Re: [PATCH V5 4/4] arm64: dts: qcom: sm8550: Add max-sd-hs-hz property

[Krzysztof Kozlowski <krzk@kernel.org>]

On 04/09/2025 14:27, Konrad Dybcio wrote:
> On 9/4/25 12:52 PM, Krzysztof Kozlowski wrote:
>> On 04/09/2025 12:51, Krzysztof Kozlowski wrote:
>>> On 04/09/2025 10:36, Konrad Dybcio wrote:
>>>> On 9/3/25 10:21 AM, 'Krzysztof Kozlowski' via kernel wrote:
>>>>> On 03/09/2025 10:04, Sarthak Garg wrote:
>>>>>> Due to board-specific hardware constraints particularly related
>>>>>> to level shifter in this case the maximum frequency for SD High-Speed
>>>>>> (HS) mode must be limited to 37.5 MHz to ensure reliable operation of SD
>>>>>> card in HS mode.
>>>>>>
>>>>>> This is achieved by introducing the `max-sd-hs-hz` property in the
>>>>>> device tree, allowing the controller to operate within safe frequency
>>>>>> limits for HS mode.
>>>>>>
>>>>>
>>>>> Probably we will now replicate the same discussion... And it will be
>>>>> happening every time you send the same and not reflect it in commit msg.
>>
>> Just to emphasize this - it will happen EVERY time.
>>
>>>>>
>>>>> Bindings say board setup, this commit msg says board config, but the
>>>>> patch says SoC. This is not correct.
>>>>
>>>> Both are correct, looking at the problem from two perspectives.
>>>>
>>>> The bindings description mentions board-specific limitations (e.g. because
>>>> "the board's electrical design does not allow one to achieve the full rated
>>>> frequency that the SoC can otherwise do, in a stable way")
>>>>
>>>> Here the author tries to argue that almost all SM8550 boards are broken
>>>> in this sense, because the reference design did not feature the required
>>>> passive components, making most (derivative) designs sort of "broken by
>>>> default" - and only some (if any?) vendors decided to go with the
>>>> additional components required to lift this limitation.
>>>>
>>>> This in turn makes it fair to assume the developer experience would benefit
>>>> from having the SD card high speed modes always work (with the slight speed
>>>> cap which may not be required for the 1 or 2 designs that took the extra
>>>> step) without each board DT creator having to track down this property
>>>> separately.
>>>
>>> And then if you send same v3, I will ask the same. Can the author
>>
>> v3 -> v6
> 
> So, would you be accepting of this patch if the commit message was:
> 
> arm64: dts: qcom: sm8550: Limit max SD HS mode frequency by default
> 
> Due to an implementation detail in this SoC, additional passive
> electrical components are required to achieve the maximum rated speed
> of the SD controller when paired with a High-Speed SD Card. Without them,
> the clock frequency must be limited to 37.5 MHz for link stability.
> 
> Because the reference design does not contain these components, most
> (derivative) boards do not have them either. To accommodate for that,
> apply the frequency limit by default and delegate lifting it to the
> odd boards that do contain the necessary onboard hardware.
Yes, it is an excellent explanation.

Best regards,
Krzysztof