Re: [PATCH v6 12/26] drm/bridge: allow bridges to be informed about added and removed bridges

[Dmitry Baryshkov]

On Thu, Feb 06, 2025 at 07:14:27PM +0100, Luca Ceresoli wrote:
> In preparation for allowing bridges to be added to and removed from a DRM
> card without destroying the whole card, add a new DRM bridge function
> called on addition and removal of bridges.
> 
> Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
> 
> ---
> 
> Changed in v6:
>  - rebased fixing conflicts
> 
> Changed in v5:
>  - fixed kerneldoc errors
> 
> This patch was added in v4.
> ---
>  drivers/gpu/drm/drm_bridge.c | 12 ++++++++++++
>  include/drm/drm_bridge.h     | 23 +++++++++++++++++++++++
>  2 files changed, 35 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> index b0834b8644284e5f7751cec81724af849b4180e7..1955a231378050abf1071d74a145831b425c47c5 100644
> --- a/drivers/gpu/drm/drm_bridge.c
> +++ b/drivers/gpu/drm/drm_bridge.c
> @@ -207,12 +207,18 @@ LIST_HEAD(bridge_list);
>   */
>  void drm_bridge_add(struct drm_bridge *bridge)
>  {
> +	struct drm_bridge *br, *tmp;
> +
>  	mutex_init(&bridge->hpd_mutex);
>  
>  	if (bridge->ops & DRM_BRIDGE_OP_HDMI)
>  		bridge->ycbcr_420_allowed = !!(bridge->supported_formats &
>  					       BIT(HDMI_COLORSPACE_YUV420));
>  
> +	list_for_each_entry_safe(br, tmp, &bridge_list, list)
> +		if (br->funcs->bridge_event_notify)
> +			br->funcs->bridge_event_notify(br, DRM_EVENT_BRIDGE_ADD, bridge);
> +
>  	mutex_lock(&bridge_lock);
>  	list_add_tail(&bridge->list, &bridge_list);
>  	mutex_unlock(&bridge_lock);
> @@ -249,10 +255,16 @@ EXPORT_SYMBOL(devm_drm_bridge_add);
>   */
>  void drm_bridge_remove(struct drm_bridge *bridge)
>  {
> +	struct drm_bridge *br, *tmp;
> +
>  	mutex_lock(&bridge_lock);
>  	list_del_init(&bridge->list);
>  	mutex_unlock(&bridge_lock);
>  
> +	list_for_each_entry_safe(br, tmp, &bridge_list, list)
> +		if (br->funcs->bridge_event_notify)
> +			br->funcs->bridge_event_notify(br, DRM_EVENT_BRIDGE_REMOVE, bridge);
> +

I think the order should be different: notify about the added bridge
after adding to the list, notify about bridge removal before removing it
from the list.

>  	mutex_destroy(&bridge->hpd_mutex);
>  }
>  EXPORT_SYMBOL(drm_bridge_remove);
> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> index 1561347c4991dac6022319774510f9560c9283c3..ad7ba444a13e5ecf16f996de3742e4ac67dc21f1 100644
> --- a/include/drm/drm_bridge.h
> +++ b/include/drm/drm_bridge.h
> @@ -56,6 +56,11 @@ enum drm_bridge_attach_flags {
>  	DRM_BRIDGE_ATTACH_NO_CONNECTOR = BIT(0),
>  };
>  
> +enum drm_bridge_event_type {
> +	DRM_EVENT_BRIDGE_ADD,
> +	DRM_EVENT_BRIDGE_REMOVE,
> +};
> +
>  /**
>   * struct drm_bridge_funcs - drm_bridge control functions
>   */
> @@ -729,6 +734,24 @@ struct drm_bridge_funcs {
>  				      struct drm_bridge *bridge,
>  				      bool enable, int direction);
>  
> +	/**
> +	 * @bridge_event_notify:
> +	 *
> +	 * Notify that another bridge is being added or removed.
> +	 *
> +	 * This callback is optional. Bridges implementing it must always
> +	 * check whether the event refers to a bridge they actually need to
> +	 * interact with.
> +	 *
> +	 * @bridge: bridge being notified
> +	 * @event: event happened (add/remove bridge)
> +	 * @event_bridge: the bridge mentioned by the event (i.e. the
> +	 * bridge being added or removed)
> +	 */
> +	void (*bridge_event_notify)(struct drm_bridge *bridge,
> +				    enum drm_bridge_event_type event,
> +				    struct drm_bridge *event_bridge);
> +

This creates a small issue. It requires drivers to have a bridge, even
if they don't need one - e.g. the drm_encoder doesn't get notifications
about the added bridges.

I'm not sure if that can be solved in an efficient way.

>  	/**
>  	 * @debugfs_init:
>  	 *
> 
> -- 
> 2.34.1
> 

-- 
With best wishes
Dmitry