ffmpeg -framerate 1 -input_format mjpeg -i /dev/video0 -r 30 -c:v copy -an out.mp4

[video4linux2,v4l2 @ 0x55ece63ac360] The driver changed the time per frame from 1/1 to 1/30
Input #0, video4linux2,v4l2, from '/dev/video0':
  Duration: N/A, start: 37064.594605, bitrate: N/A
    Stream #0:0: Video: mjpeg, yuvj422p(pc, bt470bg/unknown/unknown), 1920x1080, 30 fps, 30 tbr, 1000k tbn, 1000k tbc