Linux系统USB设备枚举基础

在Linux&UVC驱动栈（https://www.usbzh.com/article/detail-1322.html ）一文中,描述符UVC设备的驱动栈。

从图可以知道，当USB物理设备插入USB集线器中后，USB主机控制器识别到USB物理设备，然后调用USBCore驱动的usb_new_dvice回调函数注册usb_device结构体，usb_device结构体会根据”drivers\usb\core\generic.c”来匹配，在其probe函数中会做如下操作。

• usb_choose_configuration
• usb_set_configuration
  • malloc usb_device
    • 设备usb_interface
    • 注册usb_interface

注册的usb_interface属于USB的逻辑设备，会根据其class,subclass等兼容ID信息或者PID_VID硬件信息注册其对应的类驱动程序。

文章参考：https://ldd.100ask.net/zh/12_USB/08_1.html

usbcore

usbcore驱动配置如下：

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,
};

可以看到，当有新的设备来临时也即当有新的USB物理设备插入时，会调用.probe的generic_probe函数。

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\n");
    else {
        //1.set_configuration
        c = usb_choose_configuration(udev);
        if (c >= 0) {
        //2.set configuration
            err = usb_set_configuration(udev, c);
            if (err && err != -ENODEV) {
                dev_err(&udev->dev, "can't set config #%d, error %d\n",
                    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);

    return 0;
}

选择配置

一个USB可以有多个配置描述符，但一次只能选择一个。
这里返回选择的配置描述符的索引

int usb_choose_configuration(struct usb_device *udev)
{
    struct usb_host_config  *best = null;
    struct usb_host_config* c = udev->config;
    int  num_configs = udev->descriptor.bNumConfigurations;
    for (i = 0; i < num_configs; (i++, c++)) {
        struct usb_interface_descriptor    *desc = NULL;

        /* It's possible that a config has no interfaces! */
        if (c->desc.bNumInterfaces > 0)
            desc = &c->intf_cache[0]->altsetting->desc;

        //uac3的特殊处理
        //微软非标准的网卡特殊处理
        //自定义设备处理
        else if (udev->descriptor.bDeviceClass !=USB_CLASS_VENDOR_SPEC &&
                (desc && desc->bInterfaceClass !=USB_CLASS_VENDOR_SPEC)){
            best = c;
            break;
        }
        else if (!best)
            best = c;
    }

    if (best) {
        i = best->desc.bConfigurationValue;
    } else {
        i = -1;
    }
    return i;
}

选择配置

选择配置详见usb_set_configuration函数。