linux-kernel - Re: [PATCH v2 02/13] dt-bindings: PCI: qcom: Add iommu properties

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Date:   Wed, 1 Mar 2023 08:58:51 -0600
From:   Rob Herring <robh@...nel.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Robin Murphy <Robin.Murphy@....com>
Cc:     andersson@...nel.org, lpieralisi@...nel.org, kw@...ux.com,
        krzysztof.kozlowski+dt@...aro.org, vkoul@...nel.org,
        konrad.dybcio@...aro.org, bhelgaas@...gle.com, kishon@...nel.org,
        linux-arm-msm@...r.kernel.org, linux-phy@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v2 02/13] dt-bindings: PCI: qcom: Add iommu properties

+Robin

On Tue, Feb 28, 2023 at 2:20 AM Manivannan Sadhasivam
<manivannan.sadhasivam@...aro.org> wrote:
>
> On Mon, Feb 27, 2023 at 01:55:35PM -0600, Rob Herring wrote:
> > On Fri, Feb 24, 2023 at 04:28:55PM +0530, Manivannan Sadhasivam wrote:
> > > Most of the PCIe controllers require iommu support to function properly.
> > > So let's add them to the binding.
> > >
> > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > > ---
> > >  Documentation/devicetree/bindings/pci/qcom,pcie.yaml | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > >
> > > diff --git a/Documentation/devicetree/bindings/pci/qcom,pcie.yaml b/Documentation/devicetree/bindings/pci/qcom,pcie.yaml
> > > index a3639920fcbb..f48d0792aa57 100644
> > > --- a/Documentation/devicetree/bindings/pci/qcom,pcie.yaml
> > > +++ b/Documentation/devicetree/bindings/pci/qcom,pcie.yaml
> > > @@ -64,6 +64,11 @@ properties:
> > >
> > >    dma-coherent: true
> > >
> > > +  iommus:
> > > +    maxItems: 1
> > > +
> > > +  iommu-map: true
> > > +
> >
> > I think both properties together doesn't make sense unless the PCI host
> > itself does DMA in addition to PCI bus devices doing DMA.
> >
>
> How? With "iommus", we specify the SMR mask along with the starting SID and with
> iommu-map, the individual SID<->BDF mapping is specified. This has nothing to
> do with host DMA capabilities.

I spoke with Robin offline and he agrees that having both is broken at
least in RC mode. He pointed out the issue is similar to this one on
Tegra[1].

Rob

[1] https://lore.kernel.org/all/AS8P193MB2095640357779A7F9B6026F8D2A19@AS8P193MB2095.EURP193.PROD.OUTLOOK.COM/