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From:   Prashant Malani <pmalani@chromium.org>
Date:   Fri, 19 Aug 2022 15:18:15 -0700
Message-ID: <CACeCKaeXpU+AxFNAwkutMX9LT2XLgAv1XmwJRyj7Exqxg6v8rA@mail.gmail.com>
Subject: Re: [PATCH 1/2] dt-bindings: usb: Introduce GPIO-based SBU mux
To:     Bjorn Andersson <bjorn.andersson@linaro.org>
Cc:     Stephen Boyd <swboyd@chromium.org>, Rob Herring <robh@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Heikki Krogerus <heikki.krogerus@linux.intel.com>,
        linux-usb@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org,
        Pin-yen Lin <treapking@chromium.org>
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On Fri, Aug 19, 2022 at 2:39 PM Bjorn Andersson
<bjorn.andersson@linaro.org> wrote:
>
> On Fri 19 Aug 15:49 CDT 2022, Stephen Boyd wrote:
>
> > > I like 2 endpoints to represent the usb-c-connector, but that doesn't seem
> > > to be compatible (without introducing `data-lanes`, at least) with all
> > > the various
> > > combinations on the remote side, if that remote side is a DRM bridge with DP
> > > output capability (like it6505 or anx7625).
> > > That type of DRM bridge supports 1, 2 or 4 lane DP connections.
> >
> > Why can't the remote side that's a pure DP bridge (it6505) bundle
> > however many lanes it wants into one endpoint? If it's a pure DP bridge
> > we should design the bridge binding to have up to 4 endpoints, but
> > sometimes 2 or 1 and then overlay data-lanes onto that binding so that
> > we can tell the driver how to remap the lanes if it can. If the hardware
> > can't support remapping lanes then data-lanes shouldn't be in the
> > binding.

2 endpoints sounds fine to me. The overloading of the bridge-side endpoint
to mean different things depending on what it's connected to seemed odd to
me, but if that is acceptable for the bridge binding, then great.

> The existing implementation provides the interfaces usb_role_switch,
> usb_typec_mux and usb_typec_switch. These works based on the concept
> that the USB Type-C controller will request the endpoints connected to
> the usb-c-connector about changes such as "switch to host mode", "switch
> to 2+2 USB/DP combo" and "switch orientation to reverse". We use this
> same operations to inform any endpoint at any port about these events
> and they all react accordingly.

Right, but that implementation/assumption doesn't work so well when you
have 2 Type-C ports which might route to the same bridge (2 lane from each).
The other 2 lanes from the other endpoints can go to (say) a USB HUB.

>
> Perhaps I'm misunderstanding your suggestion, but if you start
> representing each individual lane in the SuperSpeed interface I believe
> you would have to just abandon this interface and replace it with
> something like "give me USB on port@1/endpoint@0 and port@1/endpoint@1
> and give me DP on port@1/endpoint@2 and port@1/endpoint@3".

I don't think that is necessary. The switch driver can register the switches (
and it can find out which end-points map to the same usb-c-connector).

From the port driver, the port driver just needs to tell each switch
registered for it's port that "I want
DP Pin assignment C/ DP Pin assignment D / Plain USB3.x" and the
switch driver(s) can figure out what to output on its pins (since
the Type-C binding will specify ep0 = A2-A3 (TX1), ep1 = B10-B11 , etc)

orientation-switch can tell the switch if the signals need to be swapped around.

The above notwithstanding, it sounds like the 2-ep approach has more support
than 4 ep-approach, so this specific example is moot.