linux-kernel - Re: [RESEND PATCH v17 5/5] iommu/arm-smmu: Add support for qcom,smmu-v2 variant

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Message-ID: <CAFp+6iFhBRJWGY4Hj30tWedGpEUcnkBJrOvffmbpD2m9QMZB3A@mail.gmail.com>
Date:   Fri, 23 Nov 2018 15:06:29 +0530
From:   Vivek Gautam <vivek.gautam@...eaurora.org>
To:     Will Deacon <will.deacon@....com>, thor.thayer@...ux.intel.com,
        Tomasz Figa <tfiga@...omium.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Linux PM <linux-pm@...r.kernel.org>,
        sboyd@...nel.org, linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        open list <linux-kernel@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        alex.williamson@...hat.com, "robh+dt" <robh+dt@...nel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [RESEND PATCH v17 5/5] iommu/arm-smmu: Add support for
 qcom,smmu-v2 variant

Hi Tomasz,

On Fri, Nov 23, 2018 at 2:52 PM Tomasz Figa <tfiga@...omium.org> wrote:
>
> Hi Vivek, Will,
>
> On Fri, Nov 23, 2018 at 6:13 PM Vivek Gautam
> <vivek.gautam@...eaurora.org> wrote:
> >
> > Hi Will,
> >
> > On Wed, Nov 21, 2018 at 11:09 PM Will Deacon <will.deacon@....com> wrote:
> > >
> > > [+Thor]
> > >
> > > On Fri, Nov 16, 2018 at 04:54:30PM +0530, Vivek Gautam wrote:
> > > > qcom,smmu-v2 is an arm,smmu-v2 implementation with specific
> > > > clock and power requirements.
> > > > On msm8996, multiple cores, viz. mdss, video, etc. use this
> > > > smmu. On sdm845, this smmu is used with gpu.
> > > > Add bindings for the same.
> > > >
> > > > Signed-off-by: Vivek Gautam <vivek.gautam@...eaurora.org>
> > > > Reviewed-by: Rob Herring <robh@...nel.org>
> > > > Reviewed-by: Tomasz Figa <tfiga@...omium.org>
> > > > Tested-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
> > > > Reviewed-by: Robin Murphy <robin.murphy@....com>
> > > > ---
> > > >  drivers/iommu/arm-smmu.c | 13 +++++++++++++
> > > >  1 file changed, 13 insertions(+)
> > > >
> > > > diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> > > > index 2098c3141f5f..d315ca637097 100644
> > > > --- a/drivers/iommu/arm-smmu.c
> > > > +++ b/drivers/iommu/arm-smmu.c
> > > > @@ -120,6 +120,7 @@ enum arm_smmu_implementation {
> > > >       GENERIC_SMMU,
> > > >       ARM_MMU500,
> > > >       CAVIUM_SMMUV2,
> > > > +     QCOM_SMMUV2,
> > > >  };
> > > >
> > > >  struct arm_smmu_s2cr {
> > > > @@ -2026,6 +2027,17 @@ ARM_SMMU_MATCH_DATA(arm_mmu401, ARM_SMMU_V1_64K, GENERIC_SMMU);
> > > >  ARM_SMMU_MATCH_DATA(arm_mmu500, ARM_SMMU_V2, ARM_MMU500);
> > > >  ARM_SMMU_MATCH_DATA(cavium_smmuv2, ARM_SMMU_V2, CAVIUM_SMMUV2);
> > > >
> > > > +static const char * const qcom_smmuv2_clks[] = {
> > > > +     "bus", "iface",
> > > > +};
> > > > +
> > > > +static const struct arm_smmu_match_data qcom_smmuv2 = {
> > > > +     .version = ARM_SMMU_V2,
> > > > +     .model = QCOM_SMMUV2,
> > > > +     .clks = qcom_smmuv2_clks,
> > > > +     .num_clks = ARRAY_SIZE(qcom_smmuv2_clks),
> > > > +};
> > >
> > > These seems redundant if we go down the route proposed by Thor, where we
> > > just pull all of the clocks out of the device-tree. In which case, why
> > > do we need this match_data at all?
> >
> > Which is better? Driver relying solely on the device tree to tell
> > which all clocks
> > are required to be enabled,
> > or, the driver deciding itself based on the platform's match data,
> > that it should
> > have X, Y, & Z clocks that should be supplied from the device tree.
>
> The former would simplify the driver, but would also make it
> impossible to spot mistakes in DT, which would ultimately surface out
> as very hard to debug bugs (likely complete system lockups).

Thanks.
Yea, this is how I understand things presently. Relying on device tree
puts the things out of driver's control.

Hi Will,
Am I unable to understand the intentions here for Thor's clock-fetch
design change?

>
> For qcom_smmuv2, I believe we're eventually going to end up with
> platform-specific quirks anyway, so specifying the clocks too wouldn't
> hurt. Given that, I'd recommend sticking to the latter, i.e. what this
> patch does.
>
> Best regards,
> Tomasz

Best regards
Vivek

> _______________________________________________
> iommu mailing list
> iommu@...ts.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/iommu

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