Android 驱动之旅：第一章在Android 内核源代码工程中编写硬件驱动程序

在Ubuntu 上为Android 系统编写Linux 内核驱动程序

这里，我们不会为真实的硬件设备编写内核驱动程序。为了方便描述为Android 系统编写内核驱动程序的过程，我们使用一

个虚拟的硬件设备，这个设备只有一个4 字节的寄存器，它可读可写。想起我们第一次学习程序语言时，都喜欢用“Hello, World”

作为例子，这里，我们就把这个虚拟的设备命名为“hello”，而这个内核驱动程序也命名为hello 驱动程序。其实，Android 内

核驱动程序和一般Linux 内核驱动程序的编写方法是一样的，都是以Linux 模块的形式实现的，具体可参考前面Android 学习

启动篇一文中提到的Linux Device Drivers 一书。不过，这里我们还是从Android 系统的角度来描述Android 内核驱动程序的编

写和编译过程。

一.因为手边的硬件是Galaxy Nexus(maguro)，下载和编译code详见http://wiki.cyanogenmod.org/w/Maguro_Info

客官会问，为啥你要这样折腾啊？因为CM会把kernel的code一起编进来，不要再单独编译kernel。

二. 进入到kernel/drivers 目录，新建hello 目录，然后文件创建hello.h, hello.c, Makefile文件

hello.h

#ifndef _HELLO_Android_H_#define _HELLO_ANDROID_H_#include <linux/cdev.h>#include <linux/semaphore.h>#define HELLO_DEVICE_NODE_NAME "hello"#define HELLO_DEVICE_FILE_NAME "hello"#define HELLO_DEVICE_PROC_NAME "hello"#define HELLO_DEVICE_CLASS_NAME "hello"#endif

hello.c

#include <linux/module.h>#include <linux/init.h>#include <linux/types.h>#include <linux/fs.h>#include <linux/proc_fs.h>#include <linux/device.h>#include <asm/uaccess.h>#include <linux/cdev.h>    #include <linux/semaphore.h>#include "hello.h"struct hello_android_dev {    int val;    struct semaphore sem;    struct cdev dev;};static int hello_major = 0;static int hello_minor = 0;static struct class* hello_class = NULL;static struct hello_android_dev* hello_dev = NULL;static ssize_t hello_val_show(struct device* dev, struct device_attribute* attr, char* buf);static ssize_t hello_val_store(struct device* dev, struct device_attribute* attr, const char* buf, size_t count); static DEVICE_ATTR(val, S_IRUGO | S_IWUSR, hello_val_show, hello_val_store);static int hello_open(struct inode* inode, struct file* filp);static int hello_release(struct inode* inode, struct file* filp);static ssize_t hello_read(struct file* filp, char __user *buf, size_t count, loff_t* f_pos);static ssize_t hello_write(struct file* filp, const char __user *buf, size_t count, loff_t* f_pos);static struct file_operations hello_fops = {     .owner = THIS_MODULE,     .open = hello_open,     .release = hello_release,     .read = hello_read,     .write = hello_write,};static int hello_open(struct inode* inode, struct file* filp) {    struct hello_android_dev* dev;        dev = container_of(inode->i_cdev, struct hello_android_dev, dev);    filp->private_data = dev;    return 0;}static int hello_release(struct inode* inode, struct file* filp) {  return 0;}static ssize_t hello_read(struct file* filp, char __user *buf, size_t count, loff_t* f_pos){      printk("hello_read()\n");          ssize_t err = 0;    struct hello_android_dev* dev = filp->private_data;    if(down_interruptible(&(dev->sem)))     {        return -ERESTARTSYS;    }    if(count < sizeof(dev->val))     {        goto out;    }    if(copy_to_user(buf, &(dev->val), sizeof(dev->val)))     {         err = -EFAULT;         goto out;     }     err = sizeof(dev->val);out:    up(&(dev->sem));    return err;}static ssize_t hello_write(struct file* filp, const char __user *buf, size_t count, loff_t* f_pos) {    struct hello_android_dev* dev = filp->private_data;    ssize_t err = 0;        printk("hello_write()\n");    if(down_interruptible(&(dev->sem))) {        return -ERESTARTSYS;    }    if(count != sizeof(dev->val)) {        goto out;    }    if(copy_from_user(&(dev->val), buf, count)) {        err = -EFAULT;        goto out;    }    err = sizeof(dev->val);out:    up(&(dev->sem));    return err;}static ssize_t __hello_get_val(struct hello_android_dev* dev, char* buf) {      printk("__hello_get_val()\n");        int val = 0;    struct semaphore *p_sem = &(dev->sem);    if(down_interruptible(p_sem))    {          printk("sem is used,pls wait to free.\n");        return -ERESTARTSYS;    }    val = dev->val;    up(p_sem);        printk("get val: %d\n",val);    return snprintf(buf, PAGE_SIZE, "%d\n", val);}static ssize_t __hello_set_val(struct hello_android_dev* dev, const char* buf, size_t count) {      printk("__hello_set_val()\n");        int val = 0;    val = simple_strtol(buf, NULL, 10);        printk("set val: %d\n",val);    if(down_interruptible(&(dev->sem)))    {        return -ERESTARTSYS;    }        dev->val = val;    up(&(dev->sem));    return count;}static ssize_t hello_val_show(struct device* dev, struct device_attribute* attr, char* buf){      printk("hello_val_show()\n");        struct hello_android_dev* hdev = (struct hello_android_dev*)dev_get_drvdata(dev);    return __hello_get_val(hdev, buf);}static ssize_t hello_val_store(struct device* dev, struct device_attribute* attr, const char* buf, size_t count) {      printk("hello_val_store()\n");        struct hello_android_dev* hdev = (struct hello_android_dev*)dev_get_drvdata(dev);    return __hello_set_val(hdev, buf, count);}static ssize_t hello_proc_read(char* page, char** start, off_t off, int count, int* eof, void* data) {          printk("hello_proc_read()\n");        if(off > 0)     {        *eof = 1;        return 0;    }    return __hello_get_val(hello_dev, page);}static ssize_t hello_proc_write(struct file* filp, const char __user *buff, unsigned long len, void* data) {      printk("hello_proc_write()\n");         int err = 0;    char* page = NULL;    if(len > PAGE_SIZE) {    printk(KERN_ALERT"The buff is too large: %lu./n", len);        return -EFAULT;    }    page = (char*)__get_free_page(GFP_KERNEL);    if(!page) {        printk(KERN_ALERT"Failed to alloc page./n");        return -ENOMEM;    }    if(copy_from_user(page, buff, len)) {        printk(KERN_ALERT"Failed to copy buff from user./n");    err = -EFAULT;    goto out;    }    err = __hello_set_val(hello_dev, page, len);out:    free_page((unsigned long)page);    return err;}static void hello_remove_proc(void) {    remove_proc_entry(HELLO_DEVICE_PROC_NAME, NULL);}static void hello_create_proc(void){    struct proc_dir_entry* entry;    entry = create_proc_entry(HELLO_DEVICE_PROC_NAME, 0, NULL);    if(entry)    {        //entry->owner = THIS_MODULE;  entry has no "owner" member        entry->read_proc = hello_proc_read;        entry->write_proc = hello_proc_write;    }}static int __hello_setup_dev(struct hello_android_dev* dev) {    int err;    dev_t devno = MKDEV(hello_major, hello_minor);        printk("__hello_setup_dev()\n");    memset(dev, 0, sizeof(struct hello_android_dev));    cdev_init(&(dev->dev), &hello_fops);    dev->dev.owner = THIS_MODULE;    dev->dev.ops = &hello_fops;    err = cdev_add(&(dev->dev),devno, 1);    if(err)     {        return err;    }    //init_MUTEX(&(dev->sem));  2.6.25及以后的linux内核版本废除了init_MUTEX函数    sema_init(&(dev->sem),1);    dev->val = 0;    return 0; }static int __init hello_init(void){    int err = -1;    dev_t dev = 0;    struct device* temp = NULL;    printk(KERN_ALERT"Initializing hello device./n");    /*动态分配主设备和从设备号*/
    err = alloc_chrdev_region(&dev, 0, 1, HELLO_DEVICE_NODE_NAME);    if(err < 0) {        printk(KERN_ALERT"Failed to alloc char dev region./n");        goto fail;    }    hello_major = MAJOR(dev);    hello_minor = MINOR(dev);
    //     hello_dev = kmalloc(sizeof(struct hello_android_dev), GFP_KERNEL);    if(!hello_dev) {        err = -ENOMEM;        printk(KERN_ALERT"Failed to alloc hello_dev./n");        goto unregister;    }    err = __hello_setup_dev(hello_dev);    if(err) {        printk(KERN_ALERT"Failed to setup dev: %d./n", err);        goto cleanup;    }    hello_class = class_create(THIS_MODULE, HELLO_DEVICE_CLASS_NAME);    if(IS_ERR(hello_class)) {        err = PTR_ERR(hello_class);        printk(KERN_ALERT"Failed to create hello class./n");        goto destroy_cdev;    }    temp = device_create(hello_class, NULL, dev, "%s", HELLO_DEVICE_FILE_NAME);    if(IS_ERR(temp)) {        err = PTR_ERR(temp);         printk(KERN_ALERT"Failed to create hello device.");         goto destroy_class;    }    err = device_create_file(temp, &dev_attr_val);    if(err < 0) {        printk(KERN_ALERT"Failed to create attribute val.");        goto destroy_device;    }    dev_set_drvdata(temp, hello_dev);    hello_create_proc();    printk(KERN_ALERT"Succedded to initialize hello device./n");    return 0;destroy_device:    device_destroy(hello_class, dev);destroy_class:    class_destroy(hello_class);destroy_cdev:    cdev_del(&(hello_dev->dev));cleanup:    kfree(hello_dev);unregister:    unregister_chrdev_region(MKDEV(hello_major, hello_minor), 1);fail:    return err;}static void __exit hello_exit(void) {    dev_t devno = MKDEV(hello_major, hello_minor);    printk(KERN_ALERT"Destroy hello device./n");    hello_remove_proc();    if(hello_class) {        device_destroy(hello_class, MKDEV(hello_major, hello_minor));        class_destroy(hello_class);    }    if(hello_dev) {        cdev_del(&(hello_dev->dev));        kfree(hello_dev);    }    unregister_chrdev_region(devno, 1);}MODULE_LICENSE("GPL");MODULE_DESCRIPTION("First Android Driver");module_init(hello_init);module_exit(hello_exit);

kernel/drivers/hello/Makefile

obj-y    += hello.o

三、修改drivers/Makefile 文件，添加一行：

obj-$(CONFIG_HELLO) += hello/

四、验证测试

验证hello 驱动程序是否已经正常安装：

linuxidc@www.linuxdc.com:~/Android$ adb shell

进入到dev 目录，可以看到hello 设备文件：

root@Android:/ # cd dev

root@Android:/dev # ls

进入到proc 目录，可以看到hello 文件：

root@Android:/ # cd proc

root@Android:/proc # ls

访问hello 文件的值：

root@Android:/proc # cat hello

root@Android:/proc # echo '5' > hello

root@Android:/proc # cat hello

进入到sys/class 目录，可以看到hello 目录：

root@Android:/ # cd sys/class

root@Android:/sys/class # ls

进入到hello 目录，可以看到hello 目录：

root@Android:/sys/class # cd hello

root@Android:/sys/class/hello # ls

进入到下一层hello 目录，可以看到val 文件：

root@Android:/sys/class/hello # cd hello

root@Android:/sys/class/hello/hello # ls

访问属性文件val 的值：

root@Android:/sys/class/hello/hello # cat val

root@Android:/sys/class/hello/hello # echo '0' > val

root@Android:/sys/class/hello/hello # cat val

至此，我们的hello 内核驱动程序就完成了，并且验证一切正常。这里我们采用的是系统提供的方法和驱动程序进行

交互，也就是通过proc 文件系统和devfs 文件系统的方法，下一篇文章中，我们将通过自己编译的C 语言程序来访问/dev/hello

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