[FFmpeg-devel] [PATCHv6 4/4] libavcodec: v4l2: add support for v4l2 mem2mem codecs

[wm4]

On Mon, 28 Aug 2017 10:21:54 +0200
Jorge Ramirez <jorge.ramirez-ortiz at linaro.org> wrote:

> On 08/25/2017 05:35 PM, wm4 wrote:
> >> +int avpriv_v4l2m2m_reinit(V4L2Context* ctx)
> >> +{
> >> +    V4L2m2mContext *s = container_of(ctx, V4L2m2mContext, capture);
> >> +    int ret;
> >> +
> >> +    av_log(ctx->log_ctx, AV_LOG_DEBUG, "%s reinit context\n", ctx->name);
> >> +
> >> +    /* 1. streamoff */
> >> +    ret = avpriv_v4l2_context_set_status(ctx, VIDIOC_STREAMOFF);
> >> +    if (ret)
> >> +        av_log(ctx->log_ctx, AV_LOG_ERROR, "VIDIOC_STREAMOFF %s\n", ctx->name);
> >> +
> >> +    /* 2. unmap the buffers (v4l2 and ffmpeg) */
> >> +    avpriv_v4l2_context_release(ctx);  
> > What happens to AVFrames or AVPackets that are still referenced?
> >  
> 
> yes in response to your previous comment on the subject, I am trying to 
> address that in v7 not releasing the context while there are buffers on 
> the fly....
> This is what I came up with (if you can suggest something better - ie, a 
> way to  block until the AVBufferRefs are unref- please let me know)
> 
> void avpriv_v4l2_context_release(V4L2Context* ctx)
> {
>      int i, ret;
> 
>      if (!ctx->buffers)
>          return;
> 
>      /* wait until all buffers are no longer in use */
>      for (i = 0; i < ctx->num_buffers; i++) {
> 
>          if (ctx->buffers[i].status & (V4L2BUF_IN_DRIVER | 
> V4L2BUF_AVAILABLE))
>              continue;
> 
>          while (ctx->buffers[i].status & V4L2BUF_RET_USER)
>              usleep(100);
>      }
> 
>      ret = ctx->ops.release_buffers(ctx);
>      if (ret)
>          av_log(ctx->log_ctx, AV_LOG_WARNING, "V4L2 failed to unmap the 
> %s buffers\n", ctx->name);
>      else
>          av_log(ctx->log_ctx, AV_LOG_DEBUG, "%s buffers unmapped\n", 
> ctx->name);
> 
>      av_free(ctx->buffers);
>      ctx->buffers = NULL;
>      ctx->num_queued = 0;
> }
> 

No, this is not possible. You must make the V4L state reference
counted, and AVBufferRefs must implicitly reference this context.

AVHWFramesContext does this in a more elegant way - it's a frame pool
specifically made for hw decoders. I suspect the V4L code should be
ported to that. I hope Mark Thompson has some helpful remarks on this.