linux-kernel - Re: [PATCH RFC 1/3] dt-bindings: phy: qcom,sc8280xp-qmp-usb43dp-phy: Document default phy mode

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Message-ID: <k736p6h6dzy5ywepcgcr64llusqkrrsknf5h4o2e6zcpcyqpz5@7k7kmhumzcw5>
Date: Sat, 23 Aug 2025 17:02:43 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Xilin Wu <sophon@...xa.com>
Cc: Neil Armstrong <neil.armstrong@...aro.org>, Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>, linux-arm-msm@...r.kernel.org,
        linux-phy@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 1/3] dt-bindings: phy: qcom,sc8280xp-qmp-usb43dp-phy:
 Document default phy mode

On Sat, Aug 23, 2025 at 02:22:56PM +0800, Xilin Wu wrote:
> On 2025/8/22 19:02:21, Neil Armstrong wrote:
> > On 22/08/2025 12:57, Dmitry Baryshkov wrote:
> > > On Thu, Aug 21, 2025 at 03:53:26PM +0200, Neil Armstrong wrote:
> > > > The QMP USB3/DP Combo PHY hosts an USB3 phy and a DP PHY on top
> > > > of a combo glue to route either lanes to the 4 shared physical lanes.
> > > > 
> > > > The routing of the lanes can be:
> > > > - 2 DP + 2 USB3
> > > > - 4 DP
> > > > - 2 USB3
> > > > 
> > > > And the layout of the lanes can be swpped depending of an
> > > > eventual USB-C connector orientation.
> > > > 
> > > > Nevertheless those QMP Comby PHY can be statically used to
> > > > drive a DisplayPort connector, DP->HDMI bridge, USB3 A Connector...
> > > > 
> > > > But if a 4lanes DP->HDMI bridge is directly connected to the
> > > > QMP Comby PHY lanes, in the default routing 2 or the 4 lanes would
> > > > probbaly be USB3, making the DP->HDMI bridge non functional.
> > > > 
> > > > Add a property to hint in which layout mode the QMP Comby PHY
> > > > should be as startup.
> > > > 
> > > > Signed-off-by: Neil Armstrong <neil.armstrong@...aro.org>
> > > > ---
> > > >   .../bindings/phy/qcom,sc8280xp-qmp-usb43dp-phy.yaml         |
> > > > 13 ++ +++++++++++
> > > >   1 file changed, 13 insertions(+)
> > > > 
> > > > diff --git
> > > > a/Documentation/devicetree/bindings/phy/qcom,sc8280xp-qmp-
> > > > usb43dp-phy.yaml b/Documentation/devicetree/bindings/phy/
> > > > qcom,sc8280xp-qmp-usb43dp-phy.yaml
> > > > index c8bc512df08b5694c8599f475de78679a4438449..129475a1d9527733e43ded5a38aad766f9810fe7
> > > > 100644
> > > > --- a/Documentation/devicetree/bindings/phy/qcom,sc8280xp-qmp-
> > > > usb43dp-phy.yaml
> > > > +++ b/Documentation/devicetree/bindings/phy/qcom,sc8280xp-qmp-
> > > > usb43dp-phy.yaml
> > > > @@ -76,6 +76,19 @@ properties:
> > > >     mode-switch: true
> > > >     orientation-switch: true
> > > > +  qcom,combo-initial-mode:
> > > > +    description:
> > > > +      Describe the initial mode of the Combo PHY configuration.
> > > > +      The Combo PHY is a wrapper on top of a DP PHY and an USB3 PHY,
> > > > +      sharing the same SuperSpeed lanes with either DisplayPort over
> > > > +      the 4 lanes (dp), USB3 on a pair of lanes (usb3) or both
> > > > +      technologies in a 2+2 configuration (usb3+dp) as default.
> > > 
> > > SPecifying this as an initial mode means that it can be switched later.
> > > Should we generalize this and desribe it as bus-type (from
> > > video-interfaces.yaml) and allow it to be present only if there is no
> > > mode-switch property?
> > 
> > I was not sure about that, and yes we should do that instead but I'm not
> > sure about how.
> > 
> > bus-type sounds great, but the numbering doesn't match so I was thinking
> > instead
> > something like phy-type with a string or use the numbers from include/
> > dt-bindings/phy/phy.h
> > 
> > The thing is, do we want to keep the dual dp+usb3 as static ? I think
> > it's very possible
> > a board would connect the combo phy to an USB3 A connector and a 2lanes
> > DisplayPort connector.
> 
> Yes, our board (Radxa Dragon Q6A) is using such design, but with usb3 and dp
> lanes swapped. [1] I think this patch series could be extended to handle
> such configuration.
> 
> Currently I just change the default orientation to reverse in
> qmp_combo_probe. [2] It works flawlessly. But of course, it's not an
> upstreamable solution :)

I thought about using data-lanes in the QMP PHY, but then I realised
that there is no way to use that property to correctly describe all
these variants. So, I think it should be the bus-type of some kind.

> 
> FWIW, Rockchip usbdp phy binding [3] has a property called
> `rockchip,dp-lane-mux` to support such configuration.
> 
> [1] https://dl.radxa.com/q6a/hw/RADXA%20Dragon%20Q6A%20V1.20%20Schematic%2020250621.pdf
> 
> [2] https://github.com/strongtz/linux-next/commit/928cd166ce81aca7f8e051c72eccbd84ad896d98
> 
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/Documentation/devicetree/bindings/phy/phy-rockchip-usbdp.yaml#n53
> 
> > 
> > Neil
> > 
> > > 
> > > 
> > > > +    default: usb3+dp
> > > > +    enum:
> > > > +      - usb3+dp
> > > > +      - usb3
> > > > +      - dp
> > > > +
> > > >     ports:
> > > >       $ref: /schemas/graph.yaml#/properties/ports
> > > >       properties:
> > > > 
> > > > -- 
> > > > 2.34.1
> > > > 
> > > 
> > 
> 
> 
> -- 
> Best regards,
> Xilin Wu <sophon@...xa.com>

-- 
With best wishes
Dmitry