snd_ctl_add_follower
err = snd_ctl_add_follower(master, follower);
return snd_ctl_add_follower(data, follower);
err = snd_ctl_add_follower(vmaster_vol, master_vol);
err = snd_ctl_add_follower(vmaster_vol, speaker_vol);
ret = snd_ctl_add_follower(vmaster, control);