libcamera: camera: Lock the pipeline handler in acquire()

To allow more than one application using libcamera simultaneously there
can be no overlap between which cameras are in use by which user. As a
camera is part of a pipeline handler and there might be shared resources
between all cameras exposed by that pipeline handler it's not enough to
to only lock access to a single camera, all cameras from that pipeline
need to be tied to the same process.

Allow for this by locking the whole pipeline when one of its cameras
is acquired by the user. Other processes can still enumerate and list
all cameras in the system but can't acquire a camera from a locked
pipeline handler.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

1 files changed, 14 insertions, 2 deletions

diff --git a/src/libcamera/camera.cpp b/src/libcamera/camera.cpp
index cb45bafe..fbc66ded 100644
--- a/src/libcamera/camera.cpp
+++ b/src/libcamera/camera.cpp
@@ -470,13 +470,17 @@ void Camera::disconnect()
  * not blocking, if the device has already been acquired (by the same or another
  * process) the -EBUSY error code is returned.
  *
+ * Acquiring a camera will limit usage of any other camera(s) provided by the
+ * same pipeline handler to the same instance of libcamera. The limit is in
+ * effect until all cameras from the pipeline handler are released. Other
+ * instances of libcamera can still list and examine the cameras but will fail
+ * if they attempt to acquire() any of them.
+ *
  * Once exclusive access isn't needed anymore, the device should be released
  * with a call to the release() function.
  *
  * This function affects the state of the camera, see \ref camera_operation.
  *
- * \todo Implement exclusive access across processes.
- *
  * \return 0 on success or a negative error code otherwise
  * \retval -ENODEV The camera has been disconnected from the system
  * \retval -EBUSY The camera is not free and can't be acquired by the caller
@@ -489,6 +493,12 @@ int Camera::acquire()
 	if (!stateIs(CameraAvailable))
 		return -EBUSY;
 
+	if (!pipe_->lock()) {
+		LOG(Camera, Info)
+			<< "Pipeline handler in use by another process";
+		return -EBUSY;
+	}
+
 	state_ = CameraAcquired;
 
 	return 0;
@@ -510,6 +520,8 @@ int Camera::release()
 	if (!stateBetween(CameraAvailable, CameraConfigured))
 		return -EBUSY;
 
+	pipe_->unlock();
+
 	state_ = CameraAvailable;
 
 	return 0;