linux-kernel - Re: [PATCH V5 2/4] dt-bindings: mmc: controller: Add max-sd-hs-frequency property

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Message-ID: <c9cd6cf5-2053-4d0e-a1c3-85bdc9046390@quicinc.com>
Date: Thu, 4 Sep 2025 13:45:50 +0530
From: Sarthak Garg <quic_sartgarg@...cinc.com>
To: Rob Herring <robh@...nel.org>
CC: Ulf Hansson <ulf.hansson@...aro.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>, <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 2/4] dt-bindings: mmc: controller: Add
 max-sd-hs-frequency property



On 9/4/2025 2:09 AM, Rob Herring wrote:
> On Wed, Sep 03, 2025 at 01:34:02PM +0530, Sarthak Garg wrote:
>> Some platforms may require limiting the maximum frequency used in SD
>> High-Speed (HS) mode due to board-level hardware constraints. For
>> example, certain boards may include level shifters or other components
>> that cannot reliably operate at the default 50 MHz HS frequency.
>>
>> 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.
>>
>> Signed-off-by: Sarthak Garg <quic_sartgarg@...cinc.com>
>> ---
>>   .../devicetree/bindings/mmc/mmc-controller-common.yaml    | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/mmc/mmc-controller-common.yaml b/Documentation/devicetree/bindings/mmc/mmc-controller-common.yaml
>> index 9a7235439759..d6b785cb2bd9 100644
>> --- a/Documentation/devicetree/bindings/mmc/mmc-controller-common.yaml
>> +++ b/Documentation/devicetree/bindings/mmc/mmc-controller-common.yaml
>> @@ -93,6 +93,14 @@ properties:
>>       minimum: 400000
>>       maximum: 384000000
>>   
>> +  max-sd-hs-hz:
>> +    description: |
>> +      Maximum frequency (in Hz) to be used for SD cards operating in
>> +      High-Speed (HS) mode. This is useful for board-specific limitations,
>> +      such as level shifters or others where the card cannot reliably
>> +      operate at the default 50 MHz HS frequency.
>> +    default: 50000000
> 
> Why doesn't max-frequency work for you? I would think frequency limits
> wouldn't really depend on the mode.
> 

The reason max-frequency isn't sufficient in this case is that it 
applies globally across all operating modes of the SD card interface. 
However, the constraint we're trying to address is specific to 
High-Speed (HS) mode where the default frequency is 50 MHz.By 
introducing a mode-specific property like max-sd-hs-frequency, we can 
target only HS mode without affecting the capabilities of other modes. 
This provides more granular control and avoids performance degradation 
for other modes.

>> +
>>     disable-wp:
>>       $ref: /schemas/types.yaml#/definitions/flag
>>       description:
>> -- 
>> 2.34.1
>>