一、导读
本文将描述linux-usb子系统的核心，主要分析其核心的初始化流程，文中源码基于内核版本：4.1.15。
linux usb子系统框架的核心位于/drivers/usb/core/目录中，文件结构如下图所示：
目录中各文件功能大致如下：
二、USB核心的初始化
在linux内核的启动阶段USB模块最早输出日志信息如下：
（1）[ 0.475284] usbcore: registered new interface driver usbfs （2）[ 0.475348] usbcore: registered new interface driver hub （3）[ 0.475400] usbcore: registered new device driver usb
从上述输出日志可知：第（1）行表示成功注册USB文件系统，且在系统正常启动后，会生成对应的/sys/bus/usb/目录。第（2）行表示成功注册USB HUB驱动。第（3）行表明成功注册USB通用设备驱动，即usb_generic_driver。通常USB设备都是以设备的身份先与usb_generic_driver匹配，匹配成功后，会分裂出接口，当对接口调用 device_add()后，会触发USB接口和USB驱动的匹配。
USB核心由/drivers/usb/core/usb.c文件描述。该文件以linux内核模块的方式设计，使用subsys_initcall(usb_init);将usb核心模块导出。其中，usb核心的初始化由usb_init()完成，实现如下：
static int __init usb_init(void)
{
int retval;
if (usb_disabled()) {
pr_info("%s: USB support disabled
", usbcore_name);
return 0;
}
usb_init_pool_max(); //初始化pool_max参数
usb_debugfs_init(); //初始化usb的调试文件系统debugfs
usb_acpi_register(); //初始化acpi
retval = bus_register(&usb_bus_type); //向linux内核注册usb总线类型
if (retval)
goto bus_register_failed;
//注册usb通知器
retval = bus_register_notifier(&usb_bus_type, &usb_bus_nb);
if (retval)
goto bus_notifier_failed;
//usb设备号初始化
retval = usb_major_init();
if (retval)
goto major_init_failed;
//注册usbfs驱动程序
retval = usb_register(&usbfs_driver);
if (retval)
goto driver_register_failed;
//初始化usb的devio
retval = usb_devio_init();
if (retval)
goto usb_devio_init_failed;
//初始化usb的hub
retval = usb_hub_init();
if (retval)
goto hub_init_failed;
//注册通用usb设备驱动
retval = usb_register_device_driver(&usb_generic_driver, THIS_MODULE);
if (!retval)
goto out;
usb_hub_cleanup();
hub_init_failed:
usb_devio_cleanup();
usb_devio_init_failed:
usb_deregister(&usbfs_driver);
driver_register_failed:
usb_major_cleanup();
major_init_failed:
bus_unregister_notifier(&usb_bus_type, &usb_bus_nb);
bus_notifier_failed:
bus_unregister(&usb_bus_type);
bus_register_failed:
usb_acpi_unregister();
usb_debugfs_cleanup();
out:
return retval;
}
从上述代码可见，usb_init()中执行了如下操作：
（1）判断linux内核是否开启了对USB的支持，如果不支持则直接返回。
（2）初始化调试文件系统关于usb的目录和文件：
static int usb_debugfs_init(void)
{
//创建usb目录
usb_debug_root = debugfs_create_dir("usb", NULL);
if (!usb_debug_root)
return -ENOENT;
//在usb目录下创建devices文件
usb_debug_devices = debugfs_create_file("devices", 0444,
usb_debug_root, NULL,
&usbfs_devices_fops);
if (!usb_debug_devices)
{
debugfs_remove(usb_debug_root);
usb_debug_root = NULL;
return -ENOENT;
}
return 0;
}
（3）初始化usb与acpi相关的参数：
（4）向linux内核注册usb总线类型，usb总线类型定义如下：
struct bus_type usb_bus_type = {
.name = "usb",
.match = usb_device_match,
.uevent = usb_uevent,
};
（5）注册usb总线通知器
（6）初始化usb主设备号：
int usb_major_init(void)
{
int error;
//注册usb字符设备
error = register_chrdev(USB_MAJOR, "usb", &usb_fops);
if (error)
printk(KERN_ERR "Unable to get major %d for usb devices
",
USB_MAJOR);
return error;
}
（7）注册usbfs驱动程序，usbfs以usb驱动程序的形式进行定义：
struct usb_driver usbfs_driver = {
.name = "usbfs",
.probe = driver_probe,
.disconnect = driver_disconnect,
.suspend = driver_suspend,
.resume = driver_resume,
};
（8）初始化usb的devio，实质上是初始化USB字符设备，devio用于USB设备与用户空间进行通信：
int __init usb_devio_init(void)
{
int retval;
retval = register_chrdev_region(USB_DEVICE_DEV, USB_DEVICE_MAX,
"usb_device");
if (retval) {
printk(KERN_ERR "Unable to register minors for usb_device
");
goto out;
}
cdev_init(&usb_device_cdev, &usbdev_file_operations);
retval = cdev_add(&usb_device_cdev, USB_DEVICE_DEV, USB_DEVICE_MAX);
if (retval) {
printk(KERN_ERR "Unable to get usb_device major %d
",
USB_DEVICE_MAJOR);
goto error_cdev;
}
//注册usbdev_nb通知器
usb_register_notify(&usbdev_nb);
out:
return retval;
error_cdev:
unregister_chrdev_region(USB_DEVICE_DEV, USB_DEVICE_MAX);
goto out;
}
（9）初始化usb的hub，hub也是以USB驱动方式进行设计(/drivers/usb/core/hub.c)：
static struct usb_driver hub_driver = {
.name = "hub",
.probe = hub_probe,
.disconnect = hub_disconnect,
.suspend = hub_suspend,
.resume = hub_resume,
.reset_resume = hub_reset_resume,
.pre_reset = hub_pre_reset,
.post_reset = hub_post_reset,
.unlocked_ioctl = hub_ioctl,
.id_table = hub_id_table,
.supports_autosuspend = 1,
};
int usb_hub_init(void)
{
if (usb_register(&hub_driver) < 0) {
printk(KERN_ERR "%s: can't register hub driver
",
usbcore_name);
return -1;
}
/*
* The workqueue needs to be freezable to avoid interfering with
* USB-PERSIST port handover. Otherwise it might see that a full-speed
* device was gone before the EHCI controller had handed its port
* over to the companion full-speed controller.
*/
hub_wq = alloc_workqueue("usb_hub_wq", WQ_FREEZABLE, 0);
if (hub_wq)
return 0;
/* Fall through if kernel_thread failed */
usb_deregister(&hub_driver);
pr_err("%s: can't allocate workqueue for usb hub
", usbcore_name);
return -1;
}
（10）注册通用usb设备驱动usb_generic_driver，定义如下（/drivers/usb/core/generic.c）：
struct usb_device_driver usb_generic_driver = {
.name = "usb",
.probe = generic_probe,
.disconnect = generic_disconnect,
#ifdef CONFIG_PM
.suspend = generic_suspend,
.resume = generic_resume,
#endif
.supports_autosuspend = 1,
};
从前文描述可知：在linux启动过程中或者是在插入USB设备后，通常USB设备都是以设备的身份先与usb_generic_driver匹配，这时候由.probe指向的generic_peobe()会执行，当匹配成功后，会分裂出接口，当对接口调用device_add()后，会触发USB接口和USB接口驱动的匹配。
generic_probe()函数实现如下：
static int generic_probe(struct usb_device *udev)
{
int err, c;
/* Choose and set the configuration. This registers the interfaces
* with the driver core and lets interface drivers bind to them.
*/
if (udev->authorized == 0)
dev_err(&udev->dev, "Device is not authorized for usage
");
else {
//选择配置
c = usb_choose_configuration(udev);
if (c >= 0) {
//设置配置
err = usb_set_configuration(udev, c);
if (err && err != -ENODEV) {
dev_err(&udev->dev, "can't set config #%d, error %d
",
c, err);
/* This need not be fatal. The user can try to
* set other configurations. */
}
}
}
/* USB device state == configured ... usable */
usb_notify_add_device(udev); //通知usb设备添加，对应注册的回调函数会被执行。
return 0;
}
三、总结
本文主要描述linux-usb核心的初始化，usb核心是usb子系统的内层，其他的usb模块都基于这个内层再设计。除此之外，着重描述了usb_init()函数的具体执行步骤，如下图所示：
审核编辑：刘清
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