Re: [PATCH v3 2/3] dt-bindings: arm: Add Coresight Dummy Trace

[Hao Zhang]

Hi Rob,

On 4/26/2023 2:46 AM, Rob Herring wrote:
> On Sat, Apr 22, 2023 at 03:37:13PM +0800, Hao Zhang wrote:
> > Add new coresight-dummy.yaml file describing the bindings required
> > to define coresight dummy trace in the device trees.
> >
> > Signed-off-by: Hao Zhang <quic_hazha@xxxxxxxxxxx>
> > ---
> >   .../bindings/arm/arm,coresight-dummy.yaml     | 101 ++++++++++++++++++
> >   1 file changed, 101 insertions(+)
> >   create mode 100644 Documentation/devicetree/bindings/arm/arm,coresight-dummy.yaml
> >
> > diff --git a/Documentation/devicetree/bindings/arm/arm,coresight-dummy.yaml b/Documentation/devicetree/bindings/arm/arm,coresight-dummy.yaml
> > new file mode 100644
> > index 000000000000..48d864aefaaa
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/arm/arm,coresight-dummy.yaml
> > @@ -0,0 +1,101 @@
> > +# SPDX-License-Identifier: GPL-2.0-only or BSD-2-Clause
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/arm/arm,coresight-dummy.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: ARM Coresight Dummy component
> > +
> > +description: |
> > +  Coresight Dummy Trace Module is for the specific devices that kernel
> > +  don't have permission to access or configure, e.g., CoreSight TPDMs
> > +  on Qualcomm platforms. So there need driver to register dummy devices
> > +  as Coresight devices. It may also be used to define components that
> > +  may not have any programming interfaces (e.g, static links), so that
> > +  paths can be established in the driver. Provide Coresight API for
> > +  dummy device operations, such as enabling and disabling dummy devices.
> > +  Build the Coresight path for dummy sink or dummy source for debugging.
> > +
> > +  The primary use case of the coresight dummy is to build path in kernel
> > +  side for dummy sink and dummy source.
>
> I could imagine the OS wanting to know more information than just
> 'dummy'. Is data from an unknown source useful? Likewise, don't you want
> to know where you are sending data too?
The necessary information for Coresight is connection between different 
components, so there is in-port for dummy sink and out-port for dummy 
source. We can get the whole path from the source to sink in device tree.

> > +
> > +maintainers:
> > +  - Mao Jinlong <quic_jinlmao@xxxxxxxxxxx>
> > +  - Tao Zhang <quic_taozha@xxxxxxxxxxx>
> > +  - Hao Zhang <quic_hazha@xxxxxxxxxxx>
> > +
> > +properties:
> > +  compatible:
> > +    oneOf:
> > +      - enum:
>
> Don't need oneOf as there is only one entry.
OK, I will remove it in the next version of patch.
> > +          - arm,coresight-dummy-sink
> > +          - arm,coresight-dummy-source
> > +
> > +  out-ports:
> > +    $ref: /schemas/graph.yaml#/properties/ports
> > +
> > +    properties:
> > +      port:
> > +        description: Output connection from the source to Coresight
> > +          Trace bus.
> > +        $ref: /schemas/graph.yaml#/properties/port
> > +
> > +  in-ports:
> > +    $ref: /schemas/graph.yaml#/properties/ports
> > +
> > +    properties:
> > +      port:
> > +        description: Input connection from the Coresight Trace bus to
> > +          dummy sink, such as Embedded USB debugger(EUD).
> > +        $ref: /schemas/graph.yaml#/properties/port
> > +
> > +required:
> > +  - compatible
> > +
> > +if:
> > +  # If the compatible contains the below value
> > +  properties:
> > +    compatible:
> > +      contains:
> > +        const: arm,coresight-dummy-sink
> > +
> > +then:
> > +  required:
> > +    - in-ports
> > +
> > +else:
> > +  required:
> > +    - out-ports
>
> This still allows the nodes when they don't make sense. I think this
> needs to be 2 schema files. The only common part is 'compatible' and
> that's not even shared.
>  > Rob
Dummy driver is very simple, the only goal of it is to build a path in 
kernel for subsystem, so we want to handle dummy source and sink in a 
generic framework.

Thanks,
Hao