libcamera: camera_manager: Add environment variable to order pipelines match

To match the enumerated media devices, each registered pipeline handler
is used in no specific order. It is a limitation when several pipelines
can match the devices, and user has to select a specific pipeline.

For this purpose, environment variable LIBCAMERA_PIPELINES_MATCH_LIST is
created to give the option to define an ordered list of pipelines to
match on.

LIBCAMERA_PIPELINES_MATCH_LIST="<name1>[,<name2>[,<name3>...]]]"

Example:
LIBCAMERA_PIPELINES_MATCH_LIST="rkisp1,simple"

Signed-off-by: Julien Vuillaumier <julien.vuillaumier@nxp.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Documentation/environment_variables.rst

@@ -37,6 +37,14 @@ LIBCAMERA_IPA_MODULE_PATH
 
    Example value: ``${HOME}/.libcamera/lib:/opt/libcamera/vendor/lib``
 
+LIBCAMERA_PIPELINES_MATCH_LIST
+   Define an ordered list of pipeline names to be used to match the media
+   devices in the system. The pipeline handler names used to populate the
+   variable are the ones passed to the REGISTER_PIPELINE_HANDLER() macro in the
+   source code.
+
+   Example value: ``rkisp1,simple``
+
 LIBCAMERA_RPI_CONFIG_FILE
    Define a custom configuration file to use in the Raspberry Pi pipeline handler.
 

include/libcamera/internal/camera_manager.h

@@ -44,6 +44,7 @@ protected:
 private:
 	int init();
 	void createPipelineHandlers();
+	void pipelineFactoryMatch(const PipelineHandlerFactoryBase *factory);
 	void cleanup() LIBCAMERA_TSA_EXCLUDES(mutex_);
 
 	/*

src/libcamera/camera_manager.cpp

@@ -99,16 +99,37 @@ int CameraManager::Private::init()
 
 void CameraManager::Private::createPipelineHandlers()
 {
-	CameraManager *const o = LIBCAMERA_O_PTR();
-
 	/*
 	 * \todo Try to read handlers and order from configuration
-	 * file and only fallback on all handlers if there is no
+	 * file and only fallback on environment variable or all handlers, if
-	 * configuration file.
+	 * there is no configuration file.
 	 */
+	const char *pipesList =
+		utils::secure_getenv("LIBCAMERA_PIPELINES_MATCH_LIST");
+	if (pipesList) {
+		/*
+		 * When a list of preferred pipelines is defined, iterate
+		 * through the ordered list to match the enumerated devices.
+		 */
+		for (const auto &pipeName : utils::split(pipesList, ",")) {
+			const PipelineHandlerFactoryBase *factory;
+			factory = PipelineHandlerFactoryBase::getFactoryByName(pipeName);
+			if (!factory)
+				continue;
+
+			LOG(Camera, Debug)
+				<< "Found listed pipeline handler '"
+				<< pipeName << "'";
+			pipelineFactoryMatch(factory);
+		}
+
+		return;
+	}
+
 	const std::vector<PipelineHandlerFactoryBase *> &factories =
 		PipelineHandlerFactoryBase::factories();
 
+	/* Match all the registered pipeline handlers. */
 	for (const PipelineHandlerFactoryBase *factory : factories) {
 		LOG(Camera, Debug)
 			<< "Found registered pipeline handler '"
@@ -117,6 +138,15 @@ void CameraManager::Private::createPipelineHandlers()
 		 * Try each pipeline handler until it exhaust
 		 * all pipelines it can provide.
 		 */
+		pipelineFactoryMatch(factory);
+	}
+}
+
+void CameraManager::Private::pipelineFactoryMatch(const PipelineHandlerFactoryBase *factory)
+{
+	CameraManager *const o = LIBCAMERA_O_PTR();
+
+	/* Provide as many matching pipelines as possible. */
 	while (1) {
 		std::shared_ptr<PipelineHandler> pipe = factory->create(o);
 		if (!pipe->match(enumerator_.get()))
@@ -126,7 +156,6 @@ void CameraManager::Private::createPipelineHandlers()
 			<< "Pipeline handler \"" << factory->name()
 			<< "\" matched";
 	}
-	}
 }
 
 void CameraManager::Private::cleanup()