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

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Message-ID: <20181123183428.GD21183@arm.com>
Date:   Fri, 23 Nov 2018 18:34:29 +0000
From:   Will Deacon <will.deacon@....com>
To:     Vivek Gautam <vivek.gautam@...eaurora.org>
Cc:     thor.thayer@...ux.intel.com, Tomasz Figa <tfiga@...omium.org>,
        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

On Fri, Nov 23, 2018 at 03:06:29PM +0530, Vivek Gautam wrote:
> On Fri, Nov 23, 2018 at 2:52 PM Tomasz Figa <tfiga@...omium.org> wrote:
> > On Fri, Nov 23, 2018 at 6:13 PM Vivek Gautam
> > <vivek.gautam@...eaurora.org> wrote:
> > > On Wed, Nov 21, 2018 at 11:09 PM Will Deacon <will.deacon@....com> wrote:
> > > > On Fri, Nov 16, 2018 at 04:54:30PM +0530, Vivek Gautam wrote:
> > > > > @@ -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.

But it also has the undesirable effect of having to update the driver
code whenever we want to add support for a new SMMU implementation. If
we do this all in the DT, as Thor is trying to do, then older kernels
will work well with new hardware.

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

I'm having trouble parsing your question, sorry. Please work with Thor
so that we have a single way to get the clock information. My preference
is to take it from the firmware, for the reason I stated above.

Will