Re: [PATCH v4 01/10] dt-bindings: iommu: Add Translation Buffer Unit bindings

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On 2024-02-02 9:17 pm, Rob Herring wrote:
On Thu, Feb 01, 2024 at 01:05:20PM -0800, Georgi Djakov wrote:
Add common bindings for the TBUs to describe their properties. The
TBUs are modelled as child devices of the IOMMU and each of them is
described with their compatible, reg and stream-id-range properties.
There could be other implementation specific properties to describe
any resources like clocks, regulators, power-domains, interconnects
that would be needed for TBU operation. Such properties will be
documented in a separate vendor-specific TBU schema.

Signed-off-by: Georgi Djakov <quic_c_gdjako@xxxxxxxxxxx>
  .../devicetree/bindings/iommu/arm,smmu.yaml   | 14 ++++++++++
  .../devicetree/bindings/iommu/tbu-common.yaml | 28 +++++++++++++++++++
  2 files changed, 42 insertions(+)
  create mode 100644 Documentation/devicetree/bindings/iommu/tbu-common.yaml

diff --git a/Documentation/devicetree/bindings/iommu/arm,smmu.yaml b/Documentation/devicetree/bindings/iommu/arm,smmu.yaml
index a4042ae24770..ba3237023b39 100644
--- a/Documentation/devicetree/bindings/iommu/arm,smmu.yaml
+++ b/Documentation/devicetree/bindings/iommu/arm,smmu.yaml
@@ -235,6 +235,20 @@ properties:
        enabled for any given device.
      $ref: /schemas/types.yaml#/definitions/phandle
+ '#address-cells':
+    enum: [ 1, 2 ]
+  '#size-cells':
+    enum: [ 1, 2 ]
+  ranges: true
+  "^tbu@[0-9a-f]+$":
+    description: TBU child nodes
+    type: object
+    $ref: tbu-common.yaml#

        additionalProperties: false

However, that's going to break with the extra QCom properties. In
json-schema, you can't have 2 schemas and extend the properties of
their child nodes. The validator doesn't "see" the child node schemas at
the same time. You are going to have to move QCom SMMU to its own schema
and remove it from arm,smmu.yaml.

Although this common binding is pretty pointless - sorry I missed the previous discussion, but these TBU registers are on obscure debugging feature of Qualcomm's own invention and definitely not generic. The internal topology of the unmodified Arm MMU-500 implementation isn't software-visible at all without getting into its own integration and debug registers (and maybe to a lesser extent the PMU), and even then everything is proxied through the TCU via an internal AXI stream interconnect, so there aren't really any TBU-owned resources which would warrant describing as such in DT. If anything, the way this binding is defined as an MMIO bus with "ranges" would actively *prevent* being able to describe the standard hardware this way, since the internal debug stuff all wants to refer to TBUs by numerical index.

Conversely, given that the Qualcomm TBU registers seem to be describing their own entirely independent resources and inheriting nothing from the parent node, I'm not sure it's necessarily worth all the bother of describing and supporting them them as children at all, when they could just as well be standalone nodes with a phandle to associate an SMMU instance.


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