linux-kernel - Re: [PATCH 10/20] dt-bindings: arm-smmu: Add compatible string for Adreno GPU SMMU

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Message-ID: <20200821143938.GA27918@jcrouse1-lnx.qualcomm.com>
Date:   Fri, 21 Aug 2020 08:39:38 -0600
From:   Jordan Crouse <jcrouse@...eaurora.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     Doug Anderson <dianders@...omium.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Will Deacon <will@...nel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Sibi Sankar <sibis@...eaurora.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>, Rob Herring <robh@...nel.org>,
        Rob Clark <robdclark@...omium.org>,
        Rob Herring <robh+dt@...nel.org>,
        "moderated list:ARM SMMU DRIVERS" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/20] dt-bindings: arm-smmu: Add compatible string for
 Adreno GPU SMMU

On Wed, Aug 19, 2020 at 10:36:38AM -0700, Rob Clark wrote:
> On Wed, Aug 19, 2020 at 10:03 AM Doug Anderson <dianders@...omium.org> wrote:
> >
> > Hi,
> >
> > On Mon, Aug 17, 2020 at 3:03 PM Rob Clark <robdclark@...il.com> wrote:
> > >
> > > From: Jordan Crouse <jcrouse@...eaurora.org>
> > >
> > > Every Qcom Adreno GPU has an embedded SMMU for its own use. These
> > > devices depend on unique features such as split pagetables,
> > > different stall/halt requirements and other settings. Identify them
> > > with a compatible string so that they can be identified in the
> > > arm-smmu implementation specific code.
> > >
> > > Signed-off-by: Jordan Crouse <jcrouse@...eaurora.org>
> > > Reviewed-by: Rob Herring <robh@...nel.org>
> > > Signed-off-by: Rob Clark <robdclark@...omium.org>
> > > ---
> > >  Documentation/devicetree/bindings/iommu/arm,smmu.yaml | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > >
> > > diff --git a/Documentation/devicetree/bindings/iommu/arm,smmu.yaml b/Documentation/devicetree/bindings/iommu/arm,smmu.yaml
> > > index 503160a7b9a0..5ec5d0d691f6 100644
> > > --- a/Documentation/devicetree/bindings/iommu/arm,smmu.yaml
> > > +++ b/Documentation/devicetree/bindings/iommu/arm,smmu.yaml
> > > @@ -40,6 +40,10 @@ properties:
> > >                - qcom,sm8150-smmu-500
> > >                - qcom,sm8250-smmu-500
> > >            - const: arm,mmu-500
> > > +      - description: Qcom Adreno GPUs implementing "arm,smmu-v2"
> > > +        items:
> > > +          - const: qcom,adreno-smmu
> > > +          - const: qcom,smmu-v2
> >
> > I know I'm kinda late to the game, but this seems weird to me,
> > especially given the later patches in the series like:
> >
> > https://lore.kernel.org/r/20200817220238.603465-19-robdclark@gmail.com
> >
> > Specifically in that patch you can see that this IOMMU already had a
> > compatible string and we're changing it and throwing away the
> > model-specific string?  I'm guessing that you're just trying to make
> > it easier for code to identify the adreno iommu, but it seems like a
> > better way would have been to just add the adreno compatible in the
> > middle, like:
> >
> >       - description: Qcom Adreno GPUs implementing "arm,smmu-v2"
> >         items:
> >           - enum:
> >               - qcom,msm8996-smmu-v2
> >               - qcom,msm8998-smmu-v2
> >               - qcom,sc7180-smmu-v2
> >               - qcom,sdm845-smmu-v2
> >         - const: qcom,adreno-smmu
> >         - const: qcom,smmu-v2
> >
> > Then we still have the SoC-specific compatible string in case we need
> > it but we also have the generic one?  It also means that we're not
> > deleting the old compatible string...
> 
> I did bring up the thing about removing the compat string in an
> earlier revision of the series.. but then we realized that
> qcom,sc7180-smmu-v2 was never actually used anywhere.
> 
> But I guess we could:  compatible = "qcom,sc7180-smmu-v2",
> "qcom,adreno-smmu", "qcom,smmu-v2";

I think the SoC specific string is intended for the "other" SMMU that everybody
else uses. Rarely would a workaround for that SMMU affect the GPU and vice
versa. Since these are the bindings it doesn't hurt to allow for the possibility
but I would be surprised if the occasion presented itself.

Jordan

> BR,
> -R
> 
> 
> 
> 
> >
> > -Doug
> >
> >
> > >        - description: Marvell SoCs implementing "arm,mmu-500"
> > >          items:
> > >            - const: marvell,ap806-smmu-500
> > > --
> > > 2.26.2
> > >

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