linux-kernel - Re: [PATCH] arm64: dts: qcom: sm8450: delete incorrect ufs interconnect fields

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Date:   Mon, 11 Apr 2022 21:16:13 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Jonathan Marek <jonathan@...ek.ca>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Vladimir Zapolskiy <vladimir.zapolskiy@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Vinod Koul <vkoul@...nel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: qcom: sm8450: delete incorrect ufs
 interconnect fields

On Thu 07 Apr 17:38 CDT 2022, Jonathan Marek wrote:

> On 4/7/22 5:16 PM, Krzysztof Kozlowski wrote:
> > On 07/04/2022 21:40, Vladimir Zapolskiy wrote:
> > > On 4/7/22 20:21, Jonathan Marek wrote:
> > > > Upstream sm8450.dtsi has #interconnect-cells = <2>; so these are wrong.
> > > > Ignored and undocumented with upstream UFS driver so delete for now.
> > 
> > This is the upstream and they are documented here, although as pointed
> > by Vladimir this was rather a reverse-documentation. The documentation
> > might be incorrect, but then the bindings should be corrected instead of
> > only modifying the DTS.
> > 
> > > 
> > > Basically the description was added by a commit 462c5c0aa798 ("dt-bindings: ufs:
> > > qcom,ufs: convert to dtschema").
> > > 
> > > It's questionable, if an example in the new yaml file is totally correct
> > > in connection to the discussed issue.
> > 
> > To be honest - the example probably is not correct, because it was based
> > on existing DTS without your patch. :)
> > 
> > Another question is whether the interconnect properties are here correct
> > at all. I assumed that DTS is correct because it should describe the
> > hardware, even if driver does not use it. However maybe that was a false
> > assumption...
> > 
> 
> writing-bindings.rst says it is OK to document even if it isn't used by the
> driver (seems wrong to me, at least for interconnects which are a firmware
> abstraction and not hardware).
> 

The devicetree, and hence the binding, should describe the hardware, so
that an implementation can make use of the hardware. So there's no
problem expressing the interconnect in the binding/dts even though the
driver isn't using it.

I'm not sure if I'm misunderstanding you, the interconnect paths
described here are a description of the hardware requirements for this
device. (I.e. it need the buses between ufs and ddr, and cpu and ufs to
operate).

> 462c5c0aa798 wasn't in my 5.17+ tree pulled after dts changes were merged (I
> guess doc changes come later), so my commit message is incorrect, but I
> think it makes more sense to have the documentation reflect the driver. Its
> also not an important issue, so I'll let others sort it out.
> 

I believe that the correctness of the interconnect property will ensure
that the interconnect provider doesn't hit sync_state until the ufs
driver has probed - regardless of the driver actually implementing the
interconnect voting. But perhaps I've misunderstood the magic involved?

Regards,
Bjorn