Hello,
On Thursday, December 22, 2011 3:34 PM Javier Martin wrote:
> we have a processing chain composed of three v4l2 devices:
>
> --------------------- -----------------------
> ----------------------
> | v4l2 source | | v4l2 fixer | | v4l2 encoder |
> | (capture) |---------->| (mem2mem)| ------------>| (mem2mem) |
> ------------>
> |___________| |____________| |____________|
>
>
> "v4l2 source" generates consecutive sequence numbers so that we can
> detect whether a frame has been lost or not.
> "v4l2 fixer" and "v4l2 encoder" cannot lose frames because they don't
> interact with an external sensor.
>
> How should "v4l2 fixer" and "v4l2 encoder" behave regarding frame
> sequence number? Should they just copy the sequence number from the
> input buffer to the output buffer or should they maintain their own
> count for the CAPTURE queue?
IMHO mem2mem devices, which process buffers in 1:1 way (there is always
exactly one 'capture'/destination buffer for every 'output'/source buffer)
can simply copy the sequence number from the source buffer to the
destination.
If there is no such 1:1 mapping between the buffers, drivers should maintain
their own numbers. video encoder is probably an example of such device.
A single destination ('capture') buffer with encoded video data might
contain a fraction, one or more source ('output') video buffers/frames.
> If the former option is chosen we should apply a patch like the
> following so that the sequence number of the input buffer is passed to
> the videobuf2 layer:
>
> diff --git a/drivers/media/video/videobuf2-core.c
> b/drivers/media/video/videobuf2-core.c
> index 1250662..7d8a88b 100644
> --- a/drivers/media/video/videobuf2-core.c
> +++ b/drivers/media/video/videobuf2-core.c
> @@ -1127,6 +1127,7 @@ int vb2_qbuf(struct vb2_queue *q, struct v4l2_buffer *b)
> */
> list_add_tail(&vb->queued_entry, &q->queued_list);
> vb->state = VB2_BUF_STATE_QUEUED;
> + vb->v4l2_buf.sequence = b->sequence;
>
> /*
> * If already streaming, give the buffer to driver for processing.
>
Right, such patch is definitely needed. Please resend it with 'signed-off-by'
annotation.
Best regards
--
Marek Szyprowski
Samsung Poland R&D Center
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