v4l2: v4l2_compat: Add V4L2 compatibility layer

Add libcamera V4L2 compatibility layer.

This initial implementation supports the minimal set of V4L2 operations,
which allows getting, setting, and enumerating formats, and streaming
frames from a video device. Some data about the wrapped V4L2 video
device are hardcoded.

Add a build option named 'v4l2' and adjust the build system to
selectively compile the V4L2 compatibility layer.

For now we match the V4L2 device node to a libcamera camera based on a
devnum that a pipeline handler may optionally map to a libcamera
camera.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/v4l2/v4l2_camera.h

@@ -0,0 +1,85 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright (C) 2019, Google Inc.
+ *
+ * v4l2_camera.h - V4L2 compatibility camera
+ */
+
+#ifndef __V4L2_CAMERA_H__
+#define __V4L2_CAMERA_H__
+
+#include <deque>
+#include <linux/videodev2.h>
+#include <mutex>
+
+#include <libcamera/buffer.h>
+#include <libcamera/camera.h>
+
+#include "semaphore.h"
+
+using namespace libcamera;
+
+class FrameMetadata
+{
+public:
+	FrameMetadata(Buffer *buffer);
+
+	int index() const { return index_; }
+
+	unsigned int bytesused() const { return bytesused_; }
+	uint64_t timestamp() const { return timestamp_; }
+	unsigned int sequence() const { return sequence_; }
+
+	Buffer::Status status() const { return status_; }
+
+private:
+	int index_;
+
+	unsigned int bytesused_;
+	uint64_t timestamp_;
+	unsigned int sequence_;
+
+	Buffer::Status status_;
+};
+
+class V4L2Camera : public Object
+{
+public:
+	V4L2Camera(std::shared_ptr<Camera> camera);
+	~V4L2Camera();
+
+	void open(int *ret);
+	void close();
+	void getStreamConfig(StreamConfiguration *streamConfig);
+	std::vector<FrameMetadata> completedBuffers();
+
+	void mmap(void **ret, unsigned int index);
+
+	void configure(int *ret, StreamConfiguration *streamConfigOut,
+		       const Size &size, PixelFormat pixelformat,
+		       unsigned int bufferCount);
+
+	void allocBuffers(int *ret, unsigned int count);
+	void freeBuffers();
+	void streamOn(int *ret);
+	void streamOff(int *ret);
+
+	void qbuf(int *ret, unsigned int index);
+
+	Semaphore bufferSema_;
+
+private:
+	void requestComplete(Request *request);
+
+	std::shared_ptr<Camera> camera_;
+	std::unique_ptr<CameraConfiguration> config_;
+
+	bool isRunning_;
+
+	std::mutex bufferLock_;
+
+	std::deque<std::unique_ptr<Request>> pendingRequests_;
+	std::deque<std::unique_ptr<FrameMetadata>> completedBuffers_;
+};
+
+#endif /* __V4L2_CAMERA_H__ */