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: <42f41be2-9afb-4087-9ce9-1e711df98df8@rock-chips.com>
Date: Mon, 29 Sep 2025 11:55:44 +0800
From: Chaoyi Chen <chaoyi.chen@...k-chips.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.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 v4 5/7] drm/rockchip: cdn-dp: Add multiple bridges to
 support PHY port selection

On 9/29/2025 5:27 AM, Dmitry Baryshkov wrote:

> On Sun, Sep 28, 2025 at 05:52:35PM +0800, Chaoyi Chen wrote:
>> 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.
> Well... consider a your board, but add onnn,nb7vpq904m retimer between
> the CDP and usb-c connector (it's not an uncommon device nowadays). Or
> add fsa4480 analog audio switch. Build all the drivers as modules. You
> should not need any changes to your drivers to handle such boards and
> such kernel config.
>
> With those devices you can't handle everything inside the DP driver,
> since there are two "streams" of probe events: the DRM bridge needs the
> "next" bridge (in the direction from the SoC to the connector), but the
> USB-C events code needs "previous" mux, switch or retirmer. After some
> trial and error we have ended up with having a chain of drm_aux_bridge
> devices ending up with the drm_aux_hpd_bridge inside the Type-C port
> manager driver. This way the typec_* depetencies are resolved first,
> going from the SoC to the Type-C controller driver then the DRM bridge
> devices probe backwards, creating the chain, which is finally consumer
> by the DP driver inside the SoC.

Sorry, I kept trying to look for the "next bridge" in "drm_hpd_aux_bridge", and I didn't notice that "drm_aux_bridge" already had a similar implementation about "next bridge". Thanks again for your patience.


BTW the devm_drm_of_get_bridge(&auxdev->dev, auxdev->dev.of_node, 0, 0) in drm_aux_bridge cannot be used directly with tcphy->dev. I may need to create a device for the dp-port child node, and then use drm_aux_bridge_register(). But this is no longer a big issue :)

-- 
Best,
Chaoyi