linux-kernel - Re: [PATCH 2/3] arm64: dts: qcom: sa8155: Add gear and rate limit properties to UFS

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Message-ID: <11ea828a-6d35-4ac6-a207-0284870c28fc@oss.qualcomm.com>
Date: Tue, 5 Aug 2025 15:16:33 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Manivannan Sadhasivam <mani@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>
Cc: Ram Kumar Dwivedi <quic_rdwivedi@...cinc.com>, alim.akhtar@...sung.com,
        avri.altman@....com, bvanassche@....org, robh@...nel.org,
        krzk+dt@...nel.org, conor+dt@...nel.org, andersson@...nel.org,
        konradybcio@...nel.org, James.Bottomley@...senpartnership.com,
        martin.petersen@...cle.com, agross@...nel.org,
        linux-arm-msm@...r.kernel.org, linux-scsi@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] arm64: dts: qcom: sa8155: Add gear and rate limit
 properties to UFS

On 8/1/25 2:19 PM, Manivannan Sadhasivam wrote:
> On Fri, Aug 01, 2025 at 11:12:42AM GMT, Krzysztof Kozlowski wrote:
>> On 01/08/2025 11:10, Ram Kumar Dwivedi wrote:
>>>
>>>
>>> On 01-Aug-25 1:58 PM, Manivannan Sadhasivam wrote:
>>>> On Thu, Jul 24, 2025 at 09:48:53AM GMT, Krzysztof Kozlowski wrote:
>>>>> On 22/07/2025 18:11, Ram Kumar Dwivedi wrote:
>>>>>> Add optional limit-hs-gear and limit-rate properties to the UFS node to
>>>>>> support automotive use cases that require limiting the maximum Tx/Rx HS
>>>>>> gear and rate due to hardware constraints.
>>>>>
>>>>> What hardware constraints? This needs to be clearly documented.
>>>>>
>>>>
>>>> Ram, both Krzysztof and I asked this question, but you never bothered to reply,
>>>> but keep on responding to other comments. This won't help you to get this series
>>>> merged in any form.
>>>>
>>>> Please address *all* review comments before posting next iteration.
>>>
>>> Hi Mani,
>>>
>>> Apologies for the delay in responding. 
>>> I had planned to explain the hardware constraints in the next patchset’s commit message, which is why I didn’t reply earlier. 
>>>
>>> To clarify: the limitations are due to customer board designs, not our SoC. Some boards can't support higher gear operation, hence the need for optional limit-hs-gear and limit-rate properties.
>>>
>>
>> That's vague and does not justify the property. You need to document
>> instead hardware capabilities or characteristic. Or explain why they
>> cannot. With such form I will object to your next patch.
>>
> 
> I had an offline chat with Ram and got clarified on what these properties are.
> The problem here is not with the SoC, but with the board design. On some Qcom
> customer designs, both the UFS controller in the SoC and the UFS device are
> capable of operating at higher gears (say G5). But due to board constraints like
> poor thermal dissipation, routing loss, the board cannot efficiently operate at
> the higher speeds.
> 
> So the customers wanted a way to limit the gear speed (say G3) and rate
> (say Mode-A) on the specific board DTS.

I'm not necessarily saying no, but have you explored sysfs for this?

I suppose it may be too late (if the driver would e.g. init the UFS
at max gear/rate at probe time, it could cause havoc as it tries to
load the userland)..

Konrad