Re: [PATCH] drm/connector: Cache out-of-band hotplug events

[Chaoyi Chen]

Hello Sebastian,

On 8/21/2026 10:55 PM, Sebastian Reichel wrote:
> When the USB-C state machine finished negotiating DP AltMode before the
> DRM device has been probed, the out-of-band hotplug events fired to
> early and are lost. Without replugging the display or reloading the
> USB-C driver, the DRM driver assumes nothing is plugged.
> 
> Reproducing this race condition at boot time depends on kernel
> configuration and exact USB-C equipment due to timing, but it can easily
> be reproduced by reloading the DRM driver consuming the out-of-band
> hotplug events without reloading the USB-C driver.
> 
> Signed-off-by: Sebastian Reichel <sebastian.reichel@xxxxxxxxxxxxx>
> ---
> This has been tested together with the patch series adding USB-C DP
> AltMode support for Rockchip RK3588/RK3576. This is mostly independent
> and should also affect other platforms, so I'm sending it separately.
> 
> I ran into this during development via the module reload path, but it
> seems Heiko [0] and Igor [1] managed to hit the race condition with a
> normal boot.
> 
> [0] https://lore.kernel.org/dri-devel/20767137.geO5KgaWL5@diego/
> [1] https://lore.kernel.org/dri-devel/20260811211534.8618-1-royalnet026@xxxxxxxxx/
> ---
>  drivers/gpu/drm/drm_connector.c     | 106 ++++++++++++++++++++++++++++++++++++
>  drivers/gpu/drm/drm_crtc_internal.h |   1 +
>  drivers/gpu/drm/drm_drv.c           |   1 +
>  3 files changed, 108 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index 8b4baed060f3..b69e3776b153 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -33,6 +33,7 @@
>  #include <drm/drm_sysfs.h>
>  #include <drm/drm_utils.h>
>  
> +#include <linux/cleanup.h>
>  #include <linux/export.h>
>  #include <linux/platform_device.h>
>  #include <linux/property.h>
> @@ -81,6 +82,22 @@
>  static DEFINE_MUTEX(connector_list_lock);
>  static LIST_HEAD(connector_list);
>  
> +/*
> + * List of connector fwnodes with their last out-of-band hotplug status
> + * required to forward them to a connector on registration. This ensures
> + * the connector sees a HPD event, if the event arrived before the DRM
> + * driver was probed (either due to module reload, or because of bad
> + * timing during bootup).
> + */
> +struct drm_oob_hotplug_state {
> +	struct list_head head;
> +	struct fwnode_handle *fwnode;
> +	enum drm_connector_status status;
> +};
> +
> +static DEFINE_MUTEX(oob_hotplug_list_lock);
> +static LIST_HEAD(oob_hotplug_list);
> +
>  struct drm_conn_prop_enum_list {
>  	int type;
>  	const char *name;
> @@ -130,6 +147,19 @@ void drm_connector_ida_destroy(void)
>  		ida_destroy(&drm_connector_enum_list[i].ida);
>  }
>  
> +void drm_connector_oob_hotplug_cleanup(void)
> +{
> +	struct drm_oob_hotplug_state *e, *tmp;
> +
> +	scoped_guard(mutex, &oob_hotplug_list_lock) {
> +		list_for_each_entry_safe(e, tmp, &oob_hotplug_list, head) {
> +			list_del(&e->head);
> +			fwnode_handle_put(e->fwnode);
> +			kfree(e);
> +		}
> +	}
> +}
> +
>  /**
>   * drm_get_connector_type_name - return a string for connector type
>   * @type: The connector type (DRM_MODE_CONNECTOR_*)
> @@ -829,6 +859,36 @@ void drm_connector_cleanup(struct drm_connector *connector)
>  }
>  EXPORT_SYMBOL(drm_connector_cleanup);
>  
> +/**
> + * drm_connector_replay_oob_hotplug_event - send cached OOB HPD event
> + * @connector: the connector that should receive the event
> + *
> + * Send the cached out-of-band hotplug as a new out-of-band hotplug event.
> + */
> +static void drm_connector_replay_oob_hotplug_event(struct drm_connector *connector)
> +{
> +	struct fwnode_handle *fwnode = connector->fwnode;
> +	enum drm_connector_status status;
> +	struct drm_oob_hotplug_state *e;
> +	bool found = false;
> +
> +	if (!fwnode || !connector->funcs->oob_hotplug_event)
> +		return;
> +
> +	scoped_guard(mutex, &oob_hotplug_list_lock) {
> +		list_for_each_entry(e, &oob_hotplug_list, head) {
> +			if (e->fwnode == fwnode || fwnode->secondary == e->fwnode) {
> +				status = e->status;
> +				found = true;
> +				break;
> +			}
> +		}
> +	}
> +
> +	if (found)
> +		connector->funcs->oob_hotplug_event(connector, status);
> +}
> +
>  /**
>   * drm_connector_register - register a connector
>   * @connector: the connector to register
> @@ -849,6 +909,7 @@ EXPORT_SYMBOL(drm_connector_cleanup);
>   */
>  int drm_connector_register(struct drm_connector *connector)
>  {
> +	bool replay_oob_hotplug = false;
>  	int ret = 0;
>  
>  	if (!connector->dev->registered)
> @@ -888,6 +949,7 @@ int drm_connector_register(struct drm_connector *connector)
>  	mutex_lock(&connector_list_lock);
>  	list_add_tail(&connector->global_connector_list_entry, &connector_list);
>  	mutex_unlock(&connector_list_lock);
> +	replay_oob_hotplug = true;
>  	goto unlock;
>  
>  err_late_register:
> @@ -898,6 +960,10 @@ int drm_connector_register(struct drm_connector *connector)
>  	drm_sysfs_connector_remove(connector);
>  unlock:
>  	mutex_unlock(&connector->mutex);
> +
> +	if (replay_oob_hotplug)
> +		drm_connector_replay_oob_hotplug_event(connector);
> +
>  	return ret;
>  }
>  EXPORT_SYMBOL(drm_connector_register);
> @@ -3671,6 +3737,41 @@ struct drm_connector *drm_connector_find_by_fwnode(struct fwnode_handle *fwnode)
>  	return found;
>  }
>  
> +/**
> + * drm_connector_record_oob_hotplug_status - Cache OOB hotplug status
> + * @fwnode - fwnode for the DRM connector
> + * @status - out-of-band status info
> + *
> + * Cache the latest out-of-band hotplug status for a fwnode so it can be
> + * (re)played from when the DRM device is (re)registered after this event
> + * arrived.
> + */
> +static void drm_connector_record_oob_hotplug_status(struct fwnode_handle *fwnode,
> +						    enum drm_connector_status status)
> +{
> +	struct drm_oob_hotplug_state *e;
> +
> +	if (!fwnode)
> +		return;
> +
> +	guard(mutex)(&oob_hotplug_list_lock);
> +
> +	list_for_each_entry(e, &oob_hotplug_list, head) {
> +		if (e->fwnode == fwnode) {
> +			e->status = status;
> +			return;
> +		}
> +	}
> +
> +	e = kzalloc(sizeof(*e), GFP_KERNEL);
> +	if (!e)
> +		return;
> +
> +	e->fwnode = fwnode_handle_get(fwnode);
> +	e->status = status;
> +	list_add_tail(&e->head, &oob_hotplug_list);
> +}
> +
>  /**
>   * drm_connector_oob_hotplug_event - Report out-of-band hotplug event to connector
>   * @connector_fwnode: fwnode_handle to report the event on
> @@ -3683,12 +3784,17 @@ struct drm_connector *drm_connector_find_by_fwnode(struct fwnode_handle *fwnode)
>   *
>   * This function can be used to report these out-of-band events after obtaining
>   * a drm_connector reference through calling drm_connector_find_by_fwnode().
> + *
> + * The last status for each fwnode is cached and replayed when a matching DRM
> + * connector device is (re)registered.
>   */
>  void drm_connector_oob_hotplug_event(struct fwnode_handle *connector_fwnode,
>  				     enum drm_connector_status status)
>  {
>  	struct drm_connector *connector;
>  
> +	drm_connector_record_oob_hotplug_status(connector_fwnode, status);
> +
>  	connector = drm_connector_find_by_fwnode(connector_fwnode);
>  	if (IS_ERR(connector))
>  		return;


Sorry for late reply.

Should we call drm_connector_record_oob_hotplug_status() only when 
drm_connector_find_by_fwnode() can't find the connector?

> diff --git a/drivers/gpu/drm/drm_crtc_internal.h b/drivers/gpu/drm/drm_crtc_internal.h
> index 83146ffef00c..c2714ea256a7 100644
> --- a/drivers/gpu/drm/drm_crtc_internal.h
> +++ b/drivers/gpu/drm/drm_crtc_internal.h
> @@ -188,6 +188,7 @@ int drm_mode_getencoder(struct drm_device *dev,
>  /* drm_connector.c */
>  void drm_connector_ida_init(void);
>  void drm_connector_ida_destroy(void);
> +void drm_connector_oob_hotplug_cleanup(void);
>  void drm_connector_unregister_all(struct drm_device *dev);
>  int drm_connector_register_all(struct drm_device *dev);
>  int drm_connector_set_obj_prop(struct drm_mode_object *obj,
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index 3c570f9393b9..c1aeab297ff9 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -1239,6 +1239,7 @@ static void drm_core_exit(void)
>  	drm_sysfs_destroy();
>  	WARN_ON(!xa_empty(&drm_minors_xa));
>  	drm_connector_ida_destroy();
> +	drm_connector_oob_hotplug_cleanup();
>  }
>  
>  static int __init drm_core_init(void)
> 
> ---
> base-commit: d3d1e0c4343385fb343a552e4f3b6b97d5762fc2
> change-id: 20260821-drm-connector-oob-hotplug-cache-13386679f852
> 
> Best regards,
> --  
> Sebastian Reichel <sebastian.reichel@xxxxxxxxxxxxx>

-- 
Best, 
Chaoyi