[RFC] delayed_controls: avoid reading undefined control value

[Milan Zamazal]

Hi Stanislaw,

Stanislaw Gruszka <stanislaw.gruszka at linux.intel.com> writes:

> Hi all, thanks for review and comments.
>
> On Wed, Dec 11, 2024 at 09:43:29AM +0200, Laurent Pinchart wrote:
>> (CC'ing Naush for the question related to the Raspberry Pi
>> implementation)
>> 
>> On Tue, Dec 10, 2024 at 10:39:50PM +0000, Kieran Bingham wrote:
>> > Quoting Stanislaw Gruszka (2024-12-06 11:27:43)
>> > > Limit ControlRingBuffer index by number of queued entries in order do
>> > > avoid reading undefined control value with type ControlTypeNone .
>> > > 
>> > > It can happen at the begging of streaming when ControlRingBuffer is
>> > 
>> > s/begging/beginning/
>> > 
>> > > not yet filled and there are errors on CSI-2 bus resulting dropping
>> > > frames. Then we can call to DelayedControls::get() with frame number
>> > > that exceed number of saved entries and get below assertion failure:
>> > > 
>> > > ../src/libcamera/delayed_controls.cpp:227:
>> > > libcamera::ControlList libcamera::DelayedControls::get(uint32_t):
>> > > Assertion `info.type() != ControlTypeNone' failed
>> > > 
>> > > Bug: https://bugs.libcamera.org/show_bug.cgi?id=241
>> > > Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka at linux.intel.com>
>> > > ---
>> > > Notes: When debugging this I notice that the push() and get() are
>> > > done in different threads. Maybe mutex should be
>> > > used? Not sure how synchronization works for mojom signals
>> > > handlers.
>> 
>> The comment related to the SoftwareISP::inputBufferReady signal, in the
>> SimpleCameraData::init() function, possibly applies to other signals of
>> the SoftwareISP class, although I wouldn't expect it would affect the
>> setSensorControls signal. Stanislaw, could you provide more information
>> about which threads the push and get functions are called from ?
>> Everything should be called from the camera manager thread.
>
> I added debug prints with backtraces to delayedControls push and get.
> Here is the output:
> https://gist.github.com/sgruszka/cfe3832a907887512a4953a3353c8806
>
> Threads are different - have different tid, but they seems to be
> serialized. I added extra:
>
> std::this_thread::sleep_for(std::chrono::milliseconds(500)); 
>
> to push and it did not change the ordering - get() was followed by push().

Thank you for testing.  As I understand the source code comment:
  
  The inputBufferReady signal is emitted from the soft ISP thread,
  and needs to be handled in the pipeline handler thread. Signals
  implement queued delivery, but this works transparently only if
  the receiver is bound to the target thread. As the
  SimpleCameraData class doesn't inherit from the Object class, it
  is not bound to any thread, and the signal would be delivered
  synchronously. Instead, connect the signal to a lambda function
  bound explicitly to the pipe, which is bound to the pipeline
  handler thread. The function then simply forwards the call to
  conversionInputDone().

and since ispStatsReady and outputBufferReady handlers are initiated in
the same place:

  stats_->finishFrame(frame, 0);
  outputBufferReady.emit(output);

they are indeed run in the same thread and thus serialized.  But they
block the caller.  I'm not sure how much it matters here; in any case if
I redirect them to the pipeline handler thread similarly to
inputBufferReady, it works and is probably more correct.  I post a patch
tomorrow and we can discuss it there further.

>> Looking at this more deeply, I think the outputBufferReady and
>> statsReady signals may be mishandled, as they seem to be emitted from
>> the soft ISP worker thread but not treated the same way as the
>> inputBufferReady signal. We shouldn't have to deal with it when
>> connecting the signals in the user, the SoftwareISP class should handle
>> thread crossing, and emit all its signals from the camera manager
>> thread. Milan, is this something you could address ?
> <snip>
>> > I'm not sure what 'valid' control values would be ? I think just
>> > clamping to the most recent data is probably the best we can do ? And 'I
>> > think' would reflect the most up to date information we have about
>> > what's configured on the sensor - so I think it's still even the 'most
>> > correct' (and probably even 'is' correct) data we can return in the
>> > get() call in this event.
>> > 
>> > Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
>> > 
>> > >  src/libcamera/delayed_controls.cpp | 2 +-
>> > >  1 file changed, 1 insertion(+), 1 deletion(-)
>> > > 
>> > > diff --git a/src/libcamera/delayed_controls.cpp b/src/libcamera/delayed_controls.cpp
>> > > index 94d0a575..78b0b8f7 100644
>> > > --- a/src/libcamera/delayed_controls.cpp
>> > > +++ b/src/libcamera/delayed_controls.cpp
>> > > @@ -202,7 +202,7 @@ bool DelayedControls::push(const ControlList &controls)
>> > >   */
>> > >  ControlList DelayedControls::get(uint32_t sequence)
>> > >  {
>> > > -       unsigned int index = std::max<int>(0, sequence - maxDelay_);
>> > > +       unsigned int index = std::clamp<int>(sequence - maxDelay_, 0, queueCount_ - 1);
>
> There is some drawback except additional cycles pointed by Naush.
> I can see possibility of queueCount_ overlap (for 30 FPS this would be
> approximately after 2 years of using camera non-stop :-) )
> Then we can have clamp<int> call with high value smaller than low value,
> what is undefined behaviour. To prevent overlap issue this need to
> be coded differently, what will add additional complexity (since
> sequence variable can overlap as well).
>
> After further debug, the actual problem is not handling startFrame
> signal and not calling DelayedControls::applyControls(). Having
> DeleyadControls::applyControls() will prevent the assertion as
> DeleyadControls::get() will have proper control values on requested
> frame number, even when some frames are missing.
>
> One issue in current simple pipeline code is that we lack enabling
> events by setFrameStartEnabled(true).
> Second, we have to enable events for proper device. On my setup this is
> CSI-2 receiver. Using video_ results in inappropriate ioctl error.
>
> Below is a draft patch that try to address those problems.
>
> Regards
> Stanislaw
>
> From 08d4caa36702da7efe7611365df8d417fedf38ee Mon Sep 17 00:00:00 2001
> From: Stanislaw Gruszka <stanislaw.gruszka at linux.intel.com>
> Date: Thu, 12 Dec 2024 10:55:44 +0100
> Subject: [PATCH] pipeline: simple: Use subdevice for frame start events
>
> Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka at linux.intel.com>
> ---
>  src/libcamera/pipeline/simple/simple.cpp | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/src/libcamera/pipeline/simple/simple.cpp b/src/libcamera/pipeline/simple/simple.cpp
> index 8ac24e6e..64525b81 100644
> --- a/src/libcamera/pipeline/simple/simple.cpp
> +++ b/src/libcamera/pipeline/simple/simple.cpp
> @@ -277,6 +277,7 @@ public:
>  	std::list<Entity> entities_;
>  	std::unique_ptr<CameraSensor> sensor_;
>  	V4L2VideoDevice *video_;
> +	V4L2Subdevice *subdev_;
>  
>  	std::vector<Configuration> configs_;
>  	std::map<PixelFormat, std::vector<const Configuration *>> formats_;
> @@ -561,6 +562,13 @@ int SimpleCameraData::init()
>  	video_ = pipe->video(entities_.back().entity);
>  	ASSERT(video_);
>  
> +	for (auto it = entities_.rbegin(); it != entities_.rend(); ++it) {
> +		if (it->entity->type() == MediaEntity::Type::V4L2Subdevice) {
> +			subdev_ = pipe->subdev(it->entity);
> +			break;
> +		}
> +	}
> +
>  	/*
>  	 * Setup links first as some subdev drivers take active links into
>  	 * account to propagate TRY formats. Such is life :-(
> @@ -1299,7 +1307,7 @@ int SimplePipelineHandler::configure(Camera *camera, CameraConfiguration *c)
>  	data->delayedCtrls_ =
>  		std::make_unique<DelayedControls>(data->sensor_->device(),
>  						  params);
> -	data->video_->frameStart.connect(data->delayedCtrls_.get(),
> +	data->subdev_->frameStart.connect(data->delayedCtrls_.get(),
>  					 &DelayedControls::applyControls);
>  
>  	StreamConfiguration inputCfg;
> @@ -1339,6 +1347,7 @@ int SimplePipelineHandler::start(Camera *camera, [[maybe_unused]] const ControlL
>  {
>  	SimpleCameraData *data = cameraData(camera);
>  	V4L2VideoDevice *video = data->video_;
> +	V4L2Subdevice *subdev = data->subdev_;
>  	int ret;
>  
>  	const MediaPad *pad = acquirePipeline(data);
> @@ -1367,6 +1376,12 @@ int SimplePipelineHandler::start(Camera *camera, [[maybe_unused]] const ControlL
>  	}
>  
>  	video->bufferReady.connect(data, &SimpleCameraData::imageBufferReady);
> +	if (subdev) {
> +		LOG(SimplePipeline, Debug)
> +			<< "Enable frame start event on "
> +			<< subdev->entity()->name();
> +		subdev->setFrameStartEnabled(true);
> +	}
>  
>  	ret = video->streamOn();
>  	if (ret < 0) {