linux-kernel - Re: [PATCH v4 5/7] drm/rockchip: cdn-dp: Add multiple bridges to support PHY port selection

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Message-ID: <ede70598-c451-4352-ab3e-0e278ce33ad7@rock-chips.com>
Date: Sun, 28 Sep 2025 17:52:35 +0800
From: Chaoyi Chen <chaoyi.chen@...k-chips.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>,
 Chaoyi Chen <kernel@...kyi.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>,
 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 v4 5/7] drm/rockchip: cdn-dp: Add multiple bridges to
 support PHY port selection

On 9/23/2025 9:50 AM, Dmitry Baryshkov wrote:

[...]


>> +	/* One endpoint may correspond to one HPD bridge. */
>> +	for_each_of_graph_port_endpoint(port, dp_ep) {
>> +		/* Try to get "port" node of correspond PHY device */
>> +		struct device_node *phy_ep __free(device_node) =
>> +			of_graph_get_remote_endpoint(dp_ep);
>> +		struct device_node *phy_port __free(device_node) =
>> +			of_get_parent(phy_ep);
>> +
>> +		if (!phy_port) {
>> +			continue;
>> +		}
>> +
>> +		/*
>> +		 * A PHY port may contain two endpoints: USB connector port or CDN-DP port.
>> +		 * Try to find the node of USB connector.
> And then there can be a retimer between PHY and the USB-C connector. Or
> some signal MUX. Or DP-to-HDMI bridge. Please, don't parse DT for other
> devices. Instead you can add drm_aux_bridge to your PHY and let DRM core
> build the bridge chain following OF graph.
>
I think building a bridge chain across multiple drm_aux_hpd_bridge may be difficult. First, drm_dp_hpd_bridge_register() cannot register the bridge immediately; instead, it is deferred until drm_aux_hpd_bridge_probe(). When it is added to the bridge_list, it may not yet be attached, and attempting to attach it at that point is too late.

But, if I only use drm_aux_bridge on the USB-C connector, and use my own custom bridge on the PHY device and managing the alloc and attach bridge process myself, then things would become much easier.

-- 
Best,
Chaoyi