Recall that callback is executed before and after the endpoint specified by type, endpoint, and direction transfers data. Below is its function prototype:

typedef void (usb_callback_t)(struct usb_xfer *, usb_error_t);

Here, struct usb_xfer * contains the transfer state:

struct usb_xfer { ... uint8_t usb_state; /* Set when callback is executed before a data transfer. */ #define USB_ST_SETUP 0 /* Set when callback is executed after a data transfer. */ #define USB_ST_TRANSFERRED 1 /* Set when a transfer error occurs. */ #define ...