Re: [PATCH 2/3] dt-bindings: iommu: Add NVIDIA Tegra CMDQV support

[Ashish Mhetre]

On 11/20/2025 3:53 PM, Robin Murphy wrote:
> External email: Use caution opening links or attachments
>
>
> On 2025-11-20 6:07 am, Ashish Mhetre wrote:
> > On 11/3/2025 7:24 PM, Ashish Mhetre wrote:
> > > On 10/31/2025 1:44 PM, Krzysztof Kozlowski wrote:
> > > > External email: Use caution opening links or attachments
> > > >
> > > >
> > > > On Fri, Oct 31, 2025 at 06:29:58AM +0000, Ashish Mhetre wrote:
> > > > > The Command Queue Virtualization (CMDQV) hardware is part of the
> > > > > SMMUv3 implementation on NVIDIA Tegra SoCs. It assists in
> > > > > virtualizing the command queue for the SMMU.
> > > > If this is specific to Nvidia, then I think you need specific front
> > > > compatible and disallow it for other vendors.
> > >
> > > Yes, CMDQV is specific to Nvidia. There isn't currently a 
> > > vendor-specific
> > > compatible for Nvidia's arm,smmu-v3 implementation. Would it be
> > > acceptable
> > > to document this as Nvidia-specific in the description? Or can we add a
> > > new Nvidia-specific compatible string like "nvidia,smmu-v3" if that's
> > > preferred and use conditional schema to restrict the property?
> >
> > Hi Will, Robin,
> >
> > Do you have any suggestions on this? I have followed existing ACPI 
> > approach
> > for implementing DT support.
>
> No, the way the ACPI binding is implemented has the lookup going *from*
> the CMDQV node back to the SMMU instance (via the matching identifier) -
> this is entirely the opposite. The literal DT equivalent would be to use
> for_each_matching_node/for_each_compatible_node to scan the CMDQV nodes
> for a property indicating the relevant SMMU.
>
> I'm not hugely fussed either way though - since the fact is the Tegra234
> SMMU does have this custom modification, a specific
> "nvidia,tegra234-smmu", "arm,smmu-v3" compatible isn't inappropriate,
> even if it really doesn't make any difference to architectural SMMU
> operation without awareness of the other CMDQV nodes.
>
> Thanks,
> Robin.

Thanks for the suggestions Robin. Approach 2 is much simpler with current
implementation I have. I will proceed with adding Nvidia specific compatible
string "nvidia,tegra264-smmu", "arm,smmu-v3" in V2.

> > Will it be fine to add separate compatible
> > string
> > for Nvidia Tegra264 SMMU to restrict the usage of CMDQV?
> >
> > > > > Add a new device tree binding document for nvidia,tegra264-cmdqv.
> > > > >
> > > > > Also update the arm,smmu-v3 binding to include an optional 
> > > > > nvidia,cmdqv
> > > > > property. This property is a phandle to the CMDQV device node, 
> > > > > allowing
> > > > > the SMMU driver to associate with its corresponding CMDQV instance.
> > > > >
> > > > > Signed-off-by: Ashish Mhetre <amhetre@xxxxxxxxxx>
> > > > > ---
> > > > >   .../bindings/iommu/arm,smmu-v3.yaml           | 10 ++++
> > > > >   .../bindings/iommu/nvidia,tegra264-cmdqv.yaml | 46 +++++++++++++++
> > > > > ++++
> > > > >   2 files changed, 56 insertions(+)
> > > > >   create mode 100644 Documentation/devicetree/bindings/iommu/
> > > > > nvidia,tegra264-cmdqv.yaml
> > > > >
> > > > > diff --git a/Documentation/devicetree/bindings/iommu/arm,smmu-
> > > > > v3.yaml b/Documentation/devicetree/bindings/iommu/arm,smmu-v3.yaml
> > > > > index 75fcf4cb52d9..edc0c20a0c80 100644
> > > > > --- a/Documentation/devicetree/bindings/iommu/arm,smmu-v3.yaml
> > > > > +++ b/Documentation/devicetree/bindings/iommu/arm,smmu-v3.yaml
> > > > > @@ -58,6 +58,15 @@ properties:
> > > > >
> > > > >     msi-parent: true
> > > > >
> > > > > +  nvidia,cmdqv:
> > > > > +    description: |
> > > > > +      A phandle to its pairing CMDQV extension for an
> > > > > implementation on NVIDIA
> > > > > +      Tegra SoC.
> > > > > +
> > > > > +      If this property is absent, CMDQ-Virtualization won't be used
> > > > > and SMMU
> > > > > +      will only use its own CMDQ.
> > > > > +    $ref: /schemas/types.yaml#/definitions/phandle
> > > > > +
> > > > >     hisilicon,broken-prefetch-cmd:
> > > > >       type: boolean
> > > > >       description: Avoid sending CMD_PREFETCH_* commands to the SMMU.
> > > > > @@ -92,4 +101,5 @@ examples:
> > > > >               dma-coherent;
> > > > >               #iommu-cells = <1>;
> > > > >               msi-parent = <&its 0xff0000>;
> > > > > +            nvidia,cmdqv = <&cmdqv>;
> > > > >       };
> > > > > diff --git a/Documentation/devicetree/bindings/iommu/
> > > > > nvidia,tegra264-cmdqv.yaml b/Documentation/devicetree/bindings/
> > > > > iommu/nvidia,tegra264-cmdqv.yaml
> > > > > new file mode 100644
> > > > > index 000000000000..f22c370278a3
> > > > > --- /dev/null
> > > > > +++ b/Documentation/devicetree/bindings/iommu/nvidia,tegra264-
> > > > > cmdqv.yaml
> > > > > @@ -0,0 +1,46 @@
> > > > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > > > > +%YAML 1.2
> > > > > +---
> > > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > > +
> > > > > +title: NVIDIA Tegra264 CMDQV
> > > > Missing blank line
> > >
> > > Ack, I will correct this in V2.
> > > > > +description: |
> > > > Do not need '|' unless you need to preserve formatting.
> > >
> > > Okay, I'll remove this in next version.
> > > > > +  The CMDQ-Virtualization hardware block is part of the SMMUv3
> > > > > implementation
> > > > > +  on Tegra264 SoCs. It assists in virtualizing the command queue
> > > > > for the SMMU.
> > > > > +
> > > > > +maintainers:
> > > > > +  - NVIDIA Corporation <linux-tegra@xxxxxxxxxx>
> > > > No. It should be a person. If entire Nvidia cannot find a person, I
> > > > don't think we are interested in having this in the kernel.
> > >
> > > Okay, I'll add Nicolin as maintainer.
> > > > > +
> > > > > +properties:
> > > > > +  compatible:
> > > > > +    const: nvidia,tegra264-cmdqv
> > > > > +
> > > > > +  reg:
> > > > > +    maxItems: 1
> > > > > +
> > > > > +  interrupts:
> > > > > +    maxItems: 1
> > > > > +
> > > > > +  interrupt-names:
> > > > > +    items:
> > > > > +      - const: cmdqv
> > > > Drop interript names, obvious.
> > >
> > > Sure, I will update in V2.
> > > > > +
> > > > > +required:
> > > > > +  - compatible
> > > > > +  - reg
> > > > > +  - interrupts
> > > > > +  - interrupt-names
> > > > > +
> > > > > +additionalProperties: false
> > > > > +
> > > > > +examples:
> > > > > +  - |
> > > > > +    #include <dt-bindings/interrupt-controller/arm-gic.h>
> > > > > +    #include <dt-bindings/interrupt-controller/irq.h>
> > > > > +
> > > > > +    cmdqv: cmdqv@8105200000 {
> > > > Drop unused label
> > >
> > > Okay, I will remove the label.
> > > > Best regards,
> > > > Krzysztof