[libcamera-devel] [PATCH 2/2] pipeline: simple: Use uniform-cost search algorithm to setup media pipeline

[Marian Cichy]

When the SimplePipeline is setting up its data and media pipeline in the
SimpleCameraData constructor, it merely tries to find the first valid
pad and link to the next entity, starting from the camera sensor.
Following this path may not always lead to a valid capture device and
therefore the setup will fail on some machines. This is for example an
issue when using the SimplePipeline on an i.MX-6Q with its IMX IPU.

This commit implements a different approach to setup the media-pipeline
by finding the shortest path to a valid capture device, using the
uniform-cost search algorithm. The shortest path has a good chance to be
the path from the sensor to the CSI capture device, as other paths may
involve image converters, encoders or other IPU blocks and will have
therefore more nodes.

Signed-off-by: Marian Cichy <m.cichy at pengutronix.de>
---
 src/libcamera/pipeline/simple/simple.cpp | 104 +++++++++++------------
 1 file changed, 52 insertions(+), 52 deletions(-)

diff --git a/src/libcamera/pipeline/simple/simple.cpp b/src/libcamera/pipeline/simple/simple.cpp
index 1fcf3671..82a3b402 100644
--- a/src/libcamera/pipeline/simple/simple.cpp
+++ b/src/libcamera/pipeline/simple/simple.cpp
@@ -274,62 +274,62 @@ SimpleCameraData::SimpleCameraData(SimplePipelineHandler *pipe,
 	int ret;
 
 	/*
-	 * Walk the pipeline towards the video node and store all entities
-	 * along the way.
+	 * Use the uniform-cost search algorithm to find the shortest path to a
+	 * capture device. The shortest path has a good chance to end in a
+	 * CSI capture device, as this is the most direct path for a camera.
+	 * There are other capture devices which ends in encoders or image
+	 * converters which will write the capture image to RAM. These paths
+	 * would be invalid for the purpose of SimplePipeline. These paths are
+	 * also usually longer as they would naturally contain IC- or encoder
+	 * IPU-blocks. Therefore, this algorithm will avoid these paths.
 	 */
-	MediaEntity *source = sensor;
-
-	while (source) {
-		/* If we have reached a video node, we're done. */
-		if (source->function() == MEDIA_ENT_F_IO_V4L)
-			break;
-
-		/*
-		 * Use the first output pad that has links and follow its first
-		 * link.
-		 */
-		MediaPad *sourcePad = nullptr;
-		MediaLink *sourceLink = nullptr;
-		for (MediaPad *pad : source->pads()) {
-			if ((pad->flags() & MEDIA_PAD_FL_SOURCE) &&
-			    !pad->links().empty()) {
-				sourcePad = pad;
-				sourceLink = pad->links().front();
-				break;
+	std::unordered_set<MediaEntity*> visited;
+	std::queue<MediaEntity*> queue;
+
+	/* Remember at each entity where we came from */
+	std::unordered_map<MediaEntity*, std::pair<MediaEntity*, MediaLink*>> parentList;
+	queue.push(sensor);
+
+	while (!queue.empty()) {
+		MediaEntity *entity = queue.back();
+		queue.pop();
+		/* Found the capture device */
+		if (entity->function() == MEDIA_ENT_F_IO_V4L) {
+			video_ = pipe->video(entity);
+			LOG(SimplePipeline, Debug) << "Capture Device: "
+						   << entity->name();
+			/* With the parentList, we can follow back our way from
+			 * the capture device to the sensor. */
+			while (parentList.find(entity) != parentList.end()) {
+				std::pair p = parentList[entity];
+				Entity e { p.first, p.second };
+				entities_.push_front(e);
+				entity = p.first;
+			}
+			/* Finally also remember the sensor */
+			sensor_ = std::make_unique<CameraSensor>(sensor);
+			ret = sensor_->init();
+			if (ret) {
+				sensor_.reset();
 			}
-		}
-
-		if (!sourcePad)
-			return;
-
-		entities_.push_back({ source, sourceLink });
-
-		source = sourceLink->sink()->entity();
-
-		/* Avoid infinite loops. */
-		auto iter = std::find_if(entities_.begin(), entities_.end(),
-					 [&](const Entity &entity) {
-						 return entity.entity == source;
-					 });
-		if (iter != entities_.end()) {
-			LOG(SimplePipeline, Info) << "Loop detected in pipeline";
 			return;
 		}
-	}
-
-	/*
-	 * We have a valid pipeline, get the video device and create the camera
-	 * sensor.
-	 */
-	video_ = pipe->video(source);
-	if (!video_)
-		return;
-
-	sensor_ = std::make_unique<CameraSensor>(sensor);
-	ret = sensor_->init();
-	if (ret) {
-		sensor_.reset();
-		return;
+		/* The actual uniform-cost search algorithm */
+		visited.insert(entity);
+		for (MediaPad *pad : entity->pads()) {
+			if (pad->flags() & MEDIA_PAD_FL_SOURCE) {
+				for (MediaLink *link : pad->links()) {
+					if ((link->flags() & MEDIA_LNK_FL_ENABLED) ||
+					    !(link->flags() & MEDIA_LNK_FL_IMMUTABLE)) {
+						MediaEntity *next = link->sink()->entity();
+						if (visited.find(next) == visited.end()) {
+							queue.push(next);
+							parentList.insert( { next, { entity, link } });
+						}
+					}
+				}
+			}
+		}
 	}
 }
 
-- 
2.29.2