leo@chessman ~ :) $ ffmpeg -f v4l2 -input_format mjpeg -video_size 1280x720 -i /dev/video0 -c copy -f mjpeg -bsf mjpeg2jpeg -y /dev/null
ffmpeg version N-57228-g71f73ee Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04)
  configuration: --enable-gpl --enable-version3 --enable-nonfree --enable-shared --disable-static --enable-avresample --enable-vda --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libcaca --enable-libcdio --enable-libdc1394 --enable-libfdk-aac --enable-libflite --enable-libfreetype --enable-libgme --enable-libgsm --enable-libiec61883 --enable-libmp3lame --enable-libopus --enable-libpulse --enable-libquvi --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-openal --enable-opencl --enable-opengl --enable-openssl --enable-sdl --enable-x11grab --enable-zlib --enable-pic
  libavutil      55. 13.100 / 55. 13.100
  libavcodec     57. 22.100 / 57. 22.100
  libavformat    57. 21.101 / 57. 21.101
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6. 23.100 /  6. 23.100
  libavresample   3.  0.  0 /  3.  0.  0
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
[mjpeg @ 0x222cea0] Changing bps to 8
Input #0, video4linux2,v4l2, from '/dev/video0':
  Duration: N/A, start: 670773.008387, bitrate: N/A
    Stream #0:0: Video: mjpeg, yuvj422p(pc, bt470bg/unknown/unknown), 1280x720, -5 kb/s, 30 fps, 30 tbr, 1000k tbn, 1000k tbc
Output #0, mjpeg, to '/dev/null':
  Metadata:
    encoder         : Lavf57.21.101
    Stream #0:0: Video: mjpeg, yuvj422p, 1280x720, q=2-31, -5 kb/s, 30 fps, 30 tbr, 1000k tbn, 1000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
[NULL @ 0x222f280] input is not MJPEG/AVI1
[NULL @ 0x222f280] Failed to open bitstream filter mjpeg2jpeg for stream 0 with codec copy: Invalid data found when processing input
[NULL @ 0x222f280] input is not MJPEG/AVI1
[NULL @ 0x222f280] Failed to open bitstream filter mjpeg2jpeg for stream 0 with codec copy: Invalid data found when processing input