[libcamera-devel] [PATCH v2 1/1] pipeline: simple: Use breadth-first search 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 i.MX IPU.

This commit implements a different approach to setup the media-pipeline
by finding the shortest path to a valid capture device, using the
breadth-first 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>
---
changelog:

- Renamed uniform-cost search to breadth-first search in commit message
- Renamed uniform-cost search to breadth-first search in code comment
- Rephrased comment for more generic cases
- Fixed code-style issues
- Include <unordered_map>
- Replace std::pair<> with struct Entity
- Rename parentList to parents
- Applied various suggestions to simplify code and reduce indentation
- Make debug-message for found capture device more explicit
- Avoid double-lookup when backtracing the parents
- Drop MEDIA_LNK_FL_ENABLED/IMMUTABLE chreck
---

 src/libcamera/pipeline/simple/simple.cpp | 91 +++++++++++++-----------
 1 file changed, 50 insertions(+), 41 deletions(-)

diff --git a/src/libcamera/pipeline/simple/simple.cpp b/src/libcamera/pipeline/simple/simple.cpp
index 1fcf3671..d24d7199 100644
--- a/src/libcamera/pipeline/simple/simple.cpp
+++ b/src/libcamera/pipeline/simple/simple.cpp
@@ -15,6 +15,7 @@
 #include <set>
 #include <string>
 #include <string.h>
+#include <unordered_map>
 #include <utility>
 #include <vector>
 
@@ -274,63 +275,71 @@ SimpleCameraData::SimpleCameraData(SimplePipelineHandler *pipe,
 	int ret;
 
 	/*
-	 * Walk the pipeline towards the video node and store all entities
-	 * along the way.
+	 * Find the shortest path from the camera sensor to a video capture
+	 * device using the breadth-first search algorithm. This heuristic will
+	 * be most likely to skip paths that aren't suitable for the simple
+	 * pipeline handler on more complex devices, and is guaranteed to
+	 * produce a valid path on all devices that have a single option.
+	 *
+	 * For instance, on the IPU-based i.MX6Q, the shortest path will skip
+	 * encoders and image converters, and will end in a CSI capture device.
 	 */
-	MediaEntity *source = sensor;
+	std::unordered_set<MediaEntity *> visited;
+	std::queue<MediaEntity *> queue;
 
-	while (source) {
-		/* If we have reached a video node, we're done. */
-		if (source->function() == MEDIA_ENT_F_IO_V4L)
-			break;
+	/* Remember at each entity where we came from. */
+	std::unordered_map<MediaEntity *, Entity> parents;
+	queue.push(sensor);
 
-		/*
-		 * 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;
-			}
-		}
+	MediaEntity *entity = nullptr;
 
-		if (!sourcePad)
-			return;
+	while (!queue.empty()) {
+		entity = queue.back();
+		queue.pop();
 
-		entities_.push_back({ source, sourceLink });
+		/* Found the capture device. */
+		if (entity->function() == MEDIA_ENT_F_IO_V4L) {
+			LOG(SimplePipeline, Debug)
+				<< "Found capture device " << entity->name();
+			video_ = pipe->video(entity);
+			break;
+		}
 
-		source = sourceLink->sink()->entity();
+		/* The actual breadth-first search algorithm. */
+		visited.insert(entity);
+		for (MediaPad *pad : entity->pads()) {
+			if (!(pad->flags() & MEDIA_PAD_FL_SOURCE))
+				continue;
 
-		/* 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;
+			for (MediaLink *link : pad->links()) {
+				MediaEntity *next = link->sink()->entity();
+				if (visited.find(next) == visited.end()) {
+					queue.push(next);
+					parents.insert({ next, { entity, link } });
+				}
+			}
 		}
 	}
 
-	/*
-	 * We have a valid pipeline, get the video device and create the camera
-	 * sensor.
-	 */
-	video_ = pipe->video(source);
 	if (!video_)
 		return;
 
+	/*
+	 * With the parents, we can follow back our way from the capture device
+	 * to the sensor.
+	 */
+	for (auto it = parents.find(entity); it != parents.end();
+	     it = parents.find(entity)) {
+		const Entity &e = it->second;
+		entities_.push_front(e);
+		entity = e.entity;
+	}
+
+	/* Finally also remember the sensor. */
 	sensor_ = std::make_unique<CameraSensor>(sensor);
 	ret = sensor_->init();
-	if (ret) {
+	if (ret)
 		sensor_.reset();
-		return;
-	}
 }
 
 int SimpleCameraData::init()
-- 
2.29.2