Man page 9 : input_ff_create

input_ff_create - create force-feedback device

int input_ff_create(struct input_dev *  dev , unsigned int  max_effects );

dev
    input device supporting force-feedback

max_effects
    maximum number of effects supported by the device

This function allocates all necessary memory for a force feedback portion of an input device and installs all default handlers. dev->ffbit should be already set up before calling this function. Once ff device is created you need to setup its upload, erase, playback and other handlers before registering input device