c - Failed to use h264_v4l2 codec in ffmpeg to decode video - Stack Overflow

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I am working on an embedded Linux system (kernel-5.10.24) and I want to use ffmpeg libraries (ffmpeg-4.4.4) to do video decoding.

The C code is as follows, it uses h264_v4l2m2m decoder to decode the video,

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavutil/imgutils.h>
#include <libavutil/opt.h>
#include <libswscale/swscale.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
    if (argc < 3) {
        printf("Usage: %s <input_video> <output_video>\n", argv[0]);
        return -1;
    }

    const char *input_file = argv[1];
    const char *output_file = argv[2];

    AVFormatContext *fmt_ctx = NULL;
    AVCodecContext *codec_ctx = NULL;
    AVCodec *codec = NULL;
    AVPacket pkt;
    AVFrame *frame = NULL;
    AVFrame *rgb_frame = NULL;
    struct SwsContext *sws_ctx = NULL;

    FILE *output = NULL;
    int video_stream_index = -1;

    avformat_network_init();

    if (avformat_open_input(&fmt_ctx, input_file, NULL, NULL) < 0) {
        fprintf(stderr, "Could not open input file %s\n", input_file);
        return -1;
    }

    if (avformat_find_stream_info(fmt_ctx, NULL) < 0) {
        fprintf(stderr, "Could not find stream information\n");
        return -1;
    }

    for (int i = 0; i < fmt_ctx->nb_streams; i++) {
        if (fmt_ctx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
            video_stream_index = i;
            break;
        }
    }

    if (video_stream_index == -1) {
        fprintf(stderr, "Could not find video stream\n");
        return -1;
    }

    //// codec = avcodec_find_decoder(fmt_ctx->streams[video_stream_index]->codecpar->codec_id);
    codec = avcodec_find_decoder_by_name("h264_v4l2m2m");
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        return -1;
    }

    codec_ctx = avcodec_alloc_context3(codec);
    if (!codec_ctx) {
        fprintf(stderr, "Could not allocate codec context\n");
        return -1;
    }

    if (avcodec_parameters_to_context(codec_ctx, fmt_ctx->streams[video_stream_index]->codecpar) < 0) {
        fprintf(stderr, "Failed to copy codec parameters to decoder context\n");
        return -1;
    }

    if (avcodec_open2(codec_ctx, codec, NULL) < 0) {
        fprintf(stderr, "Could not open codec\n");
        return -1;
    }

    output = fopen(output_file, "wb");
    if (!output) {
        fprintf(stderr, "Could not open output file %s\n", output_file);
        return -1;
    }

    frame = av_frame_alloc();
    rgb_frame = av_frame_alloc();
    if (!frame || !rgb_frame) {
        fprintf(stderr, "Could not allocate frames\n");
        return -1;
    }

    int width = codec_ctx->width;
    int height = codec_ctx->height;
    int num_bytes = av_image_get_buffer_size(AV_PIX_FMT_RGB24, width, height, 1);
    uint8_t *buffer = (uint8_t *)av_malloc(num_bytes * sizeof(uint8_t));
    av_image_fill_arrays(rgb_frame->data, rgb_frame->linesize, buffer, AV_PIX_FMT_RGB24, width, height, 1);

printf("XXXXXXXXXXXX width: %d, height: %d, fmt: %d\n", width, height, codec_ctx->pix_fmt);
    sws_ctx = sws_getContext(width, height, codec_ctx->pix_fmt,
                              width, height, AV_PIX_FMT_RGB24,
                              SWS_BILINEAR, NULL, NULL, NULL);
    if (!sws_ctx) {
        fprintf(stderr, "Could not initialize the conversion context\n");
        return -1;
    }

    while (av_read_frame(fmt_ctx, &pkt) >= 0) {
        if (pkt.stream_index == video_stream_index) {
            int ret = avcodec_send_packet(codec_ctx, &pkt);
            if (ret < 0) {
                fprintf(stderr, "Error sending packet for decoding\n");
                return -1;
            }

            while (ret >= 0) {
                ret = avcodec_receive_frame(codec_ctx, frame);
                if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {
                    break;
                } else if (ret < 0) {
                    fprintf(stderr, "Error during decoding\n");
                    return -1;
                }

                sws_scale(sws_ctx, (const uint8_t *const *)frame->data, frame->linesize,
                          0, height, rgb_frame->data, rgb_frame->linesize);

                fprintf(output, "P6\n%d %d\n255\n", width, height);
                fwrite(rgb_frame->data[0], 1, num_bytes, output);
            }
        }
        av_packet_unref(&pkt);
    }

    fclose(output);
    av_frame_free(&frame);
    av_frame_free(&rgb_frame);
    avcodec_free_context(&codec_ctx);
    avformat_close_input(&fmt_ctx);
    sws_freeContext(sws_ctx);

    return 0;
}

It ran with some error logs from swscale as follows,

# ./test_ffmpeg ./test.mp4 /tmp/output
[h264_v4l2m2m @ 0x1d76320] Using device /dev/video0
[h264_v4l2m2m @ 0x1d76320] driver 'mysoc-vdec' on card 'msoc-vdec' in mplane mode
[h264_v4l2m2m @ 0x1d76320] requesting formats: output=H264 capture=NV12
[h264_v4l2m2m @ 0x1d76320] the v4l2 driver does not support end of stream VIDIOC_SUBSCRIBE_EVENT
XXXXXXXXXXXX width: 1280, height: 720, fmt: 0
[swscaler @ 0x1dadaa0] No accelerated colorspace conversion found from yuv420p to rgb24.
[h264_v4l2m2m @ 0x1d76320] VIDIOC_G_SELECTION ioctl
[swscaler @ 0x1dadaa0] bad src image pointers
[swscaler @ 0x1dadaa0] bad src image pointers
[swscaler @ 0x1dadaa0] bad src image pointers
[swscaler @ 0x1dadaa0] bad src image pointers
[swscaler @ 0x1dadaa0] bad src image pointers
[swscaler @ 0x1dadaa0] bad src image pointers
[swscaler @ 0x1dadaa0] bad src image pointers
[swscaler @ 0x1dadaa0] bad src image pointers
[swscaler @ 0x1dadaa0] bad src image pointers
[swscaler @ 0x1dadaa0] bad src image pointers
[swscaler @ 0x1dadaa0] bad src image pointers
[swscaler @ 0x1dadaa0] bad src image pointers
......

And it ran for about 4 seconds, while the test.mp4 is about 13 seconds. If I did NOT specify the h264_v4l2m2m as the decoder, there is NO bad src image pointers and its run-time is as long as the mp4 file.

What is wrong with above codes using h264_v4l2m2m and how to fix it?

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