Standard AS Interface Descriptor

struct usb_as_interface_descriptor
{
    U8      bLength;            /* Size of this descriptor in bytes */
    U8      bDescriptorType;        /* INTERFACE descriptor type */
    U8      bInterfaceNumber;   /* Number of interface. A zero-based value
identifying the index in
 
 the array of
concurrent interfaces supported by this
configuration. */
    U8      bAlternateSetting; /*Value used to select an alternate setting
for the interface identified in the prior
field.*/
    U8      bNumEndpoints;      /* Number of endpoints in this interface */
    U8      bInterfaceClass;    /* AUDIO Interface class
 
 code */
    U8      bInterfaceSubclass; /* AUDIO_STREAMING Interface subclass code */
    U8      bInterfaceProtocol; /* Not used. Must be set to 0. */
    U8      iInterface;         /* Index of a string descriptor that describes
this interface. */
};

bLength = 0x09;
bDescriptorType = 0x04; 說明這是一個Interface descriptor;
bInterfaceNumber: 此配置所支援的Interface index；
bAlternateSetting: 可變更的Interface；
bNumEndpoints: 除了endpoint0, 此interface使用的endpoint;
bInterfaceClass = 0x01; 說明是 Audio interface class;
bInterfaceSubClass = 0x02; 說明是 audio streaming intertface class;
bInterfaceProtocol: 未使用，強制設為0；
iInterface: 如果有string descriptor, 則此值是 string 的 index;