Message-ID: <pk5wecbbpxn7v4bdwtghhdnm76fmrmglelytljwfb4cgvpu2i6@rk5turgyt5xq>
Date: Sat, 2 Aug 2025 12:55:59 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Chaoyi Chen <chaoyi.chen@...k-chips.com>
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>, Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...nel.org>,
        Heiko Stuebner <heiko@...ech.de>, Sandy Huang <hjc@...k-chips.com>,
        Andy Yan <andy.yan@...k-chips.com>,
        Yubing Zhang <yubing.zhang@...k-chips.com>,
        Frank Wang <frank.wang@...k-chips.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Amit Sunil Dhamne <amitsd@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dragan Simic <dsimic@...jaro.org>, Johan Jonker <jbx6244@...il.com>,
        Diederik de Haas <didi.debian@...ow.org>,
        Peter Robinson <pbrobinson@...il.com>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-phy@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v3 0/5] Add Type-C DP support for RK3399 EVB IND board

On Thu, Jul 31, 2025 at 10:19:49AM +0800, Chaoyi Chen wrote:
> Hi Dmitry,
> 
> On 7/31/2025 3:13 AM, Dmitry Baryshkov wrote:
> > On Tue, Jul 29, 2025 at 05:00:27PM +0800, Chaoyi Chen wrote:
> > > From: Chaoyi Chen <chaoyi.chen@...k-chips.com>
> > > 
> > > This series focuses on adding Type-C DP support for USBDP PHY and DP
> > > driver. The USBDP PHY and DP will perceive the changes in cable status
> > > based on the USB PD and Type-C state machines provided by TCPM. Before
> > > this, the USBDP PHY and DP controller of RK3399 sensed cable state
> > > changes through extcon, and devices such as the RK3399 Gru-Chromebook
> > > rely on them. This series should not break them.
> > > 
> > [....]
> > 
> > > ====
> > > 2. DP HPD event notify
> > > 
> > > The RK3399 has two USB/DP combo PHY and one CDN-DP controller. And
> > > the CDN-DP can be switched to output to one of the PHYs.
> > > 
> > > USB/DP PHY0 ---+
> > >                 | <----> CDN-DP controller
> > > USB/DP PHY1 ---+
> > Could you please clarify this, can you switch DP stream between two
> > USB-C outputs? What happens if user plugs in DP dongles in both USB-C
> > ports?
> 
> Currently, the software simply selects the first available port. So if user
> plugs in DP dongles in both USB-C ports, the DP driver will select the first
> port. This process is implemented in cdn_dp_connected_port() .
> 

I think Stephen Boyd has been looking on similar issues for Chromebooks,
which were sharing DP controller between several USB-C ports. I don't
remember what was his last status. I think there it was easier since the
bifurcation point was the EC.

I think, CDN-DP needs to register up to two encoders and up to two
connectors, having a separate drm_bridge chain for each of the DP
signals paths (in the end, you can not guarantee that both branches will
have the same simple CDN-DP -> PHY -> USB-C configuration: there can be
different retimers, etc).

Both encoders should list the same CRTC in possible_crtcs, etc. Of
course, it should not be possible to enable both of them.

This way if the user plugs in two DP dongles, it would be possible to
select, which output actually gets a signal.

> 
> 
> > 
> > > BTW, one of the important things to do is to implement extcon-like
> > > notifications. I found include/drm/bridge/aux-bridge.h , but if the
> > > aux-bridge is used, the bridge chain would look like this:
> > > 
> > > PHY0 aux-bridge ---+
> > >                     | ----> CDN-DP bridge
> > > PHY1 aux-bridge ---+
> > > 
> > > Oh, CDN-DP bridge has two previous aux-bridge!
> > > 
> > > Now, I try to use drm_connector_oob_hotplug_event() to notify HPD
> > > state between PHY and CDN-DP controller.
> > Does it actually work? The OOB HPD event will be repoted for the usb-c
> > connector's fwnode, but the DP controller isn't connected to that node
> > anyhow. If I'm not mistaken, the HPD signal will not reach DP driver in
> > this case.
> 
> Yes.  What you mentioned is the case in
> drivers/usb/typec/altmodes/displayport.c . I have also added a new OOB event
> notify in the PHY driver in Patch 3, where the expected fwnode is used in
> the PHY. So now we have two OOB HPD events, one from altmodes/displayport.c
> and the other from PHY. Only the HPD from PHY is eventually passed to the DP
> driver.

This way you will loose IRQ_HPD pulse events from the DP. They are used
by DPRX (aka DP Sink) to signal to DPTX (DP Source) that there was a
change on the DPRX side and the DPTX should reread link params and maybe
retrain the link.

> > > Patch1 add new Type-C mode switch for RK3399 USBDP phy binding.
> > > Patch2 add typec_mux and typec_switch for RK3399 USBDP PHY.
> > > Patch3 drops CDN-DP's extcon dependency when Type-C is present.
> > > Patch4 add missing dp_out port for RK3399 CDN-DP.
> > > Patch5 add Type-C DP support for RK3399 EVB IND board.
> > > 
> 
> -- 
> linux-phy mailing list
> linux-phy@...ts.infradead.org
> https://lists.infradead.org/mailman/listinfo/linux-phy

-- 
With best wishes
Dmitry

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