ffmpeg ： Could not find ref with POC 出现的流程

decode ： 函数指针

/**
 * Decode picture or subtitle data.
 *
 * @param      avctx          codec context
 * @param      outdata        codec type dependent output struct
 * @param[out] got_frame_ptr  decoder sets to 0 or 1 to indicate that a
 *                            non-empty frame or subtitle was returned in
 *                            outdata.
 * @param[in]  avpkt          AVPacket containing the data to be decoded
 * @return amount of bytes read from the packet on success, negative error
 *         code on failure
 */
int (*decode)(struct AVCodecContext *avctx, void *outdata,
              int *got_frame_ptr, struct AVPacket *avpkt);

ff_hevc_decoder ： 视频解码器 : 注意里面的 decode变量

AVCodec ff_hevc_decoder = {
    .name                  = "hevc",
    .long_name             = NULL_IF_CONFIG_SMALL("HEVC (High Efficiency Video Coding)"),
    .type                  = AVMEDIA_TYPE_VIDEO,
    .id                    = AV_CODEC_ID_HEVC,
    .priv_data_size        = sizeof(HEVCContext),
    .priv_class            = &hevc_decoder_class,
    .init                  = hevc_decode_init,
    .close                 = hevc_decode_free,
    .decode                = hevc_decode_frame,
    .flush                 = hevc_decode_flush,
    .update_thread_context = ONLY_IF_THREADS_ENABLED(hevc_update_thread_context),
    .capabilities          = AV_CODEC_CAP_DR1 | AV_CODEC_CAP_DELAY |
                             AV_CODEC_CAP_SLICE_THREADS | AV_CODEC_CAP_FRAME_THREADS,
    .caps_internal         = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_EXPORTS_CROPPING |
                             FF_CODEC_CAP_ALLOCATE_PROGRESS | FF_CODEC_CAP_INIT_CLEANUP,
    .profiles              = NULL_IF_CONFIG_SMALL(ff_hevc_profiles),
    .hw_configs            = (const AVCodecHWConfigInternal *const []) {
#if CONFIG_HEVC_DXVA2_HWACCEL
                               HWACCEL_DXVA2(hevc),
#endif
#if CONFIG_HEVC_D3D11VA_HWACCEL
                               HWACCEL_D3D11VA(hevc),
#endif
#if CONFIG_HEVC_D3D11VA2_HWACCEL
                               HWACCEL_D3D11VA2(hevc),
#endif
#if CONFIG_HEVC_NVDEC_HWACCEL
                               HWACCEL_NVDEC(hevc),
#endif
#if CONFIG_HEVC_VAAPI_HWACCEL
                               HWACCEL_VAAPI(hevc),
#endif
#if CONFIG_HEVC_VDPAU_HWACCEL
                               HWACCEL_VDPAU(hevc),
#endif
#if CONFIG_HEVC_VIDEOTOOLBOX_HWACCEL
                               HWACCEL_VIDEOTOOLBOX(hevc),
#endif
                               NULL
                           },
};

hevc_decode_frame

static int hevc_decode_frame(AVCodecContext *avctx, void *data, int *got_output,
                             AVPacket *avpkt)
{
    int ret;
    buffer_size_t new_extradata_size;
    uint8_t *new_extradata;
    HEVCContext *s = avctx->priv_data;

    if (!avpkt->size) {
        ret = ff_hevc_output_frame(s, data, 1);
        if (ret < 0)
            return ret;

        *got_output = ret;
        return 0;
    }

    new_extradata = av_packet_get_side_data(avpkt, AV_PKT_DATA_NEW_EXTRADATA,
                                            &new_extradata_size);
    if (new_extradata && new_extradata_size > 0) {
        ret = hevc_decode_extradata(s, new_extradata, new_extradata_size, 0);
        if (ret < 0)
            return ret;
    }

    s->ref = NULL;
    //  开始调用 decode_nal_units ----------------------
    //  avpkt->data 是 AVPacket  ----------------------
    ret    = decode_nal_units(s, avpkt->data, avpkt->size);
    if (ret < 0)
        return ret;

    if (avctx->hwaccel) {
        if (s->ref && (ret = avctx->hwaccel->end_frame(avctx)) < 0) {
            av_log(avctx, AV_LOG_ERROR,
                   "hardware accelerator failed to decode picture\n");
            ff_hevc_unref_frame(s, s->ref, ~0);
            return ret;
        }
    } else {
        /* verify the SEI checksum */
        if (avctx->err_recognition & AV_EF_CRCCHECK && s->ref && s->is_decoded &&
            s->sei.picture_hash.is_md5) {
            ret = verify_md5(s, s->ref->frame);
            if (ret < 0 && avctx->err_recognition & AV_EF_EXPLODE) {
                ff_hevc_unref_frame(s, s->ref, ~0);
                return ret;
            }
        }
    }
    s->sei.picture_hash.is_md5 = 0;

    if (s->is_decoded) {
        av_log(avctx, AV_LOG_DEBUG, "Decoded frame with POC %d.\n", s->poc);
        s->is_decoded = 0;
    }

    if (s->output_frame->buf[0]) {
        av_frame_move_ref(data, s->output_frame);
        *got_output = 1;
    }

    return avpkt->size;
}

decode_nal_units

static int decode_nal_units(HEVCContext *s, const uint8_t *buf, int length)
{
    int i, ret = 0;
    int eos_at_start = 1;

    s->ref = NULL;
    s->last_eos = s->eos;
    s->eos = 0;
    s->overlap = 0;

    /* split the input packet into NAL units, so we know the upper bound on the
     * number of slices in the frame */
    ret = ff_h2645_packet_split(&s->pkt, buf, length, s->avctx, s->is_nalff,
                                s->nal_length_size, s->avctx->codec_id, 1, 0);
    if (ret < 0) {
        av_log(s->avctx, AV_LOG_ERROR,
               "Error splitting the input into NAL units.\n");
        return ret;
    }

    for (i = 0; i < s->pkt.nb_nals; i++) {
        if (s->pkt.nals[i].type == HEVC_NAL_EOB_NUT ||
            s->pkt.nals[i].type == HEVC_NAL_EOS_NUT) {
            if (eos_at_start) {
                s->last_eos = 1;
            } else {
                s->eos = 1;
            }
        } else {
            eos_at_start = 0;
        }
    }

    /* decode the NAL units */
    for (i = 0; i < s->pkt.nb_nals; i++) {
        H2645NAL *nal = &s->pkt.nals[i];

        if (s->avctx->skip_frame >= AVDISCARD_ALL ||
            (s->avctx->skip_frame >= AVDISCARD_NONREF
            && ff_hevc_nal_is_nonref(nal->type)) || nal->nuh_layer_id > 0)
            continue;

        // 调用 decode_nal_unit ------------------------------------------
        ret = decode_nal_unit(s, nal);
        if (ret >= 0 && s->overlap > 2)
            ret = AVERROR_INVALIDDATA;
        if (ret < 0) {
            av_log(s->avctx, AV_LOG_WARNING,
                   "Error parsing NAL unit #%d.\n", i);
            goto fail;
        }
    }

fail:
    if (s->ref && s->threads_type == FF_THREAD_FRAME)
        ff_thread_report_progress(&s->ref->tf, INT_MAX, 0);

    return ret;
}

decode_nal_unit

static int decode_nal_unit(HEVCContext *s, const H2645NAL *nal)
{
    HEVCLocalContext *lc = s->HEVClc;
    GetBitContext *gb    = &lc->gb;
    int ctb_addr_ts, ret;

    *gb              = nal->gb;
    s->nal_unit_type = nal->type;
    s->temporal_id   = nal->temporal_id;

    switch (s->nal_unit_type) {
    case HEVC_NAL_VPS:
        if (s->avctx->hwaccel && s->avctx->hwaccel->decode_params) {
            ret = s->avctx->hwaccel->decode_params(s->avctx,
                                                   nal->type,
                                                   nal->raw_data,
                                                   nal->raw_size);
            if (ret < 0)
                goto fail;
        }
        ret = ff_hevc_decode_nal_vps(gb, s->avctx, &s->ps);
        if (ret < 0)
            goto fail;
        break;
    case HEVC_NAL_SPS:
        if (s->avctx->hwaccel && s->avctx->hwaccel->decode_params) {
            ret = s->avctx->hwaccel->decode_params(s->avctx,
                                                   nal->type,
                                                   nal->raw_data,
                                                   nal->raw_size);
            if (ret < 0)
                goto fail;
        }
        ret = ff_hevc_decode_nal_sps(gb, s->avctx, &s->ps,
                                     s->apply_defdispwin);
        if (ret < 0)
            goto fail;
        break;
    case HEVC_NAL_PPS:
        if (s->avctx->hwaccel && s->avctx->hwaccel->decode_params) {
            ret = s->avctx->hwaccel->decode_params(s->avctx,
                                                   nal->type,
                                                   nal->raw_data,
                                                   nal->raw_size);
            if (ret < 0)
                goto fail;
        }
        ret = ff_hevc_decode_nal_pps(gb, s->avctx, &s->ps);
        if (ret < 0)
            goto fail;
        break;
    case HEVC_NAL_SEI_PREFIX:
    case HEVC_NAL_SEI_SUFFIX:
        if (s->avctx->hwaccel && s->avctx->hwaccel->decode_params) {
            ret = s->avctx->hwaccel->decode_params(s->avctx,
                                                   nal->type,
                                                   nal->raw_data,
                                                   nal->raw_size);
            if (ret < 0)
                goto fail;
        }
        ret = ff_hevc_decode_nal_sei(gb, s->avctx, &s->sei, &s->ps, s->nal_unit_type);
        if (ret < 0)
            goto fail;
        break;
    case HEVC_NAL_TRAIL_R:
    case HEVC_NAL_TRAIL_N:
    case HEVC_NAL_TSA_N:
    case HEVC_NAL_TSA_R:
    case HEVC_NAL_STSA_N:
    case HEVC_NAL_STSA_R:
    case HEVC_NAL_BLA_W_LP:
    case HEVC_NAL_BLA_W_RADL:
    case HEVC_NAL_BLA_N_LP:
    case HEVC_NAL_IDR_W_RADL:
    case HEVC_NAL_IDR_N_LP:
    case HEVC_NAL_CRA_NUT:
    case HEVC_NAL_RADL_N:
    case HEVC_NAL_RADL_R:
    case HEVC_NAL_RASL_N:
    case HEVC_NAL_RASL_R:
        ret = hls_slice_header(s);
        if (ret < 0) {
            // hls_slice_header() does not cleanup on failure thus the state now is inconsistant so we cannot use it on depandant slices
            s->slice_initialized = 0;
            return ret;
        }
        if (ret == 1) {
            ret = AVERROR_INVALIDDATA;
            goto fail;
        }


        if (
            (s->avctx->skip_frame >= AVDISCARD_BIDIR && s->sh.slice_type == HEVC_SLICE_B) ||
            (s->avctx->skip_frame >= AVDISCARD_NONINTRA && s->sh.slice_type != HEVC_SLICE_I) ||
            (s->avctx->skip_frame >= AVDISCARD_NONKEY && !IS_IRAP(s))) {
            break;
        }

        if (s->sh.first_slice_in_pic_flag) {
            if (s->max_ra == INT_MAX) {
                if (s->nal_unit_type == HEVC_NAL_CRA_NUT || IS_BLA(s)) {
                    s->max_ra = s->poc;
                } else {
                    if (IS_IDR(s))
                        s->max_ra = INT_MIN;
                }
            }

            if ((s->nal_unit_type == HEVC_NAL_RASL_R || s->nal_unit_type == HEVC_NAL_RASL_N) &&
                s->poc <= s->max_ra) {
                s->is_decoded = 0;
                break;
            } else {
                if (s->nal_unit_type == HEVC_NAL_RASL_R && s->poc > s->max_ra)
                    s->max_ra = INT_MIN;
            }

            s->overlap ++;
            // 调用 hevc_frame_start --------------------------------
            ret = hevc_frame_start(s);
            if (ret < 0)
                return ret;
        } else if (!s->ref) {
            av_log(s->avctx, AV_LOG_ERROR, "First slice in a frame missing.\n");
            goto fail;
        }

        if (s->nal_unit_type != s->first_nal_type) {
            av_log(s->avctx, AV_LOG_ERROR,
                   "Non-matching NAL types of the VCL NALUs: %d %d\n",
                   s->first_nal_type, s->nal_unit_type);
            return AVERROR_INVALIDDATA;
        }

        if (!s->sh.dependent_slice_segment_flag &&
            s->sh.slice_type != HEVC_SLICE_I) {
            ret = ff_hevc_slice_rpl(s);
            if (ret < 0) {
                av_log(s->avctx, AV_LOG_WARNING,
                       "Error constructing the reference lists for the current slice.\n");
                goto fail;
            }
        }

        if (s->sh.first_slice_in_pic_flag && s->avctx->hwaccel) {
            ret = s->avctx->hwaccel->start_frame(s->avctx, NULL, 0);
            if (ret < 0)
                goto fail;
        }

        if (s->avctx->hwaccel) {
            ret = s->avctx->hwaccel->decode_slice(s->avctx, nal->raw_data, nal->raw_size);
            if (ret < 0)
                goto fail;
        } else {
            if (s->threads_number > 1 && s->sh.num_entry_point_offsets > 0)
                ctb_addr_ts = hls_slice_data_wpp(s, nal);
            else
                ctb_addr_ts = hls_slice_data(s);
            if (ctb_addr_ts >= (s->ps.sps->ctb_width * s->ps.sps->ctb_height)) {
                s->is_decoded = 1;
            }

            if (ctb_addr_ts < 0) {
                ret = ctb_addr_ts;
                goto fail;
            }
        }
        break;
    case HEVC_NAL_EOS_NUT:
    case HEVC_NAL_EOB_NUT:
        s->seq_decode = (s->seq_decode + 1) & 0xff;
        s->max_ra     = INT_MAX;
        break;
    case HEVC_NAL_AUD:
    case HEVC_NAL_FD_NUT:
        break;
    default:
        av_log(s->avctx, AV_LOG_INFO,
               "Skipping NAL unit %d\n", s->nal_unit_type);
    }

    return 0;
fail:
    if (s->avctx->err_recognition & AV_EF_EXPLODE)
        return ret;
    return 0;
}

hevc_frame_start

static int hevc_frame_start(HEVCContext *s)
{
    HEVCLocalContext *lc = s->HEVClc;
    int pic_size_in_ctb  = ((s->ps.sps->width  >> s->ps.sps->log2_min_cb_size) + 1) *
                           ((s->ps.sps->height >> s->ps.sps->log2_min_cb_size) + 1);
    int ret;

    memset(s->horizontal_bs, 0, s->bs_width * s->bs_height);
    memset(s->vertical_bs,   0, s->bs_width * s->bs_height);
    memset(s->cbf_luma,      0, s->ps.sps->min_tb_width * s->ps.sps->min_tb_height);
    memset(s->is_pcm,        0, (s->ps.sps->min_pu_width + 1) * (s->ps.sps->min_pu_height + 1));
    memset(s->tab_slice_address, -1, pic_size_in_ctb * sizeof(*s->tab_slice_address));

    s->is_decoded        = 0;
    s->first_nal_type    = s->nal_unit_type;

    s->no_rasl_output_flag = IS_IDR(s) || IS_BLA(s) || (s->nal_unit_type == HEVC_NAL_CRA_NUT && s->last_eos);

    if (s->ps.pps->tiles_enabled_flag)
        lc->end_of_tiles_x = s->ps.pps->column_width[0] << s->ps.sps->log2_ctb_size;

    ret = ff_hevc_set_new_ref(s, &s->frame, s->poc);
    if (ret < 0)
        goto fail;

    // 开始调用 ff_hevc_frame_rps ------------------
    ret = ff_hevc_frame_rps(s);
    if (ret < 0) {
        av_log(s->avctx, AV_LOG_ERROR, "Error constructing the frame RPS.\n");
        goto fail;
    }

    s->ref->frame->key_frame = IS_IRAP(s);

    ret = set_side_data(s);
    if (ret < 0)
        goto fail;

    s->frame->pict_type = 3 - s->sh.slice_type;

    if (!IS_IRAP(s))
        ff_hevc_bump_frame(s);

    av_frame_unref(s->output_frame);
    ret = ff_hevc_output_frame(s, s->output_frame, 0);
    if (ret < 0)
        goto fail;

    if (!s->avctx->hwaccel)
        ff_thread_finish_setup(s->avctx);

    return 0;

fail:
    if (s->ref)
        ff_hevc_unref_frame(s, s->ref, ~0);
    s->ref = NULL;
    return ret;
}

ff_hevc_frame_rps

/**
 * Construct the reference picture sets for the current frame.
 */

int ff_hevc_frame_rps(HEVCContext *s)
{
    const ShortTermRPS *short_rps = s->sh.short_term_rps;
    const LongTermRPS  *long_rps  = &s->sh.long_term_rps;
    RefPicList               *rps = s->rps;
    int i, ret = 0;

    if (!short_rps) {
        rps[0].nb_refs = rps[1].nb_refs = 0;
        return 0;
    }

    /* clear the reference flags on all frames except the current one */
    for (i = 0; i < FF_ARRAY_ELEMS(s->DPB); i++) {
        HEVCFrame *frame = &s->DPB[i];

        if (frame == s->ref)
            continue;

        mark_ref(frame, 0);
    }

    for (i = 0; i < NB_RPS_TYPE; i++)
        rps[i].nb_refs = 0;

    /* add the short refs */
    for (i = 0; i < short_rps->num_delta_pocs; i++) {
        int poc = s->poc + short_rps->delta_poc[i];
        int list;

        if (!short_rps->used[i])
            list = ST_FOLL;
        else if (i < short_rps->num_negative_pics)
            list = ST_CURR_BEF;
        else
            list = ST_CURR_AFT;

        // case 1 ：调用 add_candidate_ref
        ret = add_candidate_ref(s, &rps[list], poc, HEVC_FRAME_FLAG_SHORT_REF, 1);
        if (ret < 0)
            goto fail;
    }

    /* add the long refs */
    for (i = 0; i < long_rps->nb_refs; i++) {
        int poc  = long_rps->poc[i];
        int list = long_rps->used[i] ? LT_CURR : LT_FOLL;
        // case 2 ：调用 add_candidate_ref
        ret = add_candidate_ref(s, &rps[list], poc, HEVC_FRAME_FLAG_LONG_REF, long_rps->poc_msb_present[i]);
        if (ret < 0)
            goto fail;
    }

fail:
    /* release any frames that are now unused */
    for (i = 0; i < FF_ARRAY_ELEMS(s->DPB); i++)
        ff_hevc_unref_frame(s, &s->DPB[i], 0);

    return ret;
}

add_candidate_ref

/* add a reference with the given poc to the list and mark it as used in DPB */
static int add_candidate_ref(HEVCContext *s, RefPicList *list,
                             int poc, int ref_flag, uint8_t use_msb)
{
    HEVCFrame *ref = find_ref_idx(s, poc, use_msb);

    if (ref == s->ref || list->nb_refs >= HEVC_MAX_REFS)
        return AVERROR_INVALIDDATA;

    if (!ref) {
        ref = generate_missing_ref(s, poc);
        if (!ref)
            return AVERROR(ENOMEM);
    }

    list->list[list->nb_refs] = ref->poc;
    list->ref[list->nb_refs]  = ref;
    list->nb_refs++;

    mark_ref(ref, ref_flag);
    return 0;
}

find_ref_idx

static HEVCFrame *find_ref_idx(HEVCContext *s, int poc, uint8_t use_msb)
{
    int mask = use_msb ? ~0 : (1 << s->ps.sps->log2_max_poc_lsb) - 1;
    int i;

    for (i = 0; i < FF_ARRAY_ELEMS(s->DPB); i++) {
        HEVCFrame *ref = &s->DPB[i];
        if (ref->frame->buf[0] && ref->sequence == s->seq_decode) {
            if ((ref->poc & mask) == poc)
                return ref;
        }
    }

    if (s->nal_unit_type != HEVC_NAL_CRA_NUT && !IS_BLA(s))
        av_log(s->avctx, AV_LOG_ERROR,
               "Could not find ref with POC %d\n", poc);
    return NULL;
}