linux-kernel - Re: [PATCH v5 6/8] drm/rockchip: cdn-dp: Add multiple bridges to support PHY port selection

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Message-ID: <6hilafgpdbsppeeib75b5uamwf22kbu4likcp64ahb4u7zehhw@c3mtlzwx7qcp>
Date: Mon, 13 Oct 2025 05:11:58 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Chaoyi Chen <chaoyi.chen@...k-chips.com>
Cc: Chaoyi Chen <kernel@...kyi.com>, 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>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.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>, linux-usb@...r.kernel.org,
        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 v5 6/8] drm/rockchip: cdn-dp: Add multiple bridges to
 support PHY port selection

On Mon, Oct 13, 2025 at 09:26:06AM +0800, Chaoyi Chen wrote:
> On 10/12/2025 2:52 AM, Dmitry Baryshkov wrote:
> 
> > On Sat, Oct 11, 2025 at 11:32:31AM +0800, Chaoyi Chen wrote:
> > > From: Chaoyi Chen <chaoyi.chen@...k-chips.com>
> > > 
> > > 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. If both ports
> > > are plugged into DP, DP will select the first port for output.
> > > 
> > > This patch adds support for multiple bridges, enabling users to flexibly
> > > select the output port. For each PHY port, a separate encoder and bridge
> > > are registered.
> > > 
> > > The change is based on the DRM AUX HPD bridge, rather than the
> > > extcon approach. This requires the DT to correctly describe the
> > > connections between the first bridge in bridge chain and DP
> > > controller. For example, the bridge chain may be like this:
> > > 
> > > PHY aux birdge -> fsa4480 analog audio switch bridge ->
> > > onnn,nb7vpq904m USB reminder bridge -> USB-C controller AUX HPD bridge
> > > 
> > > In this case, the connection relationships among the PHY aux bridge
> > > and the DP contorller need to be described in DT.
> > > 
> > > In addition, the cdn_dp_parse_hpd_bridge_dt() will parses it and
> > > determines whether to register one or two bridges.
> > > 
> > > Since there is only one DP controller, only one of the PHY ports can
> > > output at a time. The key is how to switch between different PHYs,
> > > which is handled by cdn_dp_switch_port() and cdn_dp_enable().
> > > 
> > > There are two cases:
> > > 
> > > 1. Neither bridge is enabled. In this case, both bridges can
> > > independently read the EDID, and the PHY port may switch before
> > > reading the EDID.
> > > 
> > > 2. One bridge is already enabled. In this case, other bridges are not
> > > allowed to read the EDID. So we will try to return the cached EDID.
> > > 
> > > Since the scenario of two ports plug in at the same time is rare,
> > > I don't have a board which support two TypeC connector to test this.
> > > Therefore, I tested forced switching on a single PHY port, as well as
> > > output using a fake PHY port alongside a real PHY port.
> > > 
> > > Signed-off-by: Chaoyi Chen <chaoyi.chen@...k-chips.com>
> > > ---
> > > 
> > > +	/* One endpoint may correspond to one HPD bridge. */
> > > +	for_each_of_graph_port_endpoint(port, dp_ep) {
> > > +		struct device_node *phy_bridge_node __free(device_node) =
> > > +			of_graph_get_remote_port_parent(dp_ep);
> > > +
> > > +		bridge = of_drm_find_bridge(phy_bridge_node);
> > > +		if (!bridge) {
> > > +			ret = -EPROBE_DEFER;
> > > +			goto out;
> > > +		}
> > > +
> > > +		dp->hpd_bridge_valid = true;
> > > +		dp->hpd_bridge_list[count].bridge = bridge;
> > > +		dp->hpd_bridge_list[count].parent = dp;
> > > +		dp->hpd_bridge_list[count].id = count;
> > This looks misnamed. They are not necessarily HPD bridges. There can be
> > a random chain between your controller and the actual output / connector
> > /etc.
> 
> Yes, and more precisely, this should be `pervious_bridge_list` . Will fix in v6.

I think the typical convention is around next_bridge, not previous.


-- 
With best wishes
Dmitry