hwcontext for libavfilter?

int FFVideoStream::create_filter(const char *filter_spec)
{
    double theta = get_rotation_angle();
    if( !theta && !filter_spec )
        return 0;
    avfilter_register_all();
    if( filter_spec ) {
        const char *sp = filter_spec;
        char filter_name[BCSTRLEN], *np = filter_name;
        int i = sizeof(filter_name);
        while( --i>=0 && *sp!=0 && !strchr(" \t:=,",*sp) ) *np++ = *sp++;
        *np = 0;
        const AVFilter *filter = !filter_name[0] ? 0 : avfilter_get_by_name(filter_name);
        //AVFilterContext *ctx = filter->ctx;
        int       nb_pads = filter->nb_inputs;
        if( !filter || (nb_pads>1 && avfilter_pad_get_type(filter->inputs,0)) != AVMEDIA_TYPE_VIDEO ) {
            ff_err(AVERROR(EINVAL), "FFVideoStream::create_filter: %s\n", filter_spec);
            return -1;
        }
    }
    AVCodecParameters *avpar = st->codecpar;
    int sa_num = avpar->sample_aspect_ratio.num;
    if( !sa_num ) sa_num = 1;
    int sa_den = avpar->sample_aspect_ratio.den;
    if( !sa_den ) sa_num = 1;

    int ret = 0;  char args[BCTEXTLEN];
    AVPixelFormat pix_fmt = (AVPixelFormat)avpar->format;
    snprintf(args, sizeof(args),
        "video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
        avpar->width, avpar->height, (int)pix_fmt,
        st->time_base.num, st->time_base.den, sa_num, sa_den);
    if( ret >= 0 ) {
        filt_ctx = 0;
        ret = insert_filter("buffer", args, "in");
        buffersrc_ctx = filt_ctx;
    }
    if( ret >= 0 )
        ret = flip(theta);
    AVFilterContext *fsrc = filt_ctx;
    if(hw_device_ctx)
    fsrc->hw_device_ctx = hw_device_ctx;
    if( ret >= 0 ) {
        filt_ctx = 0;
        ret = insert_filter("buffersink", 0, "out");
        buffersink_ctx = filt_ctx;
    }
    if( ret >= 0 ) {
        ret = av_opt_set_bin(buffersink_ctx, "pix_fmts",
            (uint8_t*)&pix_fmt, sizeof(pix_fmt),
            AV_OPT_SEARCH_CHILDREN);
    }
    if( ret >= 0 )
        ret = config_filters(filter_spec, fsrc);
    else
        ff_err(ret, "FFVideoStream::create_filter");
    return ret >= 0 ? 0 : -1;
}