Re: Way to request a time discontinuity in a V4L2 encoder device

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

Hi Alain,

On Wednesday 31 October 2012 14:21:30 Alain VOLMAT wrote:
> Hi all,
> 
> We have developed a V4L2 mem2mem driver for an H264 encoder running on an IP
> of one of our SoC and would like to have one more v4l2_buffer flag value
> for that.
> 
> In the context of this driver, we discovered that the current V4L2 encode
> API is missing the feature to mention to the IP that a time discontinuity
> has to be created. Time discontinuity must be triggered when there is a
> discontinuity in the stream to be encoded, which would for example happen
> when there is a seek in the data to be encoded. In such case, it means that
> the IP should reset its bitrate allocation algorithm.
> 
> Considering that this information should be triggered on a frame basis, the
> idea is to have it passed via the flags member of v4l2_buffer, with a new
> flag V4L2_BUF_FLAG_ENCODE_TIME_DISCONTINUITY.
> 
> The usage for this flag value are:
> * Queuing a "to be encoded" v4l2_buffer with flags &
> V4L2_BUF_FLAG_ENCODE_TIME_DISCONTINUITY informs the driver/IP that a time
> discontinuity (reset in the bitrate allocation algorithm) should be
> performed before encoding this frame. * The flags bit should be then
> propagated until the dequeue to let the application know when a buffer is
> the first one after a time discontinuity.

I wonder whether a buffer flag is the best technical solution for this. 
There's a very limited quantity of flags, and it seems to me like we'll need 
more flags like this one in the future. Using flags thus wouldn't scale.

Encoding a frame obviously requires the contents of the frame itself, but also 
requires meta data such as time discontinuity. Would it be better to add a 
meta data plane that would be used to pass all the frame meta data to the 
encoder ?

> Few words about the driver itself, it is available in the following context.
> 1. STLinux kernel (www.stlinux.com) rather than vanilla kernel since the
> board support is only available there for now 2. Multicom
> (http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/US
> ER_MANUAL/CD18182595.pdf) based V4L2 driver. Multicom is an ST layer to
> allow to send and serialize commands to our various IP.

-- 
Regards,

Laurent Pinchart