linux-kernel - Re: [PATCH 1/3] dt-bindings: interconnect: add clocks property to enable QoS on sa8775p

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Message-ID: <6cdeac85-a803-4b75-8fb5-c9d485ae374c@oss.qualcomm.com>
Date: Wed, 3 Sep 2025 23:31:30 +0530
From: Odelu Kukatla <odelu.kukatla@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        Georgi Djakov <djakov@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>
Cc: Raviteja Laggyshetty <quic_rlaggysh@...cinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-pm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mike Tipton <mike.tipton@....qualcomm.com>
Subject: Re: [PATCH 1/3] dt-bindings: interconnect: add clocks property to
 enable QoS on sa8775p



On 9/3/2025 6:12 PM, Konrad Dybcio wrote:
> On 8/28/25 11:59 PM, Georgi Djakov wrote:
>> On 8/28/25 9:20 PM, Krzysztof Kozlowski wrote:
>>> On 28/08/2025 20:16, Odelu Kukatla wrote:
>>>>
>>>>>> QoS configuration is essential for managing latency and bandwidth across
>>>>>> subsystems such as CPU, GPU, and multimedia engines. Without it, the
>>>>>> system may experience performance degradation, especially under
>>>>>
>>>>> So how was it working for the last 2 years?
>>>>>
>>>> The system may function normally without this feature. However, enabling
>>>
>>>
>>> Huh? So you agree but keep continuing the discussion?
>>>
>>> I don't understand what we are discussing in such case, but just to
>>> close the topic from my side and be explicit: based on above you cannot
>>> break the ABI.
>>
>> To be even more specific, if we already have some DT binding without any
>> clocks and reg properties, we can't just suddenly change them from now
>> on to be "required". But they can still be "optional" and this will not
>> break the ABI, right? The old DT is still valid and the QoS will be
>> active when the new properties are present and this is handled properly
>> by the driver.
> 
> Correct and this very approach was used to retrofit QoS onto an even older
> sc7280 icc driver.
> 
> The icc-rpmh core already ignores QoS configuration if the clocks are not
> provided.
> 
> Konrad

Thanks Krzysztof, Georgi, Konrad for the detailed discussion.
To summarize: we can’t make reg and clocks properties required now for
sa8775p, and making these properties optional ensures backward
compatibility.
I will address this in the next revision.

Thanks again for the comments.

Best regards,
Odelu