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

0

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

root@Android:/proc # cat hello

5

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

5

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

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

0

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