[libcamera-devel] [PATCH 3/8] cam: Use SensorTimestamp rather than buffer metadata

[Laurent Pinchart]

Hi Umang,

On Tue, Dec 07, 2021 at 07:07:46PM +0530, Umang Jain wrote:
> On 12/7/21 6:51 PM, Umang Jain wrote:
> > On 12/7/21 5:39 AM, Laurent Pinchart wrote:
> >> On Mon, Dec 06, 2021 at 11:39:43PM +0000, Kieran Bingham wrote:
> >>> The SensorTimestamp is defined to be the time of capture of the image.
> >>> While all streams should have the same timestamp, this is not always
> >>> defined or guaranteed as ISP drivers may not forward sequence numbers
> >>> and timestamps from their input buffer.
> >>
> >> That should then bo solved by the pipeline handler, which should store
> >> the correct timestamp in the buffer metadata.
> >>
> >>> Use the Request metadata to get the SensorTimestamp which must be
> >>> set by the pipeline handlers according to the data from the capture
> >>> device.
> >>>
> >>> Signed-off-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> >>> ---
> >>>   src/cam/camera_session.cpp | 7 ++++---
> >>>   1 file changed, 4 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/src/cam/camera_session.cpp b/src/cam/camera_session.cpp
> >>> index 1bf460fa3fb7..50170723c30f 100644
> >>> --- a/src/cam/camera_session.cpp
> >>> +++ b/src/cam/camera_session.cpp
> >>> @@ -359,10 +359,11 @@ void CameraSession::processRequest(Request 
> >>> *request)
> >>>       const Request::BufferMap &buffers = request->buffers();
> >>>         /*
> >>> -     * Compute the frame rate. The timestamp is arbitrarily retrieved from
> >>> -     * the first buffer, as all buffers should have matching timestamps.
> >>> +     * Compute the frame rate. The timestamp is retrieved from the
> >>> +     * SensorTimestamp property, though all streams should have the
> >>> +     * same timestamp.
> >>>        */
> >>> -    uint64_t ts = buffers.begin()->second->metadata().timestamp;
> >>> +    uint64_t ts = request->metadata().get(controls::SensorTimestamp);
> >>
> >> This seems reasonable. Why do we have timestamps in the buffer metadata
> >> ? :-)
> 
> Strong chance I have mis-understood the question, I later realized this 
> is a cam-related patch.
> 
> to me, the question translated :
> 
>      why do we have timestamps in the FrameBuffer.metadata_.timestamp ?

To clarify, I meant to ask why we have both controls::SensorTimestamp
*and* timestamps in individual buffers
(FrameBuffer::metadata_.timestamp). I suppose it's historical, the
timestamp member comes from

commit e94e52c0cb27f92c085da6e776af8b3d3172bbb2
Author: Niklas Söderlund <niklas.soderlund at ragnatech.se>
Date:   Thu Jan 24 23:34:51 2019 +0100

    libcamera: v4l2_device: Update dequeued buffer information

while controls::SensorTimestamp has been introduced in

commit 84e79bd8b5347d91309cbd759bbd988b5144fb8d
Author: Jacopo Mondi <jacopo at jmondi.org>
Date:   Thu Jul 30 15:18:50 2020 +0200

    libcamera: control_ids: Define draft controls

> So ignore the discussion (if you want) :-P
> 
> > Because there is no buffer assigned on the time-point where we want to 
> > capture the timestamp.
> >
> > The exact location of the timestamp is a \todo
> >
> >              * \todo The sensor timestamp should be better estimated 
> > by connecting
> >              * to the V4L2Device::frameStart signal.
> >
> > in all the pipeline-handlers as of now.
> >
> > We *want* to capture at frameStart signal, which emits in response to 
> > VIDIOC_DQEVENT, but we can't as no buffer assigned (yet)
> >
> > We probably need to assign a container taking care of timestamps with 
> > sequence numbers until a buffer is assigned down-the-line and then set 
> > the buffer metadata by reading seq# & timestamp from that container.

Should we just drop FrameBuffer::metadata_.timestamp in favour of
controls::SensorTimestamp ? What would be the drawbacks, if any ?

> >>>       double fps = ts - last_;
> >>>       fps = last_ != 0 && fps ? 1000000000.0 / fps : 0.0;
> >>>       last_ = ts;

-- 
Regards,

Laurent Pinchart