linux-kernel - Re: [PATCH V3 2/4] dt-bindings: mmc: controller: Add max-sd-hs-frequency property

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Message-ID: <5bdae07b-a7b1-49be-b843-1704981bc63b@oss.qualcomm.com>
Date: Mon, 23 Jun 2025 14:08:59 +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_cang@...cinc.com, 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 V3 2/4] dt-bindings: mmc: controller: Add
 max-sd-hs-frequency property

On 6/22/25 11:48 AM, Krzysztof Kozlowski wrote:
> On 21/06/2025 12:20, Konrad Dybcio wrote:
>> On 6/18/25 9:43 AM, Krzysztof Kozlowski wrote:
>>> On 18/06/2025 09:28, Sarthak Garg wrote:
>>>> Introduce a new optional device tree property `max-sd-hs-frequency` to
>>>> limit the maximum frequency (in Hz) used for SD cards operating in
>>>> High-Speed (HS) mode.
>>>>
>>>> This property is useful for platforms with vendor-specific hardware
>>>> constraints, such as the presence of a level shifter that cannot
>>>> reliably support the default 50 MHz HS frequency. It allows the host
>>>> driver to cap the HS mode frequency accordingly.
>>>>
>>>> Signed-off-by: Sarthak Garg <quic_sartgarg@...cinc.com>
>>>> ---
>>>>  .../devicetree/bindings/mmc/mmc-controller-common.yaml | 10 ++++++++++
>>>>  1 file changed, 10 insertions(+)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/mmc/mmc-controller-common.yaml b/Documentation/devicetree/bindings/mmc/mmc-controller-common.yaml
>>>> index 9a7235439759..1976f5f8c401 100644
>>>> --- a/Documentation/devicetree/bindings/mmc/mmc-controller-common.yaml
>>>> +++ b/Documentation/devicetree/bindings/mmc/mmc-controller-common.yaml
>>>> @@ -93,6 +93,16 @@ properties:
>>>>      minimum: 400000
>>>>      maximum: 384000000
>>>>  
>>>> +  max-sd-hs-frequency:
>>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>>> +    description: |
>>>> +      Maximum frequency (in Hz) to be used for SD cards operating in
>>>> +      High-Speed (HS) mode. This is useful for platforms with vendor-specific
>>>> +      limitations, such as the presence of a level shifter that cannot support
>>>> +      the default 50 MHz HS frequency or other.
>>>> +    minimum: 400000
>>>> +    maximum: 50000000
>>>
>>> This might be fine, but your DTS suggests clearly this is SoC compatible
>>> deducible, which I already said at v1.
>>
>> I don't understand why you're rejecting a common solution to a problem
>> that surely exists outside this one specific chip from one specific
>> vendor, which may be caused by a multitude of design choices, including
>> erratic board (not SoC) electrical design
> 
> No one brought any arguments so far that common solution is needed. The
> only argument provided - sm8550 - is showing this is soc design.
> 
> I don't reject common solution. I provided review at v1 to which no one
> responded, no one argued, no one provided other arguments.

Okay, so the specific problem that causes this observable limitation
exists on SM8550 and at least one more platform which is not upstream
today. It can be caused by various electrical issues, in our specific
case by something internal to the SoC (but external factors may apply
too)

Looking at the docs, a number of platforms have various limitations
with regards to frequency at specific speed-modes, some of which seem
to be handled implicitly by rounding in the clock framework's
round/set_rate().

I can very easily imagine there are either boards or platforms in the
wild, where the speed must be limited for various reasons, maybe some
of them currently don't advertise it (like sm8550 on next/master) to
hide that

Konrad