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

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Message-ID: <YwBei45gx4oVqSql@builder.lan>
Date:   Fri, 19 Aug 2022 23:09:47 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Prashant Malani <pmalani@...omium.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 19 Aug 17:55 CDT 2022, Prashant Malani wrote:

> On Fri, Aug 19, 2022 at 3:01 PM Bjorn Andersson
> <bjorn.andersson@...aro.org> wrote:
> >
> >
> > You can't physically connect 1, 2 or 4 lanes of DP from a DP chip to
> > your usb-c-connector at the same time as you physically connect 0, 2 or
> > 4 lanes of USB from a USB PHY.
> 
> I apologize in case I'm misunderstanding, but why is that not possible?
> anx7625 allows that configuration (2 lane DP + 2 lane USB going to
> a single USB-C-connector)
> 

Right, but you can not connect 4 lanes DP from one chip at the same time
that you connect 4 lanes USB from another chip.

> Since the discussion is to support various conceivable hardware configurations
> That same anx7625 can support 1 1-lane DP (or 2 1-lane DPs), and 1
> 2-lane Type-C output.
> The cross-point switch allows for that level of configuration.
> 

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.

> > > So, how about 4 endpoints (1 for each SS lane) in the usb-c-connector port@1?
> > > That should support every conceivable configuration and bridge/PHY hardware.
> > > and also allows a way to specify any lane remapping (similar to what
> > > "data lanes" does)
> > > if that is required.
> >
> > Wouldn't that prevent you from handling orientation switching, given
> > that the graph is static?
> 
> It depends. If the end-points from the usb-c-connector
> go to the same switch, then it should allow orientation switching
> (anx7625 allows
> this). The port driver would just tell the orientation switch(es) attached to
> it that we are in NORMAL or REVERSE orientation.
> 

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 limitation to orientation switching would depend on how the
> hardware is routed.

Regards,
Bjorn