Change capture format with --hwdec=v4l2m2m-copy?

[knuxify]

Trying to use hardware video acceleration on a tablet with the Exynos 4212 SoC on an mp4 video. Excerpt from the log:

[vd] Codec list:
[vd]     h264 - H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10
[vd]     h264_v4l2m2m (h264) - V4L2 mem2mem H.264 decoder wrapper
[vd] Opening decoder h264
[vd] Looking at hwdec h264_v4l2m2m-v4l2m2m-copy...
[vd] Trying hardware decoding via h264_v4l2m2m-v4l2m2m-copy.
[vd] Using underlying hw-decoder 'h264_v4l2m2m'
[ffmpeg/video] h264_v4l2m2m: Could not find a valid device
[ffmpeg/video] h264_v4l2m2m: can't configure decoder
[vd] Could not open codec.

I've done some debugging and it seems that it's failing because it requests a capture format of MJPG, which is not supported by the hardware video decoding chip. (Output is set to h264, capture is set to MJPG.)

Is there a way to change this "capture format"? I assume I need to pass some option to ffmpeg?