linux-kernel - Re: [PATCH 1/2] dt-bindings: usb: Introduce GPIO-based SBU mux

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Date:   Tue, 23 Aug 2022 11:54:53 -0700
From:   Prashant Malani <pmalani@...omium.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Stephen Boyd <swboyd@...omium.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        linux-usb@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Pin-yen Lin <treapking@...omium.org>
Subject: Re: [PATCH 1/2] dt-bindings: usb: Introduce GPIO-based SBU mux

On Fri, Aug 19, 2022 at 9:09 PM Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
>
>
> We're talking about the static configuration here, where you describe
> which component are connected together. We can not dynamically switch
> the Devicetree representation around to match what the Type-C controller
> negotiates.

Yes, but we don't need to switch the device tree representation at all.
The pin routing/connections from the connector (not the cable or the partner),
to the muxing hardware (QMP phy or anx7625) remains fixed always
The port driver tells what orientation the peripheral is connected with,
and the muxing/orientation hardware routes the signals according to that.

>
> But why do you need to express the relationship between these 2
> components with > 1 link in the graph?
>
> > The graph is static, since the hardware line routing between components
> > doesn't change (e.g SSTX1 from the Type-C port is always routed to Pin
> > X1,X2 on the switch hardware), but that is what the switch is for.
> > Note that in this case, the expectation is that
> > the switch driver only registers 1 switch (it can figure out that all
> > 4 endpoints
> > go to the same Type-C port).
> >
>
> Why do we need to express this with 4 endpoints and then implement code
> to discover that the 4 endpoints points to the same remote? Why not just
> describe the logical relationship between the two components in one
> endpoint reference?

The issue I see is with the "supplier" side of that graph relationship
(i.e the DRM bridge side).
Since the bridge can be directly connected to a DP panel, the
endpoints can (technically)
represent a single DP lane. So, using 4 end-points for the
usb-c-connector port@1 gives
us something which is compatible with the bridge side endpoints too
(regardless of what
the bridge is connected to on the "output" side).
Reading the discussion, I agree 4 lanes is over-specifying, and 2
endpoints is probably
enough (especially if we can use data-lanes on the bridge side
to define the number of lanes if needed for DP panel connections).

BR,

-Prashant