Re: [PATCH v3 1/5] drm/bridge: Implement generic USB Type-C DP HPD bridge

[Sebastian Reichel]

Hi,

On Fri, Jul 17, 2026 at 07:19:29PM +0200, Sebastian Reichel wrote:
> On Fri, Jul 17, 2026 at 03:23:19PM +0800, Chaoyi Chen wrote:
> > From: Chaoyi Chen <chaoyi.chen@xxxxxxxxxxxxxx>
> > 
> > The HPD function of Type-C DP is implemented through
> > drm_connector_oob_hotplug_event(). For embedded DP, it is required
> > that the DRM connector fwnode corresponds to the Type-C port fwnode.
> > 
> > To describe the relationship between the DP controller and the Type-C
> > port device, we usually using drm_bridge to build a bridge chain.
> > 
> > Now several USB-C controller drivers have already implemented the DP
> > HPD bridge function provided by aux-hpd-bridge.c, it will build a DP
> > HPD bridge on USB-C connector port device.
> > 
> > But this requires the USB-C controller driver to manually register the
> > HPD bridge. If the driver does not implement this feature, the bridge
> > will not be create.
> > 
> > So this patch implements a generic DP HPD bridge based on
> > aux-hpd-bridge.c. It will monitor Type-C bus events, and when a
> > Type-C port device containing the DP svid is registered, it will
> > create an HPD bridge for it without the need for the USB-C controller
> > driver to implement it.
> > 
> > Signed-off-by: Chaoyi Chen <chaoyi.chen@xxxxxxxxxxxxxx>
> > Reviewed-by: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx>
> > Reviewed-by: Nicolas Frattaroli <nicolas.frattaroli@xxxxxxxxxxxxx>
> > ---
> 
> Reviewed-by: Sebastian Reichel <sebastian.reichel@xxxxxxxxxxxxx>
> Tested-by: Sebastian Reichel <sebastian.reichel@xxxxxxxxxxxxx> # ArmSom Sige5
> 
> I gave this a test together with the RK3588/RK3576 USB-C DP AltMode
> patches I'm working on. As the fusb302 does a manual registration
> for the DRM bridge in its probe function, the bridge is registered
> twice:
> 
> root@sige5 # cat /sys/kernel/debug/dri/bridges
> ...
> bridge[1]: drm_aux_hpd_bridge_funcs
> 	refcount: 4
> 	type: [10] DP
> 	OF: /soc/i2c@2ac50000/typec-portc@22/connector:usb-c-connector
> 	ops: [0x4] hpd
> bridge[2]: drm_aux_hpd_bridge_funcs
> 	refcount: 2
> 	type: [10] DP
> 	OF: /soc/i2c@2ac50000/typec-portc@22/connector:usb-c-connector
> 	ops: [0x4] hpd
> ...
> 
> Apparently the USB-C DP AltMode keeps working, so this just wastes
> a few CPU cycles and some memory. So this can land and then we can
> remove the manual code from the driver as a follow-up step. I also
> gave that a try and things keep working. I won't send the fusb302
> patch for now to ensure its not applied before this patch lands.

The above test was done with a kernel having all config options
built-in (i.e. no modules). Using arm64 defconfig one ends up with

CONFIG_DRM_AUX_HPD_TYPEC_BRIDGE=m

But the resulting 'aux-hpd-typec-dp-bridge' module is not loaded
automatically resulting in missing bridge registration. Running
'modprobe aux-hpd-typec-dp-bridge' manually in the booted system
does not work either as the TypeC controller has already been
registered and no new BUS_NOTIFY_ADD_DEVICE is generated.

Greetings,

-- Sebastian

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