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

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Message-ID: <jogwisri2gs77j5cs3xwyezmfsotnizvlruzzelemdj5xadqh4@loe7fsatoass>
Date: Tue, 5 Aug 2025 22:25:15 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: Krzysztof Kozlowski <krzk@...nel.org>, 
	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 Tue, Aug 05, 2025 at 03:16:33PM GMT, Konrad Dybcio wrote:
> 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)..
> 

If the driver tries to run with unsupported max gear speed/mode, it will just
crash with the error spit.

- Mani

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