USB Core 四

 

struct urb *usb_alloc_urb(int iso_packets, gfp_t mem_flags)
{
	struct urb *urb;

	urb = kmalloc(sizeof(struct urb) +
		iso_packets * sizeof(struct usb_iso_packet_descriptor),
		mem_flags);
	if (!urb) {
		printk(KERN_ERR "alloc_urb: kmalloc failed\n");
		return NULL;
	}
	usb_init_urb(urb);
	return urb;
}

它只做了两件事情，拿kmalloc来为urb申请内存，然后调用usb_init_urb进行初始化.usb_alloc_urb它的第一个参数iso_packets ， 表示的是struct urb 结构最后那个变长数组
iso_frame_desc的元素数目，也就是应该与number_of_packets的值相同，所以对于控制/中断/批量传输，这个参数都应该为0.

第二个问题是参数mem_flags的类型gfp_t，它在include/linux/types.h里定义。
typedef unsigned __bitwise__ gfp_t;

 

void usb_free_urb(struct urb *urb)
{
	if (urb)
		kref_put(&urb->kref, urb_destroy);
}

usb_free_urb更潇洒，只调用kref_put将urb的引用计数减一，减了之后如果变为0，也就是没人再用它了，就调用urb_destroy将它销毁掉。

 

static inline void usb_fill_control_urb(struct urb *urb,
					struct usb_device *dev,
					unsigned int pipe,
					unsigned char *setup_packet,
					void *transfer_buffer,
					int buffer_length,
					usb_complete_t complete_fn,
					void *context)
{
	urb->dev = dev;
	urb->pipe = pipe;
	urb->setup_packet = setup_packet;
	urb->transfer_buffer = transfer_buffer;
	urb->transfer_buffer_length = buffer_length;
	urb->complete = complete_fn;
	urb->context = context;
}

urb 的元素有很多，这里只是填充了一部分，剩下那些不是控制传输管不着的，就是自有安排可以不用去管的。

static inline void usb_fill_bulk_urb(struct urb *urb,
				     struct usb_device *dev,
				     unsigned int pipe,
				     void *transfer_buffer,
				     int buffer_length,
				     usb_complete_t complete_fn,
				     void *context)
{
	urb->dev = dev;
	urb->pipe = pipe;
	urb->transfer_buffer = transfer_buffer;
	urb->transfer_buffer_length = buffer_length;
	urb->complete = complete_fn;
	urb->context = context;
}

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)
{
	urb->dev = dev;
	urb->pipe = pipe;
	urb->transfer_buffer = transfer_buffer;
	urb->transfer_buffer_length = buffer_length;
	urb->complete = complete_fn;
	urb->context = context;
	if (dev->speed == USB_SPEED_HIGH || dev->speed == USB_SPEED_SUPER)
		urb->interval = 1 << (interval - 1);
	else
		urb->interval = interval;
	urb->start_frame = -1;
}

负责批量传输的usb_fill_bulk_urb和负责控制传输的usb_fill_control_urb的相比，只是少初始化了一个setup_packet，因为批量传输里没有Setup包的概念，中断传输里也没有，所以usb_fill_int_urb里也没有初始化setup_packet这一说。