关于ios port driver

我在写ios port driver时,用到AEP,
然而,在AEP init时系统传递过来的参数AEP_bi_i_hdevnode,系统是根据什么确定的,因为接下来的函数调用我总是返回失败

USBSTOR_AEP_Init(PAEP_bi_init Aep)
{
    if (CM_Get_Parent(&ParentDevNode, (DEVNODE)Aep->AEP_bi_i_hdevnode, 0) != CR_SUCCESS)
{
        USBSTOR_DebugPrintf(DBG_MIN, ("Can't find our parent DevNode!\n\r"));
        return AEP_FAILURE;
}