linux-kernel - Re: [PATCH v5 1/9] dt-bindings: usb: Add Type-C switch binding

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Message-ID: <CACeCKadtmGZ5iuTHdMms6ZHGn-Uv=MbcdtqmUzqCb=5WHuPj2Q@mail.gmail.com>
Date:   Thu, 30 Jun 2022 10:10:32 -0700
From:   Prashant Malani <pmalani@...omium.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Pin-yen Lin <treapking@...omium.org>,
        Rob Herring <robh@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux USB List <linux-usb@...r.kernel.org>,
        Benson Leung <bleung@...omium.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Nícolas F . R . A . Prado 
        <nfraprado@...labora.com>, Allen Chen <allen.chen@....com.tw>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>, devicetree@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hsin-Yi Wang <hsinyi@...omium.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Jonas Karlman <jonas@...boo.se>,
        José Expósito <jose.exposito89@...il.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Maxime Ripard <maxime@...no.tech>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Xin Ji <xji@...logixsemi.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Subject: Re: [PATCH v5 1/9] dt-bindings: usb: Add Type-C switch binding

(CC+ Bjorn)

On Wed, Jun 29, 2022 at 4:55 PM Stephen Boyd <swboyd@...omium.org> wrote:
>
> Quoting Prashant Malani (2022-06-29 15:55:10)
> > On Wed, Jun 29, 2022 at 2:58 PM Stephen Boyd <swboyd@...omium.org> wrote:
> > >
> > > My understanding is there are 4 DP lanes on it6505 and two lanes are
> > > connected to one usb-c-connector and the other two lanes are connected
> > > to a different usb-c-connector. The IT6505 driver will send DP out on
> > > the associated two DP lanes depending on which usb-c-connector has DP
> > > pins assigned by the typec manager.
> [...]
> >
> > We can adopt this binding, but from what I gathered in this thread, that
> > shouldn't be done, because IT6505 isn't meant to be aware of Type-C
> > connections at all.
>
> How will the driver know which usb-c-connector to route DP to without
> making the binding aware of typec connections?

I agree with you; I'm saying my interpretation of the comments of this
thread are that it's not the intended usage of the it6505 part, so the driver
shouldn't be updated to support that.

>
> HPD can be signalled out of band, or not at all (no-hpd). I suspect it's
> valid to ignore/disconnect the HPD pin here and start/stop DP when, for
> example, the HPD pin toggles within a dp-connector. HPD could be
> signaled directly to the kernel via an out of band gpio going from the
> dp-connector to the SoC. In this case HPD for each dp-connector could be
> a different gpio and the driver may be required to arbitrate between the
> two dp-connectors with some 'first to signal wins' logic or something.

Sure, it's possible. I just didn't see anything in the anx7625 datasheet
to suggest it supported 2x1-lane DP outputs.

For that matter I don't think even it6505 supports > 1 DP sink (based
on my reading of the datasheet), but I don't have too much experience
with these parts.


> > My interpretation of the current mode-switch search code [1] is that
> > a top level property of "mode-switch" is required.
>
> Yeah that's how it is right now, but does it have to stay that way?
> Could the code search the graph and look for a matching node that's
> registered with the typec framework?

I'll have to get back to you on that after reading the code a bit more.
Maybe Heikki or Bjorn have some comments about it.
The ACPI Type-C ports do require a device handle labelled "mode-switch"
which points to the switch device.