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SubjectRe: [PATCH v3 1/4] dt-bindings: Add cros-ec Type C port driver
Hi Stephen,

On Thu, Feb 27, 2020 at 12:41:13AM -0800, Stephen Boyd wrote:
> > +examples:
> > + - |+
> > + cros_ec: ec@0 {
> > + compatible = "google,cros-ec-spi";
> > +
> > + typec {
> > + compatible = "google,cros-ec-typec";
> > +
> > + usb_con: connector {
> > + compatible = "usb-c-connector";
> > + port-number = <0>;
> > + power-role = "dual";
> > + data-role = "dual";
> > + try-power-role = "source";
> > + };
>
> I thought that perhaps this would be done with the OF graph APIs instead
> of being a child of the ec node. I don't see how the usb connector is
> anything besides a child of the top-level root node because it's
> typically on the board. We put board level components at the root.

No.

The above follows the usb-connector bindings, so it is correct:
Documentation/devicetree/bindings/connector/usb-connector.txt

So the connector is always a child of the "CC controller" with the USB
Type-C connectors, which in this case is the EC (from operating systems
perspective). The "CC controller" controls connectors, and it doesn't
actually do anything else. So placing the connectors under the
"connector controller" is also logically correct.

> Yes, the connector is intimately involved with the EC here, but I would
> think that we would have an OF graph connection from the USB controller
> on the SoC to the USB connector, traversing through anything that may be
> in that path, such as a USB hub. Maybe the connector node itself can
> point to the EC type-c controller with some property like

I think your idea here is that there should be only a single node for
each connector that is then linked with every component that it is
physically connected to (right?), but please note that that is not
enough. Every component attached to the connector must have its own
child node that represents the "port" that is physically connected to
the USB Type-C connector.

So for example, the USB controller nodes have child nodes for every
USB2 port as well as for every USB3 port. Similarly, the GPU
controllers have child node for every DisplayPort, etc. And I believe
that is already how it has been done in DT (and also in ACPI).

Those "port" nodes then just need to be linked with the "connector"
node. I think for that the idea was to use OF graph, but I'm really
sceptical about that. The problem is that with the USB Type-C
connectors we have to be able to identify the connections, i.e. which
endpoint is the USB2 port, which is the DisplayPort and so on, and OF
graph does not give any means to do that on its own. We will have to
rely on separate device properties in order to do the identification.
Currently it is not documented anywhere which property should be used
for that.

For ACPI we are going to propose that with every type of connection,
there should be a device property that returns a reference to the
appropriate port. That way there are no problems identifying the
connections. All we need to do is to define the property names for
every type of connection. "usb2-port" for the USB2 or high speed port,
"usb3-port" for USB3, etc.


thanks,

--
heikki

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