USB iso bInterval

看USB2.0 spec一直认为endpoint 的desc是如下标准格式：
struct usb_endpoint_desc {
     /* Hardware fields */
    uchar bLength;
    uchar bDescriptorType;
    uchar bEndpointAddress;
    uchar bmAttributes;
    ushort wMaxPacketSize;
    uchar bInterval;
} __attribute__ ((packed));
今天看了下耳机的endpoint 信息，有新发现：
/* USB_DT_ENDPOINT: Endpoint descriptor */
struct usb_endpoint_descriptor {
    __u8  bLength;
    __u8  bDescriptorType;
    __u8  bEndpointAddress;
    __u8  bmAttributes;
    __le16 wMaxPacketSize;
    __u8  bInterval;//in audio device spec, this is set to 1 and must be 1
    /* NOTE:  these two are _only_ in audio endpoints. */
    /* use USB_DT_ENDPOINT*_SIZE in bLength, not sizeof. */
    __u8  bRefresh; //this info is useful when the device is audio iso endpoint, for this
                    // is used as real frame interval.
    __u8  bSynchAddress;
} __attribute__ ((packed));