USB Gadget

A USB Linux Gadget is a device which has a UDC (USB Device Controller) and can
be connected to a USB Host to extend it with additional functions like a serial

A gadget is seen by its host as a set of configurations, each of which contains
a number of interfaces which, from the gadget’s perspective, are known as
functions, each function representing e.g. a serial connection or a SCSI disk.

Linux provides a number of functions for gadgets to use.

USB Linux Gadget是USB设备控制器作为从设备，连接到USB Host，通过 USB 来模拟其它类型的设备，比如UAC、UVC、HID设备、串口等等

CDC类

USB的CDC类是USB通信设备类 (Communication Device Class)的简称。CDC类是USB组织定义的一类专门给各种通信设备(电信通信设备和中速网络通信设备)使用的USB子类
CDC，USB Driver支持以下子类: ECM、NCM、ACM、OBEX、设备管理。

Configfs

Documentation/filesystems/configfs.rst

configfs is a ram-based filesystem that provides the converse of
sysfs’s functionality. Where sysfs is a filesystem-based view of
kernel objects, configfs is a filesystem-based manager of kernel
objects, or config_items.

Userspace-driven Kernel Object Configuration
configfs是一个基于ram的文件系统，通过对该文件系统的操作实现对内核对象的配置
Documentation/filesystems/configfs.rst

不只是USB可以通过configfs进行配置，有些子系统也可以。理论上只要实现了configfs接口的应该都行

driver/usb/gadget/configfs.c:

通过configfs_register_subsystem(&gadget_subsys);，在configfs中注册了usb gadget子系统

使用configfs配置usb gadget

作者: Shell