Because the host is always the bus master, it is easy to remember USB direction: OUT means from the host to the device, and IN means from the device to the host. EZ-USB nomenclature uses this naming convention. For example, an endpoint that sends data to the host is an IN endpoint. This can be confusing at first, because the EZ-USB sends data to the host by loading an IN endpoint buffer. Likewise, the EZUSB receives host data from an OUT endpoint buffer.
因为主机一直是总线主控者，所以也很好记住USB的方向(其实就是按照主机的视角看待USB设备)。OUT代表从主机到usb设备，IN代表从设备到主机(都是以主机端的视角看的)。EZ-USB的命名也使用了这种通用的命名规则。举个例子，发送数据到主机的端点是IN(输入端点)。但如果先发起会被拒绝，因为EZ-USB是通过载入输入端点的缓存来发送数据到主机。相应的，EZ-USB设备也是通过输出端点缓存来接收主机的数据。
原文地址：http://www.cnblogs.com/ARM-LINUX-WANG/p/3809571.html
