linux-kernel - Re: [PATCH V6 3/4] dt-bindings: interconnect: Add generic compatible qcom,epss-l3-perf

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Message-ID: <fff1a05c-5e7c-451d-9b08-4e835d6ab131@kernel.org>
Date: Wed, 27 Nov 2024 20:22:02 +0100
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
 Rob Herring <robh@...nel.org>
Cc: Raviteja Laggyshetty <quic_rlaggysh@...cinc.com>,
 Georgi Djakov <djakov@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Bjorn Andersson <andersson@...nel.org>,
 Konrad Dybcio <konradybcio@...nel.org>,
 Odelu Kukatla <quic_okukatla@...cinc.com>,
 Mike Tipton <quic_mdtipton@...cinc.com>, Sibi Sankar
 <quic_sibis@...cinc.com>, linux-arm-msm@...r.kernel.org,
 linux-pm@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH V6 3/4] dt-bindings: interconnect: Add generic compatible
 qcom,epss-l3-perf

On 27/11/2024 19:49, Dmitry Baryshkov wrote:
> On 27 November 2024 20:27:27 EET, Krzysztof Kozlowski <krzk@...nel.org> wrote:
>> On 27/11/2024 17:53, Dmitry Baryshkov wrote:
>>> On Wed, Nov 27, 2024 at 08:23:04AM -0600, Rob Herring wrote:
>>>> On Mon, Nov 25, 2024 at 05:45:10PM +0000, Raviteja Laggyshetty wrote:
>>>>> EPSS instance on sc7280, sm8250 SoCs, use PERF_STATE register instead of
>>>>> REG_L3_VOTE to scale L3 clocks, hence adding a new generic compatible
>>>>> "qcom,epss-l3-perf" for these targets.
>>>>
>>>> Is this a h/w difference from prior blocks or you just want to use B 
>>>> instead of A while the h/w has both A and B? The latter sounds like 
>>>> driver policy.
>>>>
>>>> It is also an ABI break for s/w that didn't understand 
>>>> qcom,epss-l3-perf.
>>>
>>> As the bindings keep old compatible strings in addition to the new
>>> qcom,epss-l3-perf, where is the ABI break? Old SW will use old entries,
>>> newer can use either of those.
>> No, this change drops qcom,epss-l3 and adds new fallback. How old
>> software can work in such case? It's broken.
> 
> Oh, I see. We had a platform-specific overrides for those two. Then I think we should completely drop the new qcom,epss-l3-perf idea and follow the sm8250 / sc7280 example. This means compatible = "qcom,sa8775p-perf", "qcom,epss-l3". 

It depends for example whether epss-l3 is valid at all. ABI is not
broken if nothing was working in the first place, assuming it is
explained in commit msg (not the case here).

Best regards,
Krzysztof