Linux kernel: USB driver编写入门（三）_usb_class_driver

在Linux kernel: USB driver编写入门（二）中写到struct usb_class_driver的变量usb_cd需要赋值。故添加代码如下：

#define MYUSB_MINOR_BASE    250
static int myusb_open(struct inode *inode, struct file *file)
{
    pr_info("%s\n", __func__);
    return 0;
}
 
static int myusb_release(struct inode *inode, struct file *file)
{
    pr_info("%s\n", __func__);
    return 0;
}
 
static ssize_t myusb_write(struct file *file, const char *user_buffer,
                          size_t count, loff_t *ppos)
{
    pr_info("%s\n", __func__);
    return count;
}
 
static ssize_t myusb_read(struct file *file, char *buffer, size_t count,
                            loff_t *ppos)
{
    pr_info("%s\n", __func__);
    return 0;
}
 
static const struct file_operations myusb_fops = {
    .owner = THIS_MODULE,
    .open = myusb_open,
    .release = myusb_release,
    .read = myusb_read,
    .write = myusb_write,
};
 
static struct usb_class_driver usb_cd = {
    .name = "ActionsUSB%d",
    .fops = &myusb_fops,
    .minor_base = MYUSB_MINOR_BASE,
};

因为C语言编译器是按照源文件的代码顺序来编译的，所以引用的变量/常量或者函数需要先声明或者定义。

按照好理解的逻辑，我们先来看最下面，usb_cd的定义。我们先复习一下在(二）中提到的struct usb_class_driver的定义：

struct usb_class_driver {
    char *name;
    char *(*devnode)(struct device *dev, umode_t *mode);
    const struct file_operations *fops;
    int minor_base;
};

这里devnode函数指针先不给它赋值，因为根据（二），在usb_register_dev函数中只用到了struct usb_class_driver中的name, fops, minor_base。其中，最重要的就是fops，它定义了文件操作，因为Linux把几乎所有设备都当成file来看待。我这里的文件操作也很简单，就是把操作对应的函数名称在kernel消息里打印出来。

然后，在当前目录下make

$ make
make -C /lib/modules/5.19.0-rc3+/build M=/home/minipc/linux_usb_driver modules
make[1]: Entering directory '/home/minipc/stable_rc/linux-5.19.0'
  CC [M]  /home/minipc/linux_usb_driver/usb_test_drv.o
  MODPOST /home/minipc/linux_usb_driver/Module.symvers
  CC [M]  /home/minipc/linux_usb_driver/usb_test_drv.mod.o
  LD [M]  /home/minipc/linux_usb_driver/usb_test_drv.ko
  BTF [M] /home/minipc/linux_usb_driver/usb_test_drv.ko
make[1]: Leaving directory '/home/minipc/stable_rc/linux-5.19.0'

理想状况下应该是如上，没有error，没有warning。如果有问题，请按照（一）的内容排查。

如果USB设备已经连接Linux主机，请拔出，并重启Linux主机。如果在Linux主机本次运行期间曾经插上过USB设备，也需要在没有连接USB设备的状态下重启主机。

然后，load这个模块，用lsmod| grep usb和dmesg|tail查看是否load成功。

然后插上USB设备。运行dmesg。

$ dmesg| tail
[  400.931170] usb 2-4: New USB device found, idVendor=10d6, idProduct=1101, bcdDevice= 1.00
[  400.931183] usb 2-4: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[  400.931189] usb 2-4: Product: USB CARDREADER
[  400.931194] usb 2-4: Manufacturer: ACTIONS
[  400.931198] usb 2-4: SerialNumber: ㉕捤稰眷㕳愳㤷湲
[  400.933716] USB info 0 now probed: (10d6:1101)
[  400.933726] ID->bNumEndpoints:02
[  400.933729] ID->bInterfaceClass:08
[  400.933874] Minor number = 1
[  401.011611] usbcore: registered new interface driver usb-storage

靠近底部的四行正是usb_drv_probe的输出kernel消息：

    printk(KERN_INFO "USB info %d now probed: (%04x:%04x)\n", interface_desc->desc.bInterfaceNumber, id->idVendor, id->idProduct);
	printk(KERN_INFO "ID->bNumEndpoints:%02x\n", interface_desc->desc.bNumEndpoints);
	printk(KERN_INFO "ID->bInterfaceClass:%02x\n", interface_desc->desc.bInterfaceClass);
 
	ret = usb_register_dev(interface,&usb_cd);
	if(ret)
	{
		printk(KERN_INFO "usb_register_dev erro: %d\n", ret);
	}
	else
	{
		printk(KERN_INFO "Minor number = %d\n", interface->minor);
	}

 从log消息来看，usb_register_dev调用成功。$ls /dev/ 可以看到ActionsUSB1, 恰恰是struct usb_class_driver中的name。

static struct usb_class_driver usb_cd = {
    .name = "ActionsUSB%d",
    .fops = &myusb_fops,
    .minor_base = MYUSB_MINOR_BASE,
};

向该设备写数据，如同一个文件write操作。

$ sudo sh -c "echo hello>/dev/ActionsUSB1"

成功后应该立即返回。然后再查看dmesg消息。

$ dmesg | tail
[  400.931194] usb 2-4: Manufacturer: ACTIONS
[  400.931198] usb 2-4: SerialNumber: ㉕捤稰眷㕳愳㤷湲
[  400.933716] USB info 0 now probed: (10d6:1101)
[  400.933726] ID->bNumEndpoints:02
[  400.933729] ID->bInterfaceClass:08
[  400.933874] Minor number = 1
[  401.011611] usbcore: registered new interface driver usb-storage
[ 1261.675975] myusb_open
[ 1261.675995] myusb_write
[ 1261.675998] myusb_release

最后三行是这个写操作，open, write , release，输出Log消息恰恰是对应文件操作函数的名称。

向这个设备读数据，如同一个文件读操作

$ sudo cat /dev/ActionsUSB1

 再查看dmesg消息：

$ dmesg | tail
[  400.933726] ID->bNumEndpoints:02
[  400.933729] ID->bInterfaceClass:08
[  400.933874] Minor number = 1
[  401.011611] usbcore: registered new interface driver usb-storage
[ 1261.675975] myusb_open
[ 1261.675995] myusb_write
[ 1261.675998] myusb_release
[ 1893.611703] myusb_open
[ 1893.611721] myusb_read
[ 1893.611736] myusb_release

同样是输出最后三行消息。

到这里，该usb driver已经实现了从Linux kernel层到user space的一个抽象。可以说具备了基本的驱动框架。

最后，把该USB设备从Linux主机中拔出。查看dmesg消息：

 $ dmesg | tail
[  400.933874] Minor number = 1
[  401.011611] usbcore: registered new interface driver usb-storage
[ 1261.675975] myusb_open
[ 1261.675995] myusb_write
[ 1261.675998] myusb_release
[ 1893.611703] myusb_open
[ 1893.611721] myusb_read
[ 1893.611736] myusb_release
[ 2146.812223] usb 2-4: USB disconnect, device number 4
[ 2146.812363] Disconneced and Release the MINOR number 1

最后一行消息 Disconneced and Release the MINOR number 1 恰恰是usb_drv_disconnect函数中的打印消息：

static void usb_drv_disconnect(struct usb_interface *interface)
{
	printk(KERN_INFO "Disconneced and Release the MINOR number %d\n", interface->minor);
	usb_deregister_dev(interface, &usb_cd);
}

至此，整个usb驱动开发的最基础步骤已经完成。