[libcamera-devel] Request and Pipeline::Stop() behavior

[Laurent Pinchart]

Hi Naush,

Replying to a bit of an old thread.

On Thu, Feb 27, 2020 at 10:00:58AM +0000, Naushir Patuck wrote:
> On Wed, 26 Feb 2020 at 15:13, Naushir Patuck <naush at raspberrypi.com> wrote:
> > On Wed, 26 Feb 2020 at 14:24, Kieran Bingham wrote:
> 
>  <snip>
> 
> > Yes, but I thought that mechanism was only to mark the buffers
> > FrameMetadata::FrameCancelled, and causes Request::complete() to set
> > the appropriate stats
> >
> > void Request::complete()
> > {
> >    ASSERT(!hasPendingBuffers());
> >    status_ = cancelled_ ? RequestCancelled : RequestComplete;
> > }
> >
> > > And once all buffers have completed (with their status as cancelled) the
> > > Request will (I think) automatically be cancelled too.
> >
> > I did not see where this might be happening.  From my brief
> > investigation, CameraData::queuedRequests_ maintains an internal list
> > of pending requests.  The only place this list gets cleared is in
> > PipelineHandler::completeRequest.  So I was under the impression that
> > my pipeline handler should also maintain a list of Requests queued,
> > and call PipelineHandler::completeReques() on all those elements
> > during a Stop().  By the way, I noticed the UVC pipeline handler does
> > not do this, so I could be completely wrong about this!
> 
> So after looking at the  UVC pipeline handler, I think I understand
> better.  In V4L2VideoDevice::streamOff(), we call
> bufferReady.emit(buffer) - the UVC pipeline handler does an explicit
> completeBuffer and completeRequest call and the queue are emptied and
> all is good.
> 
> The reason I am seeing these problems is, unlike the UVC pipeline
> handler, when I get a request in (via
> PipelineHandlerRPi::queueRequestDevice()), I do not queue the stream
> buffer immediately.  Rather, I was holding onto the request (and any
> subsequent requests) until I can action them appropriately one-by-one.
> The reason for this was to synchronise the Request (and associated
> buffers) with the Controls applied for the Request, and Metadata
> returned back for the Request.  Since my Request buffers were not
> queued, I never got the bufferReady.emit(buffer) signal for them.
> 
> I suppose there is no harm in queueing my buffers as soon as the
> Request arrives and still maintaining my existing Request list to sync
> the Controls and Metadata.

If there is no harm in doing this that's a totally fine solution.
Otherwise, it's also fully valid to manually complete all the requests
you have stored in your internal queue when PipelineHandler::stop() is
called.

-- 
Regards,

Laurent Pinchart