Re: [PATCH v5 1/6] drm/bridge: Implement generic USB Type-C DP HPD bridge

[Sebastian Reichel]

Hi,

On Thu, Jul 30, 2026 at 09:33:44AM +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>
> ---
> 
> (no changes since v5)
> 
> Changes in v4:
> - Scan the entire typec_bus and attempt to register the hpd bridge,
>   so as not to miss devices that were already added during initialization.
> 
> (no changes since v3)
> 
> Changes in v2:
> - Add copyright text.
> - Remove useless goto.
> ---
>  drivers/gpu/drm/bridge/Kconfig                | 10 +++
>  drivers/gpu/drm/bridge/Makefile               |  1 +
>  .../gpu/drm/bridge/aux-hpd-typec-dp-bridge.c  | 64 +++++++++++++++++++
>  3 files changed, 75 insertions(+)
>  create mode 100644 drivers/gpu/drm/bridge/aux-hpd-typec-dp-bridge.c
> 
> diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
> index 4a57d49b4c6d..9739b2a19758 100644
> --- a/drivers/gpu/drm/bridge/Kconfig
> +++ b/drivers/gpu/drm/bridge/Kconfig
> @@ -30,6 +30,16 @@ config DRM_AUX_HPD_BRIDGE
>  	  Simple bridge that terminates the bridge chain and provides HPD
>  	  support.
>  
> +if DRM_AUX_HPD_BRIDGE
> +config DRM_AUX_HPD_TYPEC_BRIDGE
> +	tristate
> +	depends on TYPEC || !TYPEC
> +	default TYPEC
> +	help
> +	  Simple bridge that terminates the bridge chain and provides HPD
> +	  support. It build bridge on each USB-C connector device node.
> +endif
> +
>  menu "Display Interface Bridges"
>  	depends on DRM && DRM_BRIDGE
>  
> diff --git a/drivers/gpu/drm/bridge/Makefile b/drivers/gpu/drm/bridge/Makefile
> index 15cc821d85b7..d88a9e1ccc9a 100644
> --- a/drivers/gpu/drm/bridge/Makefile
> +++ b/drivers/gpu/drm/bridge/Makefile
> @@ -1,6 +1,7 @@
>  # SPDX-License-Identifier: GPL-2.0
>  obj-$(CONFIG_DRM_AUX_BRIDGE) += aux-bridge.o
>  obj-$(CONFIG_DRM_AUX_HPD_BRIDGE) += aux-hpd-bridge.o
> +obj-$(CONFIG_DRM_AUX_HPD_TYPEC_BRIDGE) += aux-hpd-typec-dp-bridge.o
>  obj-$(CONFIG_DRM_CHIPONE_ICN6211) += chipone-icn6211.o
>  obj-$(CONFIG_DRM_CHRONTEL_CH7033) += chrontel-ch7033.o
>  obj-$(CONFIG_DRM_CROS_EC_ANX7688) += cros-ec-anx7688.o
> diff --git a/drivers/gpu/drm/bridge/aux-hpd-typec-dp-bridge.c b/drivers/gpu/drm/bridge/aux-hpd-typec-dp-bridge.c
> new file mode 100644
> index 000000000000..43af3ea20f20
> --- /dev/null
> +++ b/drivers/gpu/drm/bridge/aux-hpd-typec-dp-bridge.c
> @@ -0,0 +1,64 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * Copyright (C) 2026 Rockchip Electronics Co., Ltd.
> + *
> + * Author: Chaoyi Chen <chaoyi.chen@xxxxxxxxxxxxxx>
> + */
> +#include <linux/of.h>
> +#include <linux/usb/typec_altmode.h>
> +#include <linux/usb/typec_dp.h>
> +
> +#include <drm/bridge/aux-bridge.h>
> +
> +static int drm_typec_bus_event(struct notifier_block *nb,
> +			       unsigned long action, void *data)
> +{
> +	struct device *dev = (struct device *)data;
> +	struct typec_altmode *alt = to_typec_altmode(dev);
> +
> +	if (action != BUS_NOTIFY_ADD_DEVICE)
> +		return NOTIFY_OK;
> +
> +	/*
> +	 * alt->dev.parent->parent : USB-C controller device
> +	 * alt->dev.parent         : USB-C connector device
> +	 */
> +	if (is_typec_port_altmode(&alt->dev) && alt->svid == USB_TYPEC_DP_SID)
> +		drm_dp_hpd_bridge_register(alt->dev.parent->parent,
> +					   to_of_node(alt->dev.parent->fwnode));

So there are 3 ways to end up with duplicated hpd bridges now:

1. TypeC controller driver already registered one manually
2. There is a race in drm_aux_hpd_typec_dp_bridge_module_init, if a
   device appears between registering the notifier and looping
   through all pre-existing devices
3. Reloading the aux-hpd-typec-dp-bridge module re-registers the
   bridges

It's not a huge problem as the system works with the duplicated HPD
bridges, but it's also quite ugly.

I think it can be trivially avoided by adding this function to
drivers/gpu/drm/bridge/aux-hpd-bridge.c and then making use of it
here as an additional check (code untested):

bool drm_device_has_dp_hpd_bridge(struct device *parent)
{
	struct device *hpd_bridge = device_find_child_by_name(parent, "dp_hpd_bridge");

	if (!hpd_bridge)
		return false;

	put_device(hpd_bridge);
	return true;
}
EXPORT_SYMBOL_GPL(drm_dev_has_dp_hpd_bridge);

That would solve all of the above, but leaves one problem when this
module races against manual device creation in a TypeC controller
driver. That could be avoided by doing the check in
devm_drm_dp_hpd_bridge_alloc() under a lock, but that's probably not
worth the trouble (it would require updating all users to handle a
new error code like -EEXIST) considering manual registration can be
removed from all drivers anyways.

Greetings,

-- Sebastian

> +
> +	return NOTIFY_OK;
> +}
> +
> +static struct notifier_block drm_typec_event_nb = {
> +	.notifier_call = drm_typec_bus_event,
> +};
> +
> +static int check_device_already_added(struct device *dev, void *data)
> +{
> +	drm_typec_bus_event(NULL, BUS_NOTIFY_ADD_DEVICE, dev);
> +	return 0;
> +}
> +
> +static void drm_aux_hpd_typec_dp_bridge_module_exit(void)
> +{
> +	bus_unregister_notifier(&typec_bus, &drm_typec_event_nb);
> +}
> +
> +static int __init drm_aux_hpd_typec_dp_bridge_module_init(void)
> +{
> +	bus_register_notifier(&typec_bus, &drm_typec_event_nb);
> +	/*
> +	 * Before module initialization, some devices may have already been added.
> +         * Register the HPD bridge for these devices.
> +	 */
> +	bus_for_each_dev(&typec_bus, NULL, NULL, check_device_already_added);
> +	return 0;
> +}
> +
> +module_init(drm_aux_hpd_typec_dp_bridge_module_init);
> +module_exit(drm_aux_hpd_typec_dp_bridge_module_exit);
> +
> +MODULE_AUTHOR("Chaoyi Chen <chaoyi.chen@xxxxxxxxxxxxxx>");
> +MODULE_DESCRIPTION("DRM TYPEC DP HPD BRIDGE");
> +MODULE_LICENSE("GPL");
> -- 
> 2.53.0
>

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