Re: [PATCH v3 4/5] dt-bindings: cpufreq: cpufreq-qcom-hw: Add SDX75 compatible

[Konrad Dybcio <konrad.dybcio@linaro.org>]

On 9.06.2023 07:00, Pavan Kondeti wrote:
> On Wed, Jun 07, 2023 at 05:17:48PM +0530, Rohit Agarwal wrote:
>> Add compatible for EPSS CPUFREQ-HW on SDX75.
>>
>> Signed-off-by: Rohit Agarwal <quic_rohiagar@quicinc.com>
>> Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
>> ---
>>  Documentation/devicetree/bindings/cpufreq/cpufreq-qcom-hw.yaml | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/Documentation/devicetree/bindings/cpufreq/cpufreq-qcom-hw.yaml b/Documentation/devicetree/bindings/cpufreq/cpufreq-qcom-hw.yaml
>> index a6b3bb8..866ed2d 100644
>> --- a/Documentation/devicetree/bindings/cpufreq/cpufreq-qcom-hw.yaml
>> +++ b/Documentation/devicetree/bindings/cpufreq/cpufreq-qcom-hw.yaml
>> @@ -36,6 +36,7 @@ properties:
>>                - qcom,sa8775p-cpufreq-epss
>>                - qcom,sc7280-cpufreq-epss
>>                - qcom,sc8280xp-cpufreq-epss
>> +              - qcom,sdx75-cpufreq-epss
>>                - qcom,sm6375-cpufreq-epss
>>                - qcom,sm8250-cpufreq-epss
>>                - qcom,sm8350-cpufreq-epss
> 
> This is a very basic question, not completely related to this patch.
> Apologies in advance.
> 
> What is the rationale for adding a new soc string under compatible and
> using it in the new soc device tree? Is it meant for documentation purpose?
> i.e one know what all SoCs / boards supported by this device node.
It's two-fold:

1. The device tree describes the hardware, and for lack of better terms (e.g.
   an SoC-specific version number of the block that is identical to all other
   implementations of that revision on all SoCs that use it), we tend to
   associate it with the SoC it's been (first) found on.

2. In case we ever needed to introduce a SoC-specific quirk, we can just add
   an of_is_compatible-sorta check to the driver and not have to update the
   device trees. This is very important for keeping backwards compatibility,
   as it's assumed that not everybody may be running the latest one. This
   means we have to avoid ABI breaks (unless we have *very* good reasons, like
   "this would have never worked anyway" or "it was not described properly
   and worked on this occasion by pure luck")

Konrad
> 
> I ask this because, we don't add these compatible strings in the driver
> [1] which means there is not SoC specific handling and there is no
> module load assist (module alias matching by user space based on device
> presence).
> 
> Thanks,
> Pavan