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

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Message-ID: <18092427-3a4a-4fb2-9f75-675de98957d4@oss.qualcomm.com>
Date: Thu, 4 Sep 2025 16:50:45 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Krzysztof Kozlowski <krzk@...nel.org>,
        Sarthak Garg <quic_sartgarg@...cinc.com>,
        Ulf Hansson <ulf.hansson@...aro.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>,
        Adrian Hunter <adrian.hunter@...el.com>
Cc: linux-mmc@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        quic_nguyenb@...cinc.com, quic_rampraka@...cinc.com,
        quic_pragalla@...cinc.com, quic_sayalil@...cinc.com,
        quic_nitirawa@...cinc.com, quic_bhaskarv@...cinc.com,
        kernel@....qualcomm.com
Subject: Re: [PATCH V5 4/4] arm64: dts: qcom: sm8550: Add max-sd-hs-hz
 property

On 9/4/25 3:07 PM, Krzysztof Kozlowski wrote:
> 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.

Sarthak, if you believe what I said is accurate, feel free to copy-paste
as is

Konrad