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Date:   Fri, 26 Feb 2021 13:14:54 -0600
From:   Bjorn Andersson <bjorn.andersson@linaro.org>
To:     Rob Clark <robdclark@gmail.com>
Cc:     Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>,
        Will Deacon <will@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jordan Crouse <jcrouse@codeaurora.org>,
        Akhil P Oommen <akhilpo@codeaurora.org>,
        "list@263.net:IOMMU DRIVERS <iommu@lists.linux-foundation.org>, Joerg
        Roedel <joro@8bytes.org>," <iommu@lists.linux-foundation.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>
Subject: Re: [PATCHv2 2/2] iommu/arm-smmu-qcom: Move the adreno smmu specific
 impl earlier
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On Fri 26 Feb 12:23 CST 2021, Rob Clark wrote:

> On Fri, Feb 26, 2021 at 9:24 AM Bjorn Andersson
> <bjorn.andersson@linaro.org> wrote:
> >
> > On Fri 26 Feb 03:55 CST 2021, Sai Prakash Ranjan wrote:
> >
> > > Adreno(GPU) SMMU and APSS(Application Processor SubSystem) SMMU
> > > both implement "arm,mmu-500" in some QTI SoCs and to run through
> > > adreno smmu specific implementation such as enabling split pagetables
> > > support, we need to match the "qcom,adreno-smmu" compatible first
> > > before apss smmu or else we will be running apps smmu implementation
> > > for adreno smmu and the additional features for adreno smmu is never
> > > set. For ex: we have "qcom,sc7280-smmu-500" compatible for both apps
> > > and adreno smmu implementing "arm,mmu-500", so the adreno smmu
> > > implementation is never reached because the current sequence checks
> > > for apps smmu compatible(qcom,sc7280-smmu-500) first and runs that
> > > specific impl and we never reach adreno smmu specific implementation.
> > >
> >
> > So you're saying that you have a single SMMU instance that's compatible
> > with both an entry in qcom_smmu_impl_of_match[] and "qcom,adreno-smmu"?
> >
> > Per your proposed change we will pick the adreno ops _only_ for this
> > component, essentially disabling the non-Adreno quirks selected by the
> > qcom impl. As such keeping the non-adreno compatible in the
> > qcom_smmu_impl_init[] seems to only serve to obfuscate the situation.
> >
> > Don't we somehow need the combined set of quirks? (At least if we're
> > running this with a standard UEFI based boot flow?)
> >
> 
> are you thinking of the apps-smmu handover of display context bank?
> That shouldn't change, the only thing that changes is that gpu-smmu
> becomes an mmu-500, whereas previously only apps-smmu was..
> 

The current logic picks one of:
1) is the compatible mentioned in qcom_smmu_impl_of_match[]
2) is the compatible an adreno
3) no quirks needed

The change flips the order of these, so the only way I can see this
change affecting things is if we expected a match on #2, but we got one
on #1.

Which implies that the instance that we want to act according to the
adreno impl was listed in qcom_smmu_impl_of_match[] - which either is
wrong, or there's a single instance that needs both behaviors.

(And I believe Jordan's answer confirms the latter - there's a single
SMMU instance that needs all them quirks at once)

Regards,
Bjorn