[libcamera-devel] [PATCH v3 8/8] android: Implement flush() camera operation

[Jacopo Mondi]

Hi Niklas,

On Sat, May 22, 2021 at 11:55:36AM +0200, Niklas Söderlund wrote:
> Hi Jacopo,
>
> Thanks for your work.
>
> On 2021-05-21 17:42:27 +0200, Jacopo Mondi wrote:
> > Implement the flush() camera operation in the CameraDevice class
> > and make it available to the camera framework by implementing the
> > operation wrapper in camera_ops.cpp.
> >
> > The flush() implementation stops the Camera and the worker thread and
> > waits for all in-flight requests to be returned. Stopping the Camera
> > forces all Requests already queued to be returned immediately in error
> > state. As flush() has to wait until all of them have been returned, make it
> > wait on a newly introduced condition variable which is notified by the
> > request completion handler when the queue of pending requests has been
> > exhausted.
> >
> > As flush() can race with processCaptureRequest() protect the requests
> > queueing by introducing a new CameraState::CameraFlushing state that
> > processCaptureRequest() inspects before queuing the Request to the
> > Camera. If flush() has been called while processCaptureRequest() was
> > executing, return the current Request immediately in error state.
> >
> > Protect potentially concurrent calls to close() and configureStreams()
> > by inspecting the CameraState, and force a wait for any flush() call
> > to complete before proceeding.
> >
> > Signed-off-by: Jacopo Mondi <jacopo at jmondi.org>
> > ---
> >  src/android/camera_device.cpp | 90 +++++++++++++++++++++++++++++++++--
> >  src/android/camera_device.h   |  9 +++-
> >  src/android/camera_ops.cpp    |  8 +++-
> >  3 files changed, 100 insertions(+), 7 deletions(-)
> >
> > diff --git a/src/android/camera_device.cpp b/src/android/camera_device.cpp
> > index 3fce14035718..899afaa49439 100644
> > --- a/src/android/camera_device.cpp
> > +++ b/src/android/camera_device.cpp
> > @@ -750,16 +750,65 @@ int CameraDevice::open(const hw_module_t *hardwareModule)
> >
> >  void CameraDevice::close()
> >  {
> > -	streams_.clear();
> > +	MutexLocker cameraLock(cameraMutex_);
> > +	if (state_ == CameraFlushing) {
> > +		flushed_.wait(cameraLock, [&] { return state_ != CameraStopped; });
> > +		camera_->release();
> >
> > +		return;
> > +	}
> > +
> > +	streams_.clear();
> >  	stop();
> >
> >  	camera_->release();
> >  }
> >
> > -void CameraDevice::stop()
> > +/*
> > + * Flush is similar to stop() but sets the camera state to 'flushing' and wait
> > + * until all the in-flight requests have been returned before setting the
> > + * camera state to stopped.
> > + *
> > + * Once flushing is done it unlocks concurrent calls to camera close() and
> > + * configureStreams().
> > + */
> > +void CameraDevice::flush()
> >  {
> > +	{
> > +		MutexLocker cameraLock(cameraMutex_);
> > +
> > +		if (state_ != CameraRunning)
> > +			return;
> > +
> > +		worker_.stop();
> > +		camera_->stop();
> > +		state_ = CameraFlushing;
> > +	}
> > +
> > +	/*
> > +	 * Now wait for all the in-flight requests to be completed before
> > +	 * continuing. Stopping the Camera guarantees that all in-flight
> > +	 * requests are completed in error state.
> > +	 */
> > +	{
> > +		MutexLocker requestsLock(requestsMutex_);
> > +		flushing_.wait(requestsLock, [&] { return descriptors_.empty(); });
> > +	}
>
> I'm still uneasy about releasing the cameraMutex_ for this section. In
> patch 6/8 you add it to protect the state_ variable but here it's

I'm not changing state_ without the mutex acquired, am I ?

> ignored. I see the ASSERT() added to stop() but the patter of taking the
> lock checking state_, releasing the lock and do some work, retake the
> lock and update state_ feels like a bad idea. Maybe I'm missing

How so, apart from the fact it feels a bit unusual, I concur ?

If I keep the held the mutex for the whole duration of flush no other
concurrent method can proceed until all the queued requests have not
been completed. While flush waits for the flushing_ condition to be
signaled, processCaptureRequest() can proceed and immediately return
the newly queued requests in error state by detecting state_ ==
CameraFlushing which signals that flush in is progress.
Otherwise it would have had to wait for flush to end. But then we're back
to a situation where we could serialize all calls and that's it, we
would be done with a single mutex to be held for the whole duration of
all operations.

If it only was for close() or configureStreams() we could have locked
for the whole duration of flush(), as they anyway wait for flush to
complete before proceeding (by waiting on the flushed_ condition here
below signaled).

> something and this is not a real problem, if so maybe we can capture
> that in the comment here?
>
> > +
> > +	/*
> > +	 * Set state to stopped and unlock close() or configureStreams() that
> > +	 * might be waiting for flush to be completed.
> > +	 */
> >  	MutexLocker cameraLock(cameraMutex_);
> > +	state_ = CameraStopped;
> > +	flushed_.notify_one();
> > +}
> > +
> > +/* Calls to stop() must be protected by cameraMutex_ being held by the caller. */
> > +void CameraDevice::stop()
> > +{
> > +	ASSERT(state_ != CameraFlushing);
> > +
> >  	if (state_ == CameraStopped)
> >  		return;
> >
> > @@ -1581,8 +1630,18 @@ PixelFormat CameraDevice::toPixelFormat(int format) const
> >   */
> >  int CameraDevice::configureStreams(camera3_stream_configuration_t *stream_list)
> >  {
> > -	/* Before any configuration attempt, stop the camera. */
> > -	stop();
> > +	{
> > +		/*
> > +		 * If a flush is in progress, wait for it to complete and to
> > +		 * stop the camera, otherwise before any new configuration
> > +		 * attempt we have to stop the camera explictely.
> > +		 */
> > +		MutexLocker cameraLock(cameraMutex_);
> > +		if (state_ == CameraFlushing)
> > +			flushed_.wait(cameraLock, [&] { return state_ != CameraStopped; });
> > +		else
> > +			stop();
> > +	}
> >
> >  	if (stream_list->num_streams == 0) {
> >  		LOG(HAL, Error) << "No streams in configuration";
> > @@ -1950,6 +2009,25 @@ int CameraDevice::processCaptureRequest(camera3_capture_request_t *camera3Reques
> >  	if (ret)
> >  		return ret;
> >
> > +	/*
> > +	 * Just before queuing the request, make sure flush() has not
> > +	 * been called after this function has been executed. In that
> > +	 * case, immediately return the request with errors.
> > +	 */
> > +	MutexLocker cameraLock(cameraMutex_);
> > +	if (state_ == CameraFlushing || state_ == CameraStopped) {
> > +		for (camera3_stream_buffer_t &buffer : descriptor.buffers_) {
> > +			buffer.status = CAMERA3_BUFFER_STATUS_ERROR;
> > +			buffer.release_fence = buffer.acquire_fence;
> > +		}
> > +
> > +		notifyError(descriptor.frameNumber_,
> > +			    descriptor.buffers_[0].stream,
> > +			    CAMERA3_MSG_ERROR_REQUEST);
> > +
> > +		return 0;
> > +	}
> > +
> >  	worker_.queueRequest(descriptor.request_.get());
> >
> >  	{
> > @@ -1979,6 +2057,10 @@ void CameraDevice::requestComplete(Request *request)
> >  			return;
> >  		}
> >
> > +		/* Release flush if all the pending requests have been completed. */
> > +		if (descriptors_.empty())
> > +			flushing_.notify_one();
> > +
> >  		node = descriptors_.extract(it);
> >  	}
> >  	Camera3RequestDescriptor &descriptor = node.mapped();
> > diff --git a/src/android/camera_device.h b/src/android/camera_device.h
> > index 7cf8e8370387..e1b3bf7d30f2 100644
> > --- a/src/android/camera_device.h
> > +++ b/src/android/camera_device.h
> > @@ -7,6 +7,7 @@
> >  #ifndef __ANDROID_CAMERA_DEVICE_H__
> >  #define __ANDROID_CAMERA_DEVICE_H__
> >
> > +#include <condition_variable>
> >  #include <map>
> >  #include <memory>
> >  #include <mutex>
> > @@ -42,6 +43,7 @@ public:
> >
> >  	int open(const hw_module_t *hardwareModule);
> >  	void close();
> > +	void flush();
> >
> >  	unsigned int id() const { return id_; }
> >  	camera3_device_t *camera3Device() { return &camera3Device_; }
> > @@ -92,6 +94,7 @@ private:
> >  	enum State {
> >  		CameraStopped,
> >  		CameraRunning,
> > +		CameraFlushing,
> >  	};
> >
> >  	void stop();
> > @@ -120,8 +123,9 @@ private:
> >
> >  	CameraWorker worker_;
> >
> > -	libcamera::Mutex cameraMutex_; /* Protects access to the camera state. */
> > +	libcamera::Mutex cameraMutex_; /* Protects the camera state and flushed_. */
> >  	State state_;
> > +	std::condition_variable flushed_;
> >
> >  	std::shared_ptr<libcamera::Camera> camera_;
> >  	std::unique_ptr<libcamera::CameraConfiguration> config_;
> > @@ -134,8 +138,9 @@ private:
> >  	std::map<int, libcamera::PixelFormat> formatsMap_;
> >  	std::vector<CameraStream> streams_;
> >
> > -	libcamera::Mutex requestsMutex_; /* Protects descriptors_. */
> > +	libcamera::Mutex requestsMutex_; /* Protects descriptors_ and flushing_. */
> >  	std::map<uint64_t, Camera3RequestDescriptor> descriptors_;
> > +	std::condition_variable flushing_;
> >
> >  	std::string maker_;
> >  	std::string model_;
> > diff --git a/src/android/camera_ops.cpp b/src/android/camera_ops.cpp
> > index 696e80436821..8a3cfa175ff5 100644
> > --- a/src/android/camera_ops.cpp
> > +++ b/src/android/camera_ops.cpp
> > @@ -66,8 +66,14 @@ static void hal_dev_dump([[maybe_unused]] const struct camera3_device *dev,
> >  {
> >  }
> >
> > -static int hal_dev_flush([[maybe_unused]] const struct camera3_device *dev)
> > +static int hal_dev_flush(const struct camera3_device *dev)
> >  {
> > +	if (!dev)
> > +		return -EINVAL;
> > +
> > +	CameraDevice *camera = reinterpret_cast<CameraDevice *>(dev->priv);
> > +	camera->flush();
> > +
> >  	return 0;
> >  }
> >
> > --
> > 2.31.1
> >
>
> --
> Regards,
> Niklas Söderlund