linux-kernel - Re: [PATCH 2/5] iommu/arm-smmu: Emulate bypass by using context banks

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Message-ID: <CAF6AEGvmqSNeQSFhc4SDJ=09x_LMVz=DmzUEb6GGv=s=f8P2aA@mail.gmail.com>
Date:   Thu, 9 Jul 2020 09:56:31 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        Jonathan Marek <jonathan@...ek.ca>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/5] iommu/arm-smmu: Emulate bypass by using context banks

On Thu, Jul 9, 2020 at 9:48 AM Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
>
> On Thu 09 Jul 09:17 PDT 2020, Rob Clark wrote:
>
> > On Wed, Jul 8, 2020 at 10:01 PM Bjorn Andersson
> > <bjorn.andersson@...aro.org> wrote:
> [..]
> > > @@ -678,7 +680,11 @@ static int arm_smmu_init_domain_context(struct iommu_domain *domain,
> > >         if (smmu_domain->smmu)
> > >                 goto out_unlock;
> > >
> > > -       if (domain->type == IOMMU_DOMAIN_IDENTITY) {
> > > +       /*
> > > +        * Nothing to do for IDENTITY domains,unless disabled context banks are
> > > +        * used to emulate bypass mappings on Qualcomm platforms.
> > > +        */
> > > +       if (domain->type == IOMMU_DOMAIN_IDENTITY && !smmu->qcom_bypass_quirk) {
> >
> > maybe I'm overlooking something, but I think this would put us back to
> > allocating pgtables (and making iommu->map/unmap() no longer no-ops),
> > which I don't think we want
> >
>
> You're right, we are allocating page tables for these contexts and
> map/unmap would modify the page tables. But afaict traversal is never
> performed, given that the banks are never enabled.
>
> But as drivers probe properly, or the direct mapped drivers sets up
> their iommu domains explicitly with translation this would not be used.
>
> So afaict we're just wasting some memory - for the gain of not
> overcomplicating this function.

the problem is that it makes dma_map/unmap less of a no-op than it
should be (for the case where the driver is explicitly managing it's
own domain)..  I was hoping to get rid of the hacks to use dma_sync go
back to dma_map/unmap for cache cleaning

BR,
-R

>
> Regards,
> Bjorn
>
> > BR,
> > -R
> >
> > >                 smmu_domain->stage = ARM_SMMU_DOMAIN_BYPASS;
> > >                 smmu_domain->smmu = smmu;
> > >                 goto out_unlock;
> > > @@ -826,6 +832,10 @@ static int arm_smmu_init_domain_context(struct iommu_domain *domain,
> > >         domain->geometry.aperture_end = (1UL << ias) - 1;
> > >         domain->geometry.force_aperture = true;
> > >
> > > +       /* Enable translation for non-identity context banks */
> > > +       if (domain->type != IOMMU_DOMAIN_IDENTITY)
> > > +               cfg->m = true;
> > > +
> > >         /* Initialise the context bank with our page table cfg */
> > >         arm_smmu_init_context_bank(smmu_domain, &pgtbl_cfg);
> > >         arm_smmu_write_context_bank(smmu, cfg->cbndx);
> > > diff --git a/drivers/iommu/arm-smmu.h b/drivers/iommu/arm-smmu.h
> > > index d172c024be61..a71d193073e4 100644
> > > --- a/drivers/iommu/arm-smmu.h
> > > +++ b/drivers/iommu/arm-smmu.h
> > > @@ -305,6 +305,8 @@ struct arm_smmu_device {
> > >
> > >         /* IOMMU core code handle */
> > >         struct iommu_device             iommu;
> > > +
> > > +       bool                            qcom_bypass_quirk;
> > >  };
> > >
> > >  enum arm_smmu_context_fmt {
> > > @@ -323,6 +325,7 @@ struct arm_smmu_cfg {
> > >         };
> > >         enum arm_smmu_cbar_type         cbar;
> > >         enum arm_smmu_context_fmt       fmt;
> > > +       bool                            m;
> > >  };
> > >  #define ARM_SMMU_INVALID_IRPTNDX       0xff
> > >
> > > --
> > > 2.26.2
> > >
> > > _______________________________________________
> > > iommu mailing list
> > > iommu@...ts.linux-foundation.org
> > > https://lists.linuxfoundation.org/mailman/listinfo/iommu

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