Re: [PATCH v5 1/9] dt-bindings: usb: Add Type-C switch binding

[Prashant Malani <pmalani@chromium.org>]

On Wed, Jun 29, 2022 at 2:58 PM Stephen Boyd <swboyd@chromium.org> wrote:
>
> > What device controls the switching in this case? Again, block diagrams
> > please if you want advice on what the binding should look like.
>
> My understanding is there are 4 DP lanes on it6505 and two lanes are
> connected to one usb-c-connector and the other two lanes are connected
> to a different usb-c-connector. The IT6505 driver will send DP out on
> the associated two DP lanes depending on which usb-c-connector has DP
> pins assigned by the typec manager.
>
>                                                      +-------+
>                                                      |       |
>           +--------+                            /----+ usb-c |
>           | IT6505 |                           / /---+       |
>           |        +------------ lane 0 ------/ /    |       |
>           |        +------------ lane 1 -------/     +-------+
>  DPI -----+        |
>           |        |                                 +-------+
>           |        |                                 |       |
>           |        +------------ lane 2 -------------+ usb-c |
>           |        +------------ lane 3 -------------+       |
>           |        |                                 |       |
>           +--------+                                 +-------+
>
> The bridge is a mux that steers DP to one or the other usb-c-connector
> based on what the typec manager decides.
>
> I would expect this to be described with the existing port binding in
> the it6505 node. The binding would need to be extended to describe the
> output side.
>
>         bridge@5c {
>             compatible = "ite,it6505";

We'll need a top level "mode-switch" property here.
>             ...
>
>             ports {
>                 #address-cells = <1>;
>                 #size-cells = <0>;
>
>                 port@0 {
>                     reg = <0>;
>                     it6505_in: endpoint {
>                         remote-endpoint = <&dpi_out>;
>                     };
>                 };
>
>                 port@1 {
>                     #address-cells = <1>;
>                     #size-cells = <0>;
>                     reg = <1>;
>
>                     it6505_out_lanes_01: endpoint@0 {
>                         reg = <0>
>                         data-lanes = <0 1>;
>                         remote-endpoint = <&typec0>;
>                     };
>
>                     it6505_out_lanes_23: endpoint@1 {
>                         reg = <1>
>                         data-lanes = <2 3>;
>                         remote-endpoint = <&typec1>;
>                     };
>                 };
>             };
>         };
>
>         usb-c-connector {
>             compatible = "usb-c-connector";
>             ....
>             ports {
>                 #address-cells = <1>;
>                 #size-cells = <0>;
>
>                 port@1 {
>                     reg = <1>;
>                     typec0: endpoint {
>                         remote-endpoint = <&it6505_out_lanes_01>;
>                     };
>                 };
>             };
>         };

We can adopt this binding, but from what I gathered in this thread, that
shouldn't be done, because IT6505 isn't meant to be aware of Type-C
connections at all.

>
> I don't see the benefit to making a genericish binding for typec
> switches, even if the hardware has typec awareness like anx7625. It
> looks like the graph binding can already handle what we need. By putting
> it in the top-level ports node we have one way to describe the
> input/output of the device instead of describing it in the top-level in
> the display connector case and the child typec switch node in the usb c
> connector case.

Ack, I'll drop the generic binding for future revisions.

>
> I think the difficulty comes from the combinatorial explosion of
> possible configurations. As evidenced here, hardware engineers can take
> a DP bridge and use it as a DP mux as long as the bridge has lane
> control. Or they can take a device like anx7625 and ignore the USB
> aspect and use the internal crosspoint switch as a DP mux. The anx7625
> part could be a MIPI-to-DP display bridge plus mux that is connected to
> two dp-connectors, in which case typec isn't even involved, but we could
> mux between two dp connectors.

Each containing a single DP lane, right?
I think that will not be a valid configuration, since there is only 1 HPD
pin (so it's assuming both DP lanes go to the same DP sink).

But yes, your larger point is valid: h/w engineers can repurpose these
bridges in ways the datasheet doesn't originally anticipate.

>
> Also, the typec framework would like to simply walk the graph from the
> usb-c-connector looking for nodes that have 'mode-switch' or
> 'orientation-switch' properties and treat those devices as the typec
> switches for the connector. This means that we have to add these typec
> properties like 'mode-switch' to something like the IT6505 bridge
> binding, which is a little awkward. I wonder if those properties aren't
> really required. Would it be sufficient if the framework could walk the
> graph and look for registered typec switches in the kernel that have a
> matching of_node?

My interpretation of the current mode-switch search code [1] is that
a top level property of "mode-switch" is required.

[1] https://elixir.bootlin.com/linux/v5.19-rc4/source/drivers/usb/typec/mux.c#L347