libcamera: camera_sensor: Sort factories by priority

In order to support a default implementation for camera sensors when no
better implementation matches, libcamera needs to try "specialized"
implementations first and pick the default last. Make this possible by
adding a priority value for factories. Newly registered factories are
inserted in the factories list sorted by descending priority, and the
default factory uses a negative priority to be inserted as the last
element.

This mechanism may be a bit overkill in the sense that there is no
expected use cases for priorities other than trying the default last,
but the implementation is simple and easy to understand.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>
---
Changes since combined RFC:

- Make the factory priority mandatory

2 files changed, 32 insertions, 4 deletions

diff --git a/src/libcamera/sensor/camera_sensor.cpp b/src/libcamera/sensor/camera_sensor.cpp
index e9b383a4..0ad2ab4f 100644
--- a/src/libcamera/sensor/camera_sensor.cpp
+++ b/src/libcamera/sensor/camera_sensor.cpp
@@ -349,11 +349,13 @@ CameraSensor::~CameraSensor() = default;
 
 /**
  * \brief Construct a camera sensor factory base
+ * \param[in] priority Priority order for factory selection
  *
  * Creating an instance of the factory base registers it with the global list of
  * factories, accessible through the factories() function.
  */
-CameraSensorFactoryBase::CameraSensorFactoryBase()
+CameraSensorFactoryBase::CameraSensorFactoryBase(int priority)
+	: priority_(priority)
 {
 	registerFactory(this);
 }
@@ -362,6 +364,12 @@ CameraSensorFactoryBase::CameraSensorFactoryBase()
  * \brief Create an instance of the CameraSensor corresponding to a media entity
  * \param[in] entity The media entity on the source end of the sensor
  *
+ * When multiple factories match the same \a entity, this function selects the
+ * matching factory with the highest priority as specified to the
+ * REGISTER_CAMERA_SENSOR() macro at factory registration time. If multiple
+ * matching factories have the same highest priority value, which factory gets
+ * selected is undefined and may vary between runs.
+ *
  * \return A unique pointer to a new instance of the CameraSensor subclass
  * matching the entity, or a null pointer if no such factory exists
  */
@@ -389,7 +397,16 @@ std::unique_ptr<CameraSensor> CameraSensorFactoryBase::create(MediaEntity *entit
 }
 
 /**
+ * \fn CameraSensorFactoryBase::priority()
+ * \brief Retrieve the priority value for the factory
+ * \return The priority value for the factory
+ */
+
+/**
  * \brief Retrieve the list of all camera sensor factories
+ *
+ * The factories are sorted in decreasing priority order.
+ *
  * \return The list of camera sensor factories
  */
 std::vector<CameraSensorFactoryBase *> &CameraSensorFactoryBase::factories()
@@ -412,7 +429,12 @@ void CameraSensorFactoryBase::registerFactory(CameraSensorFactoryBase *factory)
 	std::vector<CameraSensorFactoryBase *> &factories =
 		CameraSensorFactoryBase::factories();
 
-	factories.push_back(factory);
+	auto pos = std::upper_bound(factories.begin(), factories.end(), factory,
+				    [](const CameraSensorFactoryBase *value,
+				       const CameraSensorFactoryBase *elem) {
+					    return value->priority() > elem->priority();
+				    });
+	factories.insert(pos, factory);
 }
 
 /**
@@ -438,9 +460,10 @@ void CameraSensorFactoryBase::registerFactory(CameraSensorFactoryBase *factory)
  */
 
 /**
- * \def REGISTER_CAMERA_SENSOR(sensor)
+ * \def REGISTER_CAMERA_SENSOR(sensor, priority)
  * \brief Register a camera sensor type to the sensor factory
  * \param[in] sensor Class name of the CameraSensor derived class to register
+ * \param[in] priority Priority order for factory selection
  *
  * Register a CameraSensor subclass with the factory and make it available to
  * try and match sensors. The subclass needs to implement a static match
@@ -458,6 +481,11 @@ void CameraSensorFactoryBase::registerFactory(CameraSensorFactoryBase *factory)
  *   creation succeeded ;
  * - A non-zero error code if the entity matched and the creation failed ; or
  * - A zero error code if the entity didn't match.
+ *
+ * When multiple factories can support the same MediaEntity (as in the match()
+ * function of multiple factories returning true for the same entity), the \a
+ * priority argument selects which factory will be used. See
+ * CameraSensorFactoryBase::create() for more information.
  */
 
 } /* namespace libcamera */
diff --git a/src/libcamera/sensor/camera_sensor_legacy.cpp b/src/libcamera/sensor/camera_sensor_legacy.cpp
index 6affee78..dfb297bb 100644
--- a/src/libcamera/sensor/camera_sensor_legacy.cpp
+++ b/src/libcamera/sensor/camera_sensor_legacy.cpp
@@ -1010,6 +1010,6 @@ std::string CameraSensorLegacy::logPrefix() const
 	return "'" + entity_->name() + "'";
 }
 
-REGISTER_CAMERA_SENSOR(CameraSensorLegacy)
+REGISTER_CAMERA_SENSOR(CameraSensorLegacy, -100)
 
 } /* namespace libcamera */