Konrad