Re: MEM2MEM devices: how to handle sequence number?

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Javier,

On Monday 02 January 2012 12:44:08 javier Martin wrote:
> Just one more question about this.
> 
> The v4l2 encoder, which is the last element in my processing chain, is
> an H.264 encoder that has to know about previous frames to encode.
> For these kind of devices it is very useful to know whether a frame
> has been lost to introduce a skip frame and improve the encoding
> process.
> 
> But, with the current approach we don't have any way to communicate
> this to the device.
> 
> One option would be that the user specified a sequence number when
> issuing VIDIOC_QBUF at the output queue so that the device could
> detect any discontinuity and introduce a skip frame. But this would
> break your rule that sequence number introduced at the output queue
> has to be ignored by the driver.

That's a use case I haven't thought about. Using sequence numbers could indeed 
help in that case. My concern is that most (if not all) applications don't set 
the sequence number before queuing a buffer, so requiring them to do so could 
introduce breakages. This could be limited to H.264 encoders (or more 
generally codecs) though.

Another option would be to queue a 0-bytes frame. That might be a bit hackish 
though.

-- 
Regards,

Laurent Pinchart