USB urb生命周期

1. 在probe中创建URB

  - struct urb *usb_alloc_urb(int iso_packets, gfp_t mem_flags)

  - void *usb_buffer_alloc(struct usb_device *dev, size_t size, gfp_t mem_flags, dma_addr_t *dma)

  - static inline void usb_fill_int_urb(struct urb *urb,

   struct usb_device *dev,

   unsigned int pipe,

   void *transfer_buffer,

   int buffer_length,

   usb_complete_t complete_fn,

   void *context,

   int interval)

2. 将URB分配给USB设备的指定endpoint

3. 在需要往endpoint写数据时，将URB提交到USB core

  - int usb_submit_urb(struct urb *urb, gfp_t mem_flags)

4. USB core将URB提交到特定的USB host controller

5. USB host controller发送到USB设备

6. 发送完成后，USB host controller通过回调通知到USB设备驱动