嵌入式Linux设备驱动程序开发指南8（字符设备驱动）——读书笔记

八、字符设备驱动

8.1 概述

操作系统被设计为了对设备使用者隐藏底层硬件细节。Linux需要一种能够将数据从内核态传递给用户态的机制。这种传递通过设备节点来处理&＃xff0c;即虚拟文件&＃xff08;/dev/xxx&＃xff09;。当用户获取设备节点时候&＃xff0c;内核将底层数据流拷贝到应用程序的内存空间&＃xff0c;当用户写设备节点时候&＃xff0c;内核将应用程序提供的数据拷贝到数据缓存区&＃xff0c;这些数据最终送到底层硬件&＃xff0c;这些虚拟文件可以被用户应用程序通过标准的系统调用打开、读取、写入。
Linux应用程序分为三类&＃xff1a;字符设备、块设备、网络设备。

字符设备&＃xff1a;
是常见的设备&＃xff0c;这种设备的读写是直接进行&＃xff0c;无需缓冲区&＃xff0c;如键盘、鼠标、LED灯、显示器、串口、打印机等&＃xff1b;
块设备&＃xff1a;
读写以块大小为单位&＃xff0c;一次读写整数倍的块大小&＃xff0c;如512或者1024字节。
网络设备&＃xff1a;
通过BSD套接字接口和网络子系统来访问。


从应用的角度看&＃xff0c;一个字符设备本质上是一个文件。通过file_operations数据结构来描述的各种操作并注册。其中&＃xff0c;file_operations定义在

include/linux.fs.h


其中一种从用户态和内核态实现数据交互的函数是&＃xff1a;

copy_from_user()
copy_to_user()


Linux设备读写返回值如果是负数&＃xff0c;则表示出错。出错码定义在

linux/errno.h


Linux设备通过两个设备号来标识&＃xff1a;

主设备号
从设备号


8.2 helloworld字符设备模块

准备工作&＃xff1a;
Linux设备/dev下有很多文件&＃xff0c;不管以后你就设备是否存在&＃xff0c;软件都会生成对应文件。创建/dev/xxx方式总共有&＃xff1a;

静态设备创建&＃xff1b;
使用设备文件系统和杂项框架&＃xff1b;


静态设备创建&＃xff0c;一般是由MAKEDEV&＃xff08;mknod程序&＃xff09;完成&＃xff0c;也可以手动创建如&＃xff1a;

mknod /dev/mydev c 202 108 //备注&＃xff1a;主设备号202 从设备号108


本文将通过ioctl_test应用程序和helloworld驱动调试。
在内核中&＃xff0c;cdev数据结构表示一个字符类型的设备&＃xff0c;该数据结构用于将设备注册到系统中。
字符设备的注册/注销是通过主从设备号来实现。dev_t用来保存设备的标识信息&＃xff08;主从设备号&＃xff09;&＃xff0c;主从设备号通过MKDEV宏来获取.
静态分配和释放一个设备标识&＃xff1a;

#include register_chrdev_region()
unregister_chrdev_region()


动态分配和释放一个设备标识&＃xff1a;
分配一组字符设备编号&＃xff0c;

alloc_chrdev_region()


分配完字符设备之后&＃xff0c;
初始化字符设备&＃xff1a;

cdev_init()


字符设备注册到内核&＃xff1a;

cdev_add()


通过

/proc/devices


8.2.1 helloworld_char_driver.c代码

#include /* add header files to support character devices */
#include
#include /* define mayor number */
#define MY_MAJOR_NUM 202 //主设备号static struct cdev my_dev;static int my_dev_open(struct inode *inode, struct file *file)
{pr_info("my_dev_open() is called.\n");return 0;
}static int my_dev_close(struct inode *inode, struct file *file)
{pr_info("my_dev_close() is called.\n");return 0;
}static long my_dev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{pr_info("my_dev_ioctl() is called. cmd &＃61; %d, arg &＃61; %ld\n", cmd, arg);return 0;
}/* declare a file_operations structure */
//创建一个my_dev_fops的文件file_operations数据结构体
//定义:
// *打开
// *读取
// *写入
static const struct file_operations my_dev_fops &＃61; {.owner &＃61; THIS_MODULE,.open &＃61; my_dev_open,.release &＃61; my_dev_close,.unlocked_ioctl &＃61; my_dev_ioctl,
};static int __init hello_init(void)
{int ret;dev_t dev &＃61; MKDEV(MY_MAJOR_NUM, 0);pr_info("Hello world init\n");/* Allocate device numbers */ret &＃61; register_chrdev_region(dev, 1, "my_char_device");if (ret < 0){pr_info("Unable to allocate mayor number %d\n", MY_MAJOR_NUM);return ret;}/* Initialize the cdev structure and add it to the kernel space */cdev_init(&my_dev, &my_dev_fops);ret&＃61; cdev_add(&my_dev, dev, 1);if (ret < 0){unregister_chrdev_region(dev, 1);pr_info("Unable to add cdev\n");return ret;}return 0;
}static void __exit hello_exit(void)
{pr_info("Hello world exit\n");cdev_del(&my_dev);unregister_chrdev_region(MKDEV(MY_MAJOR_NUM, 0), 1);
}module_init(hello_init);
module_exit(hello_exit);MODULE_LICENSE("GPL");
MODULE_AUTHOR(" ");
MODULE_DESCRIPTION("This is a module that interacts with the ioctl system call");


8.2.2 Makefile

obj-m &＃43;&＃61; helloworld_sam_char_driver.oKERNEL_DIR ?&＃61; $(HOME)/my-linux-samall:make -C $(KERNEL_DIR) \ARCH&＃61;arm CROSS_COMPILE&＃61;arm-poky-linux-gnueabi- \SUBDIRS&＃61;$(PWD) modulesclean:make -C $(KERNEL_DIR) \ARCH&＃61;arm CROSS_COMPILE&＃61;arm-poky-linux-gnueabi- \SUBDIRS&＃61;$(PWD) cleandeploy:scp *.ko root&＃64;10.0.0.10:


8.2.3 ioctl_char_test.c

/** A simple application to call helloworld character driver&＃39;s ioctl.**/
#include
#include
#include
#include int main(void)
{/* First you need run "mknod /dev/mydev c 202 0" to create /dev/mydev */int my_dev &＃61; open("/dev/mydev", 0);if (my_dev < 0) {perror("Fail to open device file: /dev/mydev.");} else {ioctl(my_dev, 100, 110); // cmd &＃61; 100, arg &＃61; 110.close(my_dev);}return 0;
}


8.3 将模块添加到内核

insmod helloworld_char_driver.ko
cat /proc/devices //查看申请的主设备号202 "my_char_device"
ls -l /dev
mknod /dev/mydev c 202 0 //在/dev下&＃xff0c;创建mydev
./ioctl_test
rmmod helloworld_char_driver.ko


8.4 将模块添加到内核构建

驱动构建成为可加载的内核模块。将驱动作为内核代码树的一部分构建到内核的二进制镜像中。在内核目录新增helloworld_char_driver.c&＃xff1a;

/drivers/char/


编辑~/my-linux-imx/drivers/char/Kconfig&＃xff0c;并在末尾添加&＃xff1a;

config HELLOWORLDtristate "My helloworld driver"default nhelpThe simplest driver.


编辑~/my-linux-imx/drivers/char/Makefile&＃xff0c;并在末尾添加&＃xff1a;

obj-$(CONFIG_HELLOWORLD) &＃43;&＃61; helloworld_char_driver.o


修改完Kconfig和Makefile之后&＃xff0c;helloworld_char_driver将成为内核的一部分&＃xff0c;而不是一个可加载模块。
接下来是构建内核镜像步骤&＃xff1a;
打开menuconfig窗口&＃xff0c;选择main munu -> Device Driver -> Character devices -> My simple helloworld driver 选择*&＃xff0c;然后保存退出GUI.
检查&＃xff1a;

make menuconfig ARCH&＃61;arm


打开内核根目录的.config文件将看到CONFIG_HELLOWORLD符号以及被添加进去了。
编译新镜像&＃xff1a;

source /opt/fsl-imx-x11/4.9.11-1.0.0/environment-setup-cortexa7hf
make zImage
cp /arch/arm/boot/zImage /var/lib/tftpboot/


测试调试&＃xff0c;启动系统后查看&＃xff1a;

cat /proc/devices
mknod /dev/mydev c 202 0 //申请主设备号
./ioctl_test


8.5 class字符设备

8.5.1 使用设备文件系统创建设备文件

在Linux2.6之后&＃xff0c;增加了sysfs即虚拟文件系统。sysfs的任务是方便用户态使用查看系统硬件。sysfs是通过Linux内核配置CONFIG_SYSFS来打开并准备的。内核通过设备文件系统创建设备之后&＃xff0c;会给udevd发送一个uevent.

8.5.2 class字符设备模块

在/sys/class目录下&＃xff0c;有一个类别名&＃xff0c;而每个设备都有一个设备名。
当驱动register_chrdev_region()函数执行之后&＃xff0c;并没有在/class/sys/生成设备文件&＃xff0c;而是使用如下&＃xff1a;
驱动使用内核API创建/销毁类别&＃xff1a;

#include class_create() //在/sys/class/目录下创建自己的类别名&＃xff0c;如&＃xff1a;/sys/class/hello_class
class_destroy()


创建设备节点内核API&＃xff1a;

#include device_create() //在/dev下创建对应device node
device_destory()


helloworld_class_driver.c代码&＃xff1a;


#include
#include
#include
#include #define DEVICE_NAME "mydev"
#define CLASS_NAME "hello_class"static struct class* helloClass;
static struct cdev my_dev;
dev_t dev;static int my_dev_open(struct inode *inode, struct file *file)
{pr_info("my_dev_open() is called.\n");return 0;
}static int my_dev_close(struct inode *inode, struct file *file)
{pr_info("my_dev_close() is called.\n");return 0;
}static long my_dev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{pr_info("my_dev_ioctl() is called. cmd &＃61; %d, arg &＃61; %ld\n", cmd, arg);return 0;
}/* declare a file_operations structure */
static const struct file_operations my_dev_fops &＃61; {.owner &＃61; THIS_MODULE,.open &＃61; my_dev_open,.release &＃61; my_dev_close,.unlocked_ioctl &＃61; my_dev_ioctl,
};static int __init hello_init(void)
{int ret;dev_t dev_no;int Major;struct device* helloDevice;pr_info("Hello world init\n");/* Allocate dynamically device numbers */ret &＃61; alloc_chrdev_region(&dev_no, 0, 1, DEVICE_NAME);if (ret < 0){pr_info("Unable to allocate Mayor number \n");return ret;}/* Get the device identifiers */Major &＃61; MAJOR(dev_no);dev &＃61; MKDEV(Major,0);pr_info("Allocated correctly with major number %d\n", Major);/* Initialize the cdev structure and add it to the kernel space */cdev_init(&my_dev, &my_dev_fops);ret &＃61; cdev_add(&my_dev, dev, 1);if (ret < 0){unregister_chrdev_region(dev, 1);pr_info("Unable to add cdev\n");return ret;}/* Register the device class */helloClass &＃61; class_create(THIS_MODULE, CLASS_NAME);if (IS_ERR(helloClass)){unregister_chrdev_region(dev, 1);cdev_del(&my_dev);pr_info("Failed to register device class\n");return PTR_ERR(helloClass);}pr_info("device class registered correctly\n");/* Create a device node named DEVICE_NAME associated a dev */helloDevice &＃61; device_create(helloClass, NULL, dev, NULL, DEVICE_NAME);if (IS_ERR(helloDevice)){class_destroy(helloClass);cdev_del(&my_dev);unregister_chrdev_region(dev, 1);pr_info("Failed to create the device\n");return PTR_ERR(helloDevice);}pr_info("The device is created correctly\n");return 0;
}static void __exit hello_exit(void)
{device_destroy(helloClass, dev); /* remove the device */class_destroy(helloClass); /* remove the device class */cdev_del(&my_dev);unregister_chrdev_region(dev, 1); /* unregister the device numbers */pr_info("Hello world with parameter exit\n");
}module_init(hello_init);
module_exit(hello_exit);MODULE_LICENSE("GPL");
MODULE_AUTHOR(" ");
MODULE_DESCRIPTION("This is a module that interacts with the ioctl system call");


测试调试&＃xff1a;

insmod helloworld_class_driver.ko //加载模块
ls /sys/class/
ls /sys/class/hello_class/my_dev
ls -l /dev
./ioctl_test
rmmod helloworld_clsss_driver.ko


8.6 杂项字符设备

Linux允许模块注册其各自的从设备编号。官方分配给杂项驱动的主设备编号是10。
杂项设备文件创建目录如下&＃xff1a;

/sys/class/misc


注册杂项设备编号&＃xff1a;

include/linux/miscdevice.h


杂项驱动导出的两个函数&＃xff1a;

#include
misc_register()
misc_deregister()


misc_sam_driver.c代码&＃xff1a;

#include
#include
#include static int my_dev_open(struct inode *inode, struct file *file)
{pr_info("my_dev_open() is called.\n");return 0;
}static int my_dev_close(struct inode *inode, struct file *file)
{pr_info("my_dev_close() is called.\n");return 0;
}static long my_dev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{pr_info("my_dev_ioctl() is called. cmd &＃61; %d, arg &＃61; %ld\n", cmd, arg);return 0;
}static const struct file_operations my_dev_fops &＃61; {.owner &＃61; THIS_MODULE,.open &＃61; my_dev_open,.release &＃61; my_dev_close,.unlocked_ioctl &＃61; my_dev_ioctl,
};/* declare & initialize struct miscdevice */
static struct miscdevice helloworld_miscdevice &＃61; {.minor &＃61; MISC_DYNAMIC_MINOR,.name &＃61; "mydev",.fops &＃61; &my_dev_fops,
};static int __init hello_init(void)
{int ret_val;pr_info("Hello world init\n");/* Register the device with the Kernel */ret_val &＃61; misc_register(&helloworld_miscdevice);if (ret_val !&＃61; 0) {pr_err("could not register the misc device mydev");return ret_val;}pr_info("mydev: got minor %i\n",helloworld_miscdevice.minor);return 0;
}static void __exit hello_exit(void)
{pr_info("Hello world exit\n");/* unregister the device with the Kernel */misc_deregister(&helloworld_miscdevice);}module_init(hello_init);
module_exit(hello_exit);MODULE_LICENSE("GPL");
MODULE_AUTHOR(" ");
MODULE_DESCRIPTION("This is the helloworld_char_driver using misc framework");


测试调试&＃xff1a;

insmod misc_sam_driver.ko
ls /sys/class/misc
ls /sys/class/misc/mydev/
ls /sys/class/misc/mydev/dev
./ioctl_test
rmmod misc_sam_driver.ko


感谢阅读&＃xff0c;祝君成功&＃xff01;
-by aiziyou