v4l2: v4l2_compat: Support multiple open

Previously, since we acquired the libcamera camera upon open(), it was
impossible to support multiple open, as any subsequent opens would
return error because the camera would already be acquired.

To fix this, we first initialize the camera in the first call to
V4L2CameraProxy::open(), just to heat up the stream format cache. We
then add ownership by a V4L2CameraFile of a V4L2Camera via the
V4L2CameraProxy. All vidioc ioctls prior to reqbufs > 0 (except for
s_fmt) are able to access the camera without ownership. A call to
reqbufs > 0 (and s_fmt) will take ownership, and the ownership will be
released at reqbufs = 0. While ownership is assigned, the eventfd that
should be signaled (and cleared) by V4L2Camera and V4L2CameraProxy is
set to the V4L2CameraFile that has ownership, and is cleared when the
ownership is released. In case close() is called without a
reqbufs = 0 first, the ownership is also released on close().

We also use the V4L2CameraFile to contain all the information specific
to an open instance of the file. This removes the need to keep track of
such information within V4L2CameraProxy via multiple maps from int fd to
info.

Since V4L2 does not expect reqbufs 0 to ever return error, make
V4L2CameraProxy::freeBuffers() return void.

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

src/v4l2/v4l2_camera.cpp

@@ -17,7 +17,8 @@ using namespace libcamera;
 LOG_DECLARE_CATEGORY(V4L2Compat);
 
 V4L2Camera::V4L2Camera(std::shared_ptr<Camera> camera)
-	: camera_(camera), isRunning_(false), bufferAllocator_(nullptr)
+	: camera_(camera), isRunning_(false), bufferAllocator_(nullptr),
+	  efd_(-1)
 {
 	camera_->requestCompleted.connect(this, &V4L2Camera::requestComplete);
 }
@@ -29,7 +30,6 @@ V4L2Camera::~V4L2Camera()
 
 int V4L2Camera::open()
 {
-	/* \todo Support multiple open. */
 	if (camera_->acquire() < 0) {
 		LOG(V4L2Compat, Error) << "Failed to acquire camera";
 		return -EINVAL;
@@ -59,6 +59,11 @@ void V4L2Camera::bind(int efd)
 	efd_ = efd;
 }
 
+void V4L2Camera::unbind()
+{
+	efd_ = -1;
+}
+
 void V4L2Camera::getStreamConfig(StreamConfiguration *streamConfig)
 {
 	*streamConfig = config_->at(0);

src/v4l2/v4l2_camera.h

@@ -40,6 +40,7 @@ public:
 	int open();
 	void close();
 	void bind(int efd);
+	void unbind();
 	void getStreamConfig(StreamConfiguration *streamConfig);
 	std::vector<Buffer> completedBuffers();
 

src/v4l2/v4l2_camera_proxy.cpp

@@ -35,7 +35,7 @@ LOG_DECLARE_CATEGORY(V4L2Compat);
 V4L2CameraProxy::V4L2CameraProxy(unsigned int index,
 				 std::shared_ptr<Camera> camera)
 	: refcount_(0), index_(index), bufferCount_(0), currentBuf_(0),
-	  vcam_(std::make_unique<V4L2Camera>(camera))
+	  vcam_(std::make_unique<V4L2Camera>(camera)), owner_(nullptr)
 {
 	querycap(camera);
 }
@@ -44,18 +44,29 @@ int V4L2CameraProxy::open(V4L2CameraFile *file)
 {
 	LOG(V4L2Compat, Debug) << "Servicing open fd = " << file->efd();
 
+	if (refcount_++)
+		return 0;
+
+	/*
+	 * We open the camera here, once, and keep it open until the last
+	 * V4L2CameraFile is closed. The proxy is initially not owned by any
+	 * file. The first file that calls reqbufs with count > 0 or s_fmt
+	 * will become the owner, and no other file will be allowed to call
+	 * buffer-related ioctls (except querybuf), set the format, or start or
+	 * stop the stream until ownership is released with a call to reqbufs
+	 * with count = 0.
+	 */
+
 	int ret = vcam_->open();
 	if (ret < 0) {
-		errno = -ret;
+		refcount_--;
-		return -1;
+		return ret;
 	}
 
 	vcam_->getStreamConfig(&streamConfig_);
 	setFmtFromConfig(streamConfig_);
 	sizeimage_ = calculateSizeImage(streamConfig_);
 
-	refcount_++;
-
 	return 0;
 }
 
@@ -63,6 +74,7 @@ void V4L2CameraProxy::close(V4L2CameraFile *file)
 {
 	LOG(V4L2Compat, Debug) << "Servicing close fd = " << file->efd();
 
+	release(file);
 
 	if (--refcount_ > 0)
 		return;
@@ -277,12 +289,16 @@ int V4L2CameraProxy::vidioc_s_fmt(V4L2CameraFile *file, struct v4l2_format *arg)
 	if (!validateBufferType(arg->type))
 		return -EINVAL;
 
+	int ret = acquire(file);
+	if (ret < 0)
+		return ret;
+
 	tryFormat(arg);
 
 	Size size(arg->fmt.pix.width, arg->fmt.pix.height);
-	int ret = vcam_->configure(&streamConfig_, size,
+	ret = vcam_->configure(&streamConfig_, size,
-				   v4l2ToDrm(arg->fmt.pix.pixelformat),
+			       v4l2ToDrm(arg->fmt.pix.pixelformat),
-				   bufferCount_);
+			       bufferCount_);
 	if (ret < 0)
 		return -EINVAL;
 
@@ -309,19 +325,16 @@ int V4L2CameraProxy::vidioc_try_fmt(V4L2CameraFile *file, struct v4l2_format *ar
 	return 0;
 }
 
-int V4L2CameraProxy::freeBuffers()
+void V4L2CameraProxy::freeBuffers()
 {
 	LOG(V4L2Compat, Debug) << "Freeing libcamera bufs";
 
 	int ret = vcam_->streamOff();
-	if (ret < 0) {
+	if (ret < 0)
 		LOG(V4L2Compat, Error) << "Failed to stop stream";
-		return ret;
+
-	}
 	vcam_->freeBuffers();
 	bufferCount_ = 0;
-
-	return 0;
 }
 
 int V4L2CameraProxy::vidioc_reqbufs(V4L2CameraFile *file, struct v4l2_requestbuffers *arg)
@@ -334,10 +347,17 @@ int V4L2CameraProxy::vidioc_reqbufs(V4L2CameraFile *file, struct v4l2_requestbuf
 
 	LOG(V4L2Compat, Debug) << arg->count << " buffers requested ";
 
+	if (!hasOwnership(file) && owner_)
+		return -EBUSY;
+
 	arg->capabilities = V4L2_BUF_CAP_SUPPORTS_MMAP;
 
-	if (arg->count == 0)
+	if (arg->count == 0) {
-		return freeBuffers();
+		freeBuffers();
+		release(file);
+
+		return 0;
+	}
 
 	Size size(curV4L2Format_.fmt.pix.width, curV4L2Format_.fmt.pix.height);
 	int ret = vcam_->configure(&streamConfig_, size,
@@ -386,6 +406,8 @@ int V4L2CameraProxy::vidioc_reqbufs(V4L2CameraFile *file, struct v4l2_requestbuf
 
 	LOG(V4L2Compat, Debug) << "Allocated " << arg->count << " buffers";
 
+	acquire(file);
+
 	return 0;
 }
 
@@ -409,6 +431,9 @@ int V4L2CameraProxy::vidioc_qbuf(V4L2CameraFile *file, struct v4l2_buffer *arg)
 	LOG(V4L2Compat, Debug) << "Servicing vidioc_qbuf, index = "
 			       << arg->index << " fd = " << file->efd();
 
+	if (!hasOwnership(file))
+		return -EBUSY;
+
 	if (!validateBufferType(arg->type) ||
 	    !validateMemoryType(arg->memory) ||
 	    arg->index >= bufferCount_)
@@ -428,6 +453,9 @@ int V4L2CameraProxy::vidioc_dqbuf(V4L2CameraFile *file, struct v4l2_buffer *arg)
 {
 	LOG(V4L2Compat, Debug) << "Servicing vidioc_dqbuf fd = " << file->efd();
 
+	if (!hasOwnership(file))
+		return -EBUSY;
+
 	if (!validateBufferType(arg->type) ||
 	    !validateMemoryType(arg->memory))
 		return -EINVAL;
@@ -462,6 +490,9 @@ int V4L2CameraProxy::vidioc_streamon(V4L2CameraFile *file, int *arg)
 	if (!validateBufferType(*arg))
 		return -EINVAL;
 
+	if (!hasOwnership(file))
+		return -EBUSY;
+
 	currentBuf_ = 0;
 
 	return vcam_->streamOn();
@@ -474,6 +505,9 @@ int V4L2CameraProxy::vidioc_streamoff(V4L2CameraFile *file, int *arg)
 	if (!validateBufferType(*arg))
 		return -EINVAL;
 
+	if (!hasOwnership(file) && owner_)
+		return -EBUSY;
+
 	int ret = vcam_->streamOff();
 
 	for (struct v4l2_buffer &buf : buffers_)
@@ -532,10 +566,45 @@ int V4L2CameraProxy::ioctl(V4L2CameraFile *file, unsigned long request, void *ar
 	return ret;
 }
 
-void V4L2CameraProxy::bind(int fd)
+bool V4L2CameraProxy::hasOwnership(V4L2CameraFile *file)
 {
-	efd_ = fd;
+	return owner_ == file;
-	vcam_->bind(fd);
+}
+
+/**
+ * \brief Acquire exclusive ownership of the V4L2Camera
+ *
+ * \return Zero on success or if already acquired, and negative error on
+ * failure.
+ *
+ * This is sufficient for poll()ing for buffers. Events, however, are signaled
+ * on the file level, so all fds must be signaled. poll()ing from a different
+ * fd than the one that locks the device is a corner case, and is currently not
+ * supported.
+ */
+int V4L2CameraProxy::acquire(V4L2CameraFile *file)
+{
+	if (owner_ == file)
+		return 0;
+
+	if (owner_)
+		return -EBUSY;
+
+	vcam_->bind(file->efd());
+
+	owner_ = file;
+
+	return 0;
+}
+
+void V4L2CameraProxy::release(V4L2CameraFile *file)
+{
+	if (owner_ != file)
+		return;
+
+	vcam_->unbind();
+
+	owner_ = nullptr;
 }
 
 struct PixelFormatPlaneInfo {

src/v4l2/v4l2_camera_proxy.h

@@ -33,7 +33,6 @@ public:
 	void *mmap(void *addr, size_t length, int prot, int flags, off64_t offset);
 	int munmap(void *addr, size_t length);
 
-	void bind(int fd);
 	int ioctl(V4L2CameraFile *file, unsigned long request, void *arg);
 
 private:
@@ -44,7 +43,7 @@ private:
 	void querycap(std::shared_ptr<Camera> camera);
 	void tryFormat(struct v4l2_format *arg);
 	void updateBuffers();
-	int freeBuffers();
+	void freeBuffers();
 
 	int vidioc_querycap(struct v4l2_capability *arg);
 	int vidioc_enum_fmt(V4L2CameraFile *file, struct v4l2_fmtdesc *arg);
@@ -58,6 +57,10 @@ private:
 	int vidioc_streamon(V4L2CameraFile *file, int *arg);
 	int vidioc_streamoff(V4L2CameraFile *file, int *arg);
 
+	bool hasOwnership(V4L2CameraFile *file);
+	int acquire(V4L2CameraFile *file);
+	void release(V4L2CameraFile *file);
+
 	static unsigned int bplMultiplier(uint32_t format);
 	static unsigned int imageSize(uint32_t format, unsigned int width,
 				      unsigned int height);
@@ -80,7 +83,16 @@ private:
 
 	std::unique_ptr<V4L2Camera> vcam_;
 
-	int efd_;
+	/*
+	 * This is the exclusive owner of this V4L2CameraProxy instance.
+	 * When there is no owner, anybody can call any ioctl before reqbufs.
+	 * The first file to call reqbufs with count > 0 or s_fmt will become
+	 * the owner, and when the owner calls reqbufs with count = 0 it will
+	 * release ownership. Any buffer-related ioctl (except querybuf) or
+	 * s_fmt that is called by a non-owner while there exists an owner
+	 * will return -EBUSY.
+	 */
+	V4L2CameraFile *owner_;
 };
 
 #endif /* __V4L2_CAMERA_PROXY_H__ */