I found a good overview about USB on Linux. Here is the article :
The USB subsystem is based on message passing transactions. The messages are called URBs, which stands for USB request block. URBs are sent by calling the usb_submit_urb method, struct urb *urb, int mem_flags). This is an asynchronous call, and it returns immediately. The URB is put in a queue, and later, it reaches a completion handler. The completion handler is a member of the URB structure called complete, a pointer to a function in the URB struct. In the completion handler, you can check urb -> status to see if any errors have been detected.
To cancel pending requests, use usb_unlink_urb(). URBs are allocated by calling usb_alloc_urb(), and they are freed with a call to usb_free_urb(). Three helper methods are available to help fill URBs: usb_fill_control_urb(), usb_fill_bulk_urb and usb_fill_int_urb.
The URB struct resides in include/usb/usb.h, which is one of the two most important headers that define the USB interface. The second is include/linux/usb_ch9.h. Another little file also is called usb.h and can be found in usb/core, but it is not important in this context.
The header file called usb_ch9.h hold constants and structures for USB. The name comes from chapter 9 of the USB 2.0 specification. Currently, there are two specifications for USB, 2.0 and 1.1. USB 2.0 operates at high speed, defined as 60MB/s (480Mb/s), which is 40 times faster than USB full speed. USB 1.1 operates either at full speed, which is 1.5MB/s (12Mb/s), or at low speed, which is 1.5Mb/s. When you connect high speed devices to USB 1.1 systems, they operate at USB 1.1 speeds.
The USB_SPEED_LOW, USB_SPEED_FULL and USB_SPEED_HIGH constants are defined in usb_ch9.h. The constants are encountered here and there in the USB sources in the kernel.
you can view the rest of the article on the original website