mmal fail to decode mjpeg from USB camera

[linhuifj]

Example of the frame:

I have a USB camera that outputs mjpeg frames that fail to be decoded by mmal.
I have tried to use hello_mmal_jpeg, hello_mmal_video, and FFmpeg with mmal_mjpeg decoder, none of them is ok.

I finally come out with the solution to use mjpeg2jpeg converter in FFmpeg:
./ffmpeg -i ~/raw.jpg -codec copy -bsf:v mjpeg2jpeg yy.jpg
I writer a convert by referring to the code in FFmpeg mjpeg2jpeg.

My question:

1. Is there any better solution?
2. what's the highest framerate supported by mmal to decode video with size 3840x2160? I have tested the decoding of hello_mmal_jpeg and found that it takes about 150ms to decode the image, which is too slow.
3. Is mjpeg video decoding faster than image decoding?