基于android4.0 USB gadget框架分析

    转眼间，看usb gadget代码看的有一段时间了，把其中的层次关系大致的理了一遍，就写出来和大家分享下，不对之处敬请指出，了解层次结构嘛，当然是代码越简单越好，俗话说“柿子专拣软的捏”，就拿最简单的zero.c开涮啦！

首先说下整体流程：

一个UDC(usb设备控制器)上面要挂一个usb_gadget_driver，usb_composite_dev挂到UDC的gadget上，usb_composite_driver挂到usb_composite_dev上，然后在usb_composite_dev上挂上一个usb_configuration配置，然后在usb_configuration的function链表中挂一个function，并且把function写入对应的config 的interface数组中。这就是整个流程。可能这么说有点晕，下面就看实际的代码分析吧！

首先把结构体老大贴出来，因为别人都依靠他呢！

struct s3c2410_udc {
    .........................
    struct usb_gadget        gadget;
    struct usb_gadget_driver    *driver;
    .....................
};

首先你应该准备一个usb_gadget_driver，并把它挂到上述结构体的driver成员中。在zero.c中该结构为

static struct usb_gadget_driver composite_driver = {
    .speed        = USB_SPEED_HIGH,

    .unbind        = composite_unbind,

    .setup        = composite_setup,
    .disconnect    = composite_disconnect,

    .suspend    = composite_suspend,
    .resume        = composite_resume,

    .driver    = {
        .owner        = THIS_MODULE,
    },
};

这个结构是通用的结构。其中的composite_driver.function成员和composite_driver.driver.name被

usb_composite_driver中得成员给重新赋值了。

static struct usb_composite_driver zero_driver = {
    .name        = "zero",
    .dev        = &device_desc,
    .strings    = dev_strings,
    .unbind        = zero_unbind,
    .suspend    = zero_suspend,
    .resume        = zero_resume,
};

然后分配一个usb_composite_dev，把该结构体挂到gadget上，现在结构体老大如下

struct s3c2410_udc {
    .........................
    struct usb_gadget        gadget;----------》cdev
    struct usb_gadget_driver    *driver;-------->composite_driver     .....................
};
然后在cdev上挂上usb_composite_driver，其定义如下：
static struct usb_composite_driver zero_driver = {
    .name        = "zero",
    .dev        = &device_desc,
    .strings    = dev_strings,
    .unbind        = zero_unbind,
    .suspend    = zero_suspend,
    .resume        = zero_resume,
};然后在cdev上绑定usb_configuration，usb_config的定义如下：
static struct usb_configuration loopback_driver = {
    .label        = "loopback",
    .strings    = loopback_strings,
    .bConfigurationValue = 2,
    .bmAttributes    = USB_CONFIG_ATT_SELFPOWER,
    /* .iConfiguration = DYNAMIC */
};把该config挂到cdev上。然后初始化一个function，并把它挂到config上，初始化的function如下
loop = kzalloc(sizeof *loop, GFP_KERNEL);
    loop->function.name = "loopback";
    loop->function.descriptors = fs_loopback_descs;
    loop->function.bind = loopback_bind;
    loop->function.unbind = loopback_unbind;
    loop->function.set_alt = loopback_set_alt;
    loop->function.disable = loopback_disable;然后调用该function的bind函数，把function加入到config的interface数组中。至此层次结构已经理清，这也是usb gadget的框架结构。