snd_pcm_open_substream
int snd_pcm_open_substream(struct snd_pcm *pcm, int stream, struct file *file,
err = snd_pcm_open_substream(pcm, idx, file, &substream);
EXPORT_SYMBOL(snd_pcm_open_substream);
err = snd_pcm_open_substream(pcm, stream, file, &substream);