[libcamera-devel] [PATCH 05/21] qcam: main_window: Move capture event processing to main thread

[Laurent Pinchart]

Hi Kieran,

On Mon, Mar 23, 2020 at 02:59:08PM +0000, Kieran Bingham wrote:
> On 23/03/2020 14:21, Laurent Pinchart wrote:
> > To avoid blocking the camera manager for a long amount of time, move
> > capture event processing to the main thread. Captured buffers are added
> > to a queue and an event is posted to the main window to signal
> > availability of a buffer.
> 
> Is the 'queue' necessary?
> Could the buffers not be passed through the event?

I would then have no way in processCapture() to ignore the event after
stopping the camera. I could check isCapturing_, but at least in theory,
we could stop and restart and have the pending event generated before
stopped delivered after the restart. I think a queue is cleaner as it
gives us full control.

> > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > ---
> >  src/qcam/main_window.cpp | 54 +++++++++++++++++++++++++++++++++++++++-
> >  src/qcam/main_window.h   |  8 ++++++
> >  2 files changed, 61 insertions(+), 1 deletion(-)
> > 
> > diff --git a/src/qcam/main_window.cpp b/src/qcam/main_window.cpp
> > index 354a53367d0f..afd8b9c413f5 100644
> > --- a/src/qcam/main_window.cpp
> > +++ b/src/qcam/main_window.cpp
> > @@ -19,6 +19,7 @@
> >  #include <QImage>
> >  #include <QImageWriter>
> >  #include <QInputDialog>
> > +#include <QMutexLocker>
> >  #include <QStandardPaths>
> >  #include <QTimer>
> >  #include <QToolBar>
> > @@ -31,6 +32,21 @@
> >  
> >  using namespace libcamera;
> >  
> > +class CaptureEvent : public QEvent
> > +{
> > +public:
> > +	CaptureEvent()
> > +		: QEvent(type())
> > +	{
> > +	}
> > +
> > +	static Type type()
> > +	{
> > +		static int type = QEvent::registerEventType();
> > +		return static_cast<Type>(type);
> > +	}
> > +};
> > +
> >  MainWindow::MainWindow(CameraManager *cm, const OptionsParser::Options &options)
> >  	: options_(options), cm_(cm), allocator_(nullptr), isCapturing_(false)
> >  {
> > @@ -63,6 +79,16 @@ MainWindow::~MainWindow()
> >  	}
> >  }
> >  
> > +bool MainWindow::event(QEvent *e)
> > +{
> > +	if (e->type() == CaptureEvent::type()) {
> > +		processCapture();
> > +		return true;
> > +	}
> > +
> > +	return QMainWindow::event(e);
> > +}
> > +
> >  int MainWindow::createToolbars()
> >  {
> >  	QAction *action;
> > @@ -343,6 +369,13 @@ void MainWindow::stopCapture()
> >  
> >  	config_.reset();
> >  
> > +	/*
> > +	 * A CaptureEvent may have been posted before we stopped the camera,
> > +	 * but not processed yet. Clear the queue of done buffers to avoid
> > +	 * racing with the event handler.
> > +	 */
> > +	doneQueue_.clear();
> > +
> 
> Do we not need to lock mutex_ here?

No, because at this point there can't be any call to
MainWindow::requestComplete(), as the camera is stopped. This function,
as well as MainWindow::processCapture(), run in the same thread, so they
can't run concurrently.

> >  	titleTimer_.stop();
> >  	setWindowTitle(title_);
> >  }
> > @@ -371,10 +404,29 @@ void MainWindow::requestComplete(Request *request)
> >  		return;
> >  
> >  	const std::map<Stream *, FrameBuffer *> &buffers = request->buffers();
> > +	FrameBuffer *buffer = buffers.begin()->second;
> > +
> > +	{
> > +		QMutexLocker locker(&mutex_);
> > +		doneQueue_.enqueue(buffer);
> > +	}
> > +
> > +	QCoreApplication::postEvent(this, new CaptureEvent);
> > +}
> > +
> > +void MainWindow::processCapture()
> > +{
> > +	FrameBuffer *buffer;
> 
> This scoping is to ensure the locker unlocks after the dequeue right?

Correct.

> Perhaps a new line between the FrameBuffer declaration and the start of
> the scope? It looks a bit like some sort of badly defined inline
> function otherwise :-S

OK.

> > +	{
> > +		QMutexLocker locker(&mutex_);
> > +		if (doneQueue_.isEmpty())
> > +			return;
> > +
> > +		buffer = doneQueue_.dequeue();
> > +	}
> >  
> >  	framesCaptured_++;
> >  
> > -	FrameBuffer *buffer = buffers.begin()->second;
> >  	const FrameMetadata &metadata = buffer->metadata();
> >  
> >  	double fps = metadata.timestamp - lastBufferTime_;
> > diff --git a/src/qcam/main_window.h b/src/qcam/main_window.h
> > index 720a3393e3dc..c623120d5894 100644
> > --- a/src/qcam/main_window.h
> > +++ b/src/qcam/main_window.h
> > @@ -11,7 +11,9 @@
> >  
> >  #include <QElapsedTimer>
> >  #include <QMainWindow>
> > +#include <QMutex>
> >  #include <QObject>
> > +#include <QQueue>
> >  #include <QTimer>
> >  
> >  #include <libcamera/buffer.h>
> > @@ -40,6 +42,8 @@ public:
> >  	MainWindow(CameraManager *cm, const OptionsParser::Options &options);
> >  	~MainWindow();
> >  
> > +	bool event(QEvent *e) override;
> > +
> >  private Q_SLOTS:
> >  	void quit();
> >  	void updateTitle();
> > @@ -57,6 +61,7 @@ private:
> >  	int openCamera();
> >  
> >  	void requestComplete(Request *request);
> > +	void processCapture();
> >  	int display(FrameBuffer *buffer);
> >  	void queueRequest(FrameBuffer *buffer);
> >  
> > @@ -78,6 +83,9 @@ private:
> >  	uint32_t previousFrames_;
> >  	uint32_t framesCaptured_;
> >  
> > +	QMutex mutex_;
> > +	QQueue<FrameBuffer *> doneQueue_;
> > +
> >  	QToolBar *toolbar_;
> >  	ViewFinder *viewfinder_;
> >  	std::map<int, std::pair<void *, unsigned int>> mappedBuffers_;

-- 
Regards,

Laurent Pinchart