media: v4l2-subdev: Support hybrid links in v4l2_subdev_link_validate()

The v4l2_subdev_link_validate() helper function is meant to be used as a
drop-in implementation of a V4L2 subdev entity .link_validate() handler.
It supports subdev-to-subdev links only, and complains if one end of the
link is not a subdev. This forces drivers that have video output devices
connected to subdevs to implement a custom .link_validate() handler,
calling v4l2_subdev_link_validate() for the subdev-to-subdev links, and
performing manual link validation for the video-to-subdev links.

Video devices embed a media entity, and therefore also have a
.link_validate() operation. For video capture devices, the operation
should be manually implemented by drivers for validate the
subdev-to-video links. For video output devices, on the other hand, that
operation is never called, as link validation is performed in the
context of the sink entity.

As a result, we end up forcing drivers to implement a custom
.link_validate() handler for subdevs connected to video output devices,
when the video devices provide an operation that could be used for that
purpose.

To improve that situation, make v4l2_subdev_link_validate() delegate
link validation to the source's .link_validate() operation when the link
source is a video device and the link sink is a subdev. This allows
broader usage of v4l2_subdev_link_validate(), and simplifies drivers by
making video device link validation easy to implement in the video
device .link_validate(), regardless of whether the video device is an
output device or a capture device.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>

Author: Laurent Pinchart

drivers/media/v4l2-core/v4l2-subdev.c

@@ -1450,13 +1450,46 @@ int v4l2_subdev_link_validate(struct media_link *link)
 	if (WARN_ON_ONCE(!is_media_entity_v4l2_subdev(link->sink->entity)))
 		return -EINVAL;
 
-	if (!is_media_entity_v4l2_subdev(link->source->entity)) {
-		pr_warn_once("source of link '%s':%u->'%s':%u is not a V4L2 sub-device, driver bug!\n",
-			     link->source->entity->name, link->source->index,
-			     link->sink->entity->name, link->sink->index);
-		return 0;
+	/*
+	 * If the source is a video device, delegate link validation to it. This
+	 * allows usage of this helper for subdev connected to a video output
+	 * device, provided that the driver implement the video output device's
+	 * .link_validate() operation.
+	 */
+	if (is_media_entity_v4l2_video_device(link->source->entity)) {
+		struct media_entity *source = link->source->entity;
+
+		if (!source->ops || !source->ops->link_validate) {
+			/*
+			 * Many existing drivers do not implement the required
+			 * .link_validate() operation for their video devices.
+			 * Print a warning to get the drivers fixed, and return
+			 * 0 to avoid breaking userspace. This should
+			 * eventually be turned into a WARN_ON() when all
+			 * drivers will have been fixed.
+			 */
+			pr_warn_once("video device '%s' does not implement .link_validate(), driver bug!\n",
+				     source->name);
+			return 0;
+		}
+
+		/*
+		 * Avoid infinite loops in case a video device incorrectly uses
+		 * this helper function as its .link_validate() handler.
+		 */
+		if (WARN_ON(source->ops->link_validate == v4l2_subdev_link_validate))
+			return -EINVAL;
+
+		return source->ops->link_validate(link);
 	}
 
+	/*
+	 * If the source is still not a subdev, usage of this helper is a clear
+	 * driver bug.
+	 */
+	if (WARN_ON(!is_media_entity_v4l2_subdev(link->source->entity)))
+		return -EINVAL;
+
 	sink_sd = media_entity_to_v4l2_subdev(link->sink->entity);
 	source_sd = media_entity_to_v4l2_subdev(link->source->entity);
 

include/media/v4l2-subdev.h

@@ -1250,6 +1250,12 @@ int v4l2_subdev_link_validate_default(struct v4l2_subdev *sd,
  * calls v4l2_subdev_link_validate_default() to ensure that
  * width, height and the media bus pixel code are equal on both
  * source and sink of the link.
+ *
+ * The function can be used as a drop-in &media_entity_ops.link_validate
+ * implementation for v4l2_subdev instances. It supports all links between
+ * subdevs, as well as links between subdevs and video devices, provided that
+ * the video devices also implement their &media_entity_ops.link_validate
+ * operation.
  */
 int v4l2_subdev_link_validate(struct media_link *link);