Date:   Thu, 19 Jan 2023 09:39:54 -0800
From:   Bjorn Andersson <quic_bjorande@...cinc.com>
To:     Rob Herring <robh@...nel.org>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Bjorn Andersson <andersson@...nel.org>,
        <linux-usb@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] dt-bindings: usb: Introduce GPIO-based SBU mux

On Thu, Jan 19, 2023 at 10:11:32AM -0600, Rob Herring wrote:
> On Wed, Jan 18, 2023 at 10:08:11AM -0800, Bjorn Andersson wrote:
> > On Tue, Jan 17, 2023 at 11:56:57AM -0600, Rob Herring wrote:
> > > On Thu, Jan 12, 2023 at 08:11:14PM -0800, Bjorn Andersson wrote:
> > > > From: Bjorn Andersson <bjorn.andersson@...aro.org>
> > > > 
> > > > Introduce a binding for GPIO-based mux hardware used for connecting,
> > > > disconnecting and switching orientation of the SBU lines in USB Type-C
> > > > applications.
> > > > 
> > > > Signed-off-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> > > > Signed-off-by: Bjorn Andersson <quic_bjorande@...cinc.com>
> > > > ---
> 
> 
> > > > +    tcpm {
> > > > +        connector {
> > > > +            compatible = "usb-c-connector";
> > > > +
> > > > +            ports {
> > > > +                #address-cells = <1>;
> > > > +                #size-cells = <0>;
> > > > +
> > > > +                port@0 {
> > > > +                    reg = <0>;
> > > > +                    tcpm_hs_out: endpoint {
> > > > +                        remote-endpoint = <&usb_hs_phy_in>;
> > > > +                    };
> > > > +                };
> > > > +
> > > > +                port@1 {
> > > > +                    reg = <1>;
> > > > +                    tcpm_ss_out: endpoint {
> > > > +                        remote-endpoint = <&usb_ss_phy_in>;
> > > > +                    };
> > > > +                };
> > > > +
> > > > +                port@2 {
> > > > +                    reg = <2>;
> > > > +                    tcpm_sbu_out: endpoint {
> > > > +                        remote-endpoint = <&sbu_mux_in>;
> > > > +                    };
> > > > +                };
> > > > +            };
> > > > +        };
> > > > +    };
> > > > +
> > > > +    sbu-mux {
> > > > +        compatible = "pericom,pi3usb102", "gpio-sbu-mux";
> > > > +
> > > > +        enable-gpios = <&tlmm 101 GPIO_ACTIVE_LOW>;
> > > > +        select-gpios = <&tlmm 164 GPIO_ACTIVE_HIGH>;
> > > > +
> > > > +        mode-switch;
> > > > +        orientation-switch;
> > > > +
> > > > +        port {
> > > > +            sbu_mux_in: endpoint {
> > > > +                remote-endpoint = <&tcpm_sbu_out>;
> > > > +            };
> > > 
> > > Don't you need a connection to whatever drives SBU? Maybe your case is 
> > > fixed because the phy does the DP/USB muxing? But the binding needs to 
> > > support the worst case which I guess would be all the muxing/switching 
> > > is done by separate board level components.
> > > 
> > 
> > Perhaps I'm misunderstanding your request, but I think this is the worst
> > case you're talking about.
> > 
> > &usb_ss_phy_in is a reference to the PHY, which does switching/muxing of
> > the SuperSpeed lanes in the connector, but the PHY provides no control
> > over the SBU signals.
> > 
> > So this sbu-mux is a separate component between the SBU-pads on the SoC
> > and the usb-c-connector, referenced through he &sbu_mux_in reference.
> > 
> > 
> > So upon e.g. a orientation switch, the typec_switch_set() call the tcpm
> > implementation will request orientation switching from port@1 and port@2
> > (no orientation-switch on port@...S pins).
> 
> 'port@2' is supposed to define the connection to what controls SBU. The 
> mux here switches the signals, but it doesn't control them.

The SBU signals are driven by the SS PHY, on behalf of the DisplayPort
controller. These signals are  turned on/off as a result of the TCPM
indicating the HPD state to the DisplayPort controller.

There's a such not really a direct representation today of the entity
that drives the SBU lines. It happens to be a sub-block in
&usb_ss_phy_in, but I don't envision that we need/want any signaling
between the TCPM and the SBU-"driver".


I see that I missed that in the example above, your suggestion on how to
model that relationship (TCPM - DP controller) was to add an additional
endpoint in port@1. So that's the current design (but neither ports nor
endpoints are significant from an implementation point of view).

> The mux should sit in the middle, but the graph terminates at the mux.
> You don't have a connection presumably because you know what the
> connection.

But do you suggest that the graph should reference the entity that
drives the SBU signals? What about the discrete mux?

> Perhaps because there is only 1 connector and controller.
> 

There is one SBU mux, one DP controller and one SS PHY per
usb-c-connector.

> Suppose you have 2 connectors and 2 controllers which drive SBU 
> signals. Also assume that the SBU signals are completely independent 
> from what's driving the altmode SS signals. How would you describe that?
> 

This is the setup we have on e.g. SC8280XP CRD; where the TCPM has two
usb-c-connectors defined, each with their graph referencing the SS PHY,
DP controller and respective sbu-mux.

There's an incomplete example of this published at [1] (where the SS phy
isn't represented yet - and hence there's no control over the SS lanes,
nor is the HS lanes connected to the dwc3 for role switching).

Perhaps I'm misunderstanding your concerns though?

[1] https://github.com/andersson/kernel/blob/wip/sc8280xp-next-20220720/arch/arm64/boot/dts/qcom/sc8280xp-crd.dts#L37

Regards,
Bjorn

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