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

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Message-ID: <i6eyiscdf2554znc4aaglhi22opfgyicif3y7kzjafwsrtdrtm@jjpzak64gdft>
Date: Tue, 5 Aug 2025 22:52:20 +0530
From: 'Manivannan Sadhasivam' <mani@...nel.org>
To: Alim Akhtar <alim.akhtar@...sung.com>
Cc: 'Konrad Dybcio' <konrad.dybcio@....qualcomm.com>, 
	'Krzysztof Kozlowski' <krzk@...nel.org>, 'Ram Kumar Dwivedi' <quic_rdwivedi@...cinc.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 10:49:45PM GMT, Alim Akhtar wrote:
> 
> 
> > -----Original Message-----
> > From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
> > Sent: Tuesday, August 5, 2025 10:36 PM
> > To: Manivannan Sadhasivam <mani@...nel.org>
> > 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 8/5/25 6:55 PM, Manivannan Sadhasivam wrote:
> > > 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.
> > 
> > OK
> > 
> > just a couple related nits that I won't bother splitting into separate emails
> > 
> > rate (mode? I'm seeing both names) should probably have dt-bindings
> > defines while gear doesn't have to since they're called G<number> anyway,
> > with the bindings description strongly discouraging use, unless absolutely
> > necessary (e.g. in the situation we have right there)
> > 
> > I'd also assume the code should be moved into the ufs-common code, rather
> > than making it ufs-qcom specific
> > 
> > Konrad
> Since this is a board specific constrains and not a SoC properties, have an option of handling this via bootloader is explored?

Both board and SoC specific properties *should* be described in devicetree if
they are purely describing the hardware.

- Mani

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