libcamera: camera: Use invokeMethod() for pipe_->acquire() and pipe_->release()

The uvcvideo driver needs to open / close its /dev/video# node from
pipe_->acquireDevices() / pipe_->releaseDevices().

V4L2VideoDevice::open() creates an EventNotifier and this notifier needs
to be created from the CameraManager thread.

Use invokeMethod() for pipe_->acquire() and pipe_->release() so that the
EventNotifiers are created from the CameraManager thread context.

Running pipe_->acquire() and pipe_->release() from the CameraManager
thread context serializes all calls to them. Drop PipelineHandler::lock_
this now is no longer necessary and update the "\context" part of the
documentation for acquire[Device]() and release[Device]() to match.

Note the delayed opening of /dev/video# is a special case because the
kernel uvcvideo driver powers on the USB device as soon as /dev/video#
is opened. This behavior should *not* be copied by other pipeline
handlers.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/internal/pipeline_handler.h

@@ -14,7 +14,6 @@
 #include <sys/types.h>
 #include <vector>
 
-#include <libcamera/base/mutex.h>
 #include <libcamera/base/object.h>
 
 #include <libcamera/controls.h>
@@ -99,9 +98,7 @@ private:
 	std::queue<Request *> waitingRequests_;
 
 	const char *name_;
-
-	Mutex lock_;
-	unsigned int useCount_ LIBCAMERA_TSA_GUARDED_BY(lock_);
+	unsigned int useCount_;
 
 	friend class PipelineHandlerFactoryBase;
 };