Re: [PATCH 1/2] dt-bindings: chrome: Add cros-ec-typec mux props

[Rob Herring <robh@kernel.org>]

On Wed, Jun 10, 2020 at 11:49 AM Prashant Malani <pmalani@chromium.org> wrote:
>
> Hi Rob,
>
> On Wed, Jun 10, 2020 at 9:53 AM Rob Herring <robh@kernel.org> wrote:
> >
> > On Wed, Jun 10, 2020 at 9:34 AM Heikki Krogerus
> > <heikki.krogerus@linux.intel.com> wrote:
> > >
> > > On Tue, Jun 09, 2020 at 04:57:40PM -0700, Prashant Malani wrote:
> > > > Hi Rob,
> > > >
> > > > Thanks again for the comments and feedback. Kindly see responses inline:
> > > >
> > > > (Trimming unrelated text from thread):
> > > >
> > > >
> > > >             ports {
> > > >                 #address-cells = <1>;
> > > >                 #size-cells = <0>;
> > > >
> > > >                 port@0 {
> > > >                     reg = <0>;
> > > >                     usb_con_hs: endpoint {
> > > >                         remote-endpoint = <&foo_usb_hs_controller>;
> > > >                     };
> > > >                 };
> > > >
> > > >                 port@1 {
> > > >                     reg = <1>;
> > > >                     usb_con0_ss: endpoint@0 {
> > > >                         remote-endpoint = <&mode_mux_in>;
> > > >                     };
> > > >                 };
> > > >
> > > >                 port@2 {
> > > >                     reg = <2>;
> > > >                     usb_con_sbu: endpoint {
> > > >                         remote-endpoint = <&foo_dp_aux>;
> > > >                     };
> > > >                 };
> > > >             };
> > >
> > > The pins that can be reassigned can in practice go anywhere. We can't
> > > group them in any way. What do we do for example when the two sideband
> > > pins go to different locations?
> >
> > The sideband pins from the connector go to multiple places or the
> > sideband signal from a controller go to multiple connectors? Either
> > way, that's solved with multiple endpoints. In the former case, port@2
> > would have multiple endpoints with all the possible connections. The
> > general model of the graph is each port is a separate data channel and
> > multiple endpoints are either a mux or fanout depending on the data
> > direction.
> >
> > > It looks like the OF graph for the USB Type-C connectors expects the
> > > pins to be grouped like that, which is too bad, because unfortunately
> > > it will not work. It would require that we have a separate port for
> > > every pin that can be reassigned on the connector, and let's not even
> > > consider that.
> >
> > I guess you are referring to the 4 SS signal pairs and that they could
> > be 2 USB links, 1 USB link and 1-2 Alt mode links, or 4 Alt mode
> > links. I think the grouping of SS signals is fine as I'd expect
> > there's a single entity deciding the routing. That would be 'mux' node
> > I think, but 'mux' is the wrong abstraction here. I guess we could
> > have 4 muxes (1 for each signal), but I'd hope we don't need that
> > level of detail in DT. I think we're in agreement on that.
>
> I think the updated example handles this grouping (port@1 going to a
> "SS mux") although as you said it should probably be a group of muxes,
> but I think the example illustrates the point. Is that assessment
> correct?

Yes, but let's stop calling it a mux. It's a "USB Type C signal routing blob".

> > > We need higher level description of the connections for the USB Type-C
> > > connectors. For example, a connector can be connected to this (or
> > > these) DisplayPort(s), this USB 2.0 port, this USB 3.x port, this USB4
> > > port, etc. And this is the mux that handles the pins on this
> > > connector, and these are the retimers, etc. etc.
> > >
> > > We also need a way to identify those connections, and relying on
> > > something like fixed port node addresses, so indexes in practice,
> > > feels really risky to me. A conflict may seem unlikely now, but we
> > > seen those so many times in the past other things like GPIOs, IRQs,
> > > etc. We really need to define string identifiers for these
> > > connections.
> >
> > I assume for IRQs you are referring to cases where we have a variable
> > number such as 'interrupts = <1 2 3>;' where 'interrupts = <1 3>'
> > doesn't work because we can't describe interrupt 2 is not present? The
> > graph binding doesn't suffer from that issue since we can easily omit
> > port or endpoint nodes.
> >
> > Also, the numbering is specific to a compatible string. If we need
> > different numbering, then we can do a new compatible.
> >
> > Rob
>
> Would this block the addition of the "*-switch" properties? IIUC the
> two are related but not dependent on each other.
>
> The *-switch properties are phandles which the Type C connector class
> framework expects (and uses to get handles to those switches).
> These would point to the "mux" or "group of mux" abstractions as noted earlier.

You don't need them though. Walk the graph. You get the connector
port@1 remote endpoint and then get its parent.

> I'd suggest we work on updating the Type C connector class to a model
> that can describe connections for both DT (using OF graph) and ACPI,
> if something
> like that exists, but let it not block the addition of these switches
> to usb-connector.yaml; they will be needed by the Type C connector
> class framework
> regardless of the form the connection description takes.