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

1 files changed, 9 insertions, 5 deletions

diff --git a/include/libcamera/internal/camera_sensor.h b/include/libcamera/internal/camera_sensor.h
index d18115bb..ccdb3a9b 100644
--- a/include/libcamera/internal/camera_sensor.h
+++ b/include/libcamera/internal/camera_sensor.h
@@ -78,11 +78,13 @@ public:
 class CameraSensorFactoryBase
 {
 public:
-	CameraSensorFactoryBase();
+	CameraSensorFactoryBase(int priority);
 	virtual ~CameraSensorFactoryBase() = default;
 
 	static std::unique_ptr<CameraSensor> create(MediaEntity *entity);
 
+	int priority() const { return priority_; }
+
 private:
 	LIBCAMERA_DISABLE_COPY_AND_MOVE(CameraSensorFactoryBase)
 
@@ -92,14 +94,16 @@ private:
 
 	virtual std::variant<std::unique_ptr<CameraSensor>, int>
 	match(MediaEntity *entity) const = 0;
+
+	int priority_;
 };
 
 template<typename _CameraSensor>
 class CameraSensorFactory final : public CameraSensorFactoryBase
 {
 public:
-	CameraSensorFactory()
-		: CameraSensorFactoryBase()
+	CameraSensorFactory(int priority)
+		: CameraSensorFactoryBase(priority)
 	{
 	}
 
@@ -111,7 +115,7 @@ private:
 	}
 };
 
-#define REGISTER_CAMERA_SENSOR(sensor) \
-static CameraSensorFactory<sensor> global_##sensor##Factory{};
+#define REGISTER_CAMERA_SENSOR(sensor, priority) \
+static CameraSensorFactory<sensor> global_##sensor##Factory{ priority };
 
 } /* namespace libcamera */