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

[Chaoyi Chen <chaoyi.chen@rock-chips.com>]

Hi Sebastian,

On 7/31/2026 10:20 PM, Sebastian Reichel wrote:
> Hi,
> 
> On Thu, Jul 30, 2026 at 09:33:44AM +0800, Chaoyi Chen wrote:
>> From: Chaoyi Chen <chaoyi.chen@rock-chips.com>
>>
>> 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@rock-chips.com>
>> Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
>> Reviewed-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
>> ---
>>
>> (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@rock-chips.com>
>> + */
>> +#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);
>

I think deduplication during registration is a good idea. Detecting it
during alloc seems a bit premature.

Thanks again for your suggestion. I'll try to add it in the next version.

> 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@rock-chips.com>");
>> +MODULE_DESCRIPTION("DRM TYPEC DP HPD BRIDGE");
>> +MODULE_LICENSE("GPL");
>> -- 
>> 2.53.0
>>

-- 
Best, 
Chaoyi