字符设备驱动-同步互斥阻塞

我们想来达到一个目的：同一时刻，只能有一个应用程序打开/dev/buttons

一般的，我们想达到目的会想到下面这种方法：

static int canopen = 1;
static int sixth_drv_open(struct inode *inode,struct file *file)
{
if(--canopen != 0)
{
canopen++;
return -EBUSY;
}
}
static int sixth_drv_close (struct inode *inode, struct file *file)
{
canopen++;
}

假设程序A来调用，那么进入open函数，canopen = 0,if条件不成立；如果A没有释放，程序B来调用时候,canopen = -1,if条件成立，return -EBUSY;

原则上是可以实现，但是我们进行–canopen操作实际上转化程汇编使用三条指令(读->改->写)执行的，由于Linux是多任务编程的。当我们程序A进行–canopen的读取后，程序B刚好进行那么就有可能发生程序AB都能调用驱动！看下图分析：

下面来介绍三种解决上面bug的方法

一、原子操作：

原子操作指的是在执行过程中不会被别的代码路径所中断的操作。

常用原子操作函数举例：

atomic_t v = ATOMIC_INIT(0);     //定义原子变量v并初始化为0
atomic_read(atomic_t *v);        //返回原子变量的值
void atomic_inc(atomic_t *v);    //原子变量增加1
void atomic_dec(atomic_t *v);    //原子变量减少1
int atomic_dec_and_test(atomic_t *v); //自减操作后测试其是否为0，为0则返回true，否则返回false。

Makefile

KERN_DIR = /work/system/linux-2.6.22.6

all:
make -C $(KERN_DIR) M=`pwd` modules

clean:
make -C $(KERN_DIR) M=`pwd` modules clean
rm -rf modules.order

obj-m += sixth_drv.o

驱动函数:sixth_drv.c

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/delay.h>
#include <linux/irq.h>
#include <asm/uaccess.h>
#include <asm/irq.h>
#include <asm/io.h>
#include <asm/arch/regs-gpio.h>
#include <asm/hardware.h>
#include <linux/poll.h>

static struct fasync_struct *button_async;
static struct class *sixthdrv_class;
static struct class_device *sixthdrv_class_dev;

static DECLARE_WAIT_QUEUE_HEAD(button_waitq);

/* 中断事件标志, 中断服务程序将它置1，s3c24xx_sixth_read将它清0 */
static volatile int ev_press = 0;

volatile unsigned long *gpfcon;
volatile unsigned long *gpfdat;
volatile unsigned long *gpgcon;
volatile unsigned long *gpgdat;

struct pin_desc{
unsigned int pin;
unsigned int key_val;
};

/* 键值： 按下时，0x01、0x02、0x03 */
/* 键值： 松开时，0x81、0x82、0x83 */
static unsigned char key_val;

struct pin_desc pin_desc[3] = {
{S3C2410_GPF0,0X01},
{S3C2410_GPF2,0X02},
{S3C2410_GPG3,0X03},
};
atomic_t canopen = ATOMIC_INIT(1);//定义原子变量canopen并初始化为1

static irqreturn_t buttons_irq(int irq, void *dev_id)
{
struct pin_desc *pindesc = (struct pin_desc *)dev_id;
unsigned int pinval;
pinval =  s3c2410_gpio_getpin(pindesc->pin);
if(pinval)
{
/* 松开 */
key_val = 0x80 | (pindesc->key_val);
*gpfdat |= ((1<<4) | (1<<5) | (1<<6));
}
else
{
/* 按下 */
key_val = pindesc->key_val;
*gpfdat &= ~((1<<4) | (1<<5) | (1<<6));
}
ev_press = 1;                /* 表示中断发生了 */
wake_up_interruptible(&button_waitq);   /* 唤醒休眠的进程 */
kill_fasync (&button_async, SIGIO, POLL_IN);
return IRQ_RETVAL(IRQ_HANDLED);
}

static int sixth_drv_open(struct inode *inode,struct file *file)
{
if(!atomic_dec_and_test(&canopen))
{
atomic_inc(&canopen);
return -EBUSY;

}
/* 配置GPF0,2、GPG3为中断引脚 */
request_irq(IRQ_EINT0, buttons_irq,IRQT_BOTHEDGE,"s2",&pin_desc[0]);
request_irq(IRQ_EINT2, buttons_irq,IRQT_BOTHEDGE,"s3",&pin_desc[1]);
request_irq(IRQ_EINT11,buttons_irq,IRQT_BOTHEDGE,"s4",&pin_desc[2]);
/* 配置GPF4、5、6为输入引脚 */
*gpfcon &= ~((0x3<<4*2) | (0x3<<5*2) | (0x3<<6*2));
*gpfcon |=  ((1<<4*2) | (1<<5*2) | (1<<6*2));
return 0;
}

static ssize_t sixth_drv_read (struct file *file, char __user *buf, size_t count, loff_t *ppos)
{
//看用户需要读取的空间，和这里的是否相同
if(count != 1)
return -EINVAL;
/* 如果无按键动作发生，则进行休眠状态 */
/* 如果ev_press等于0，休眠 */
wait_event_interruptible(button_waitq,ev_press);

/* 如果有按键动作发生，则返回按键的值 */
copy_to_user(buf,&key_val,1);
ev_press = 0;

return 1;
}

static int sixth_drv_close (struct inode *inode, struct file *file)
{
atomic_inc(&canopen);
free_irq(IRQ_EINT0,  &pin_desc[0]);
free_irq(IRQ_EINT2,  &pin_desc[1]);
free_irq(IRQ_EINT11, &pin_desc[2]);
return 0;

}

static unsigned int sixth_drv_poll(struct file *file, struct poll_table_struct *wait)
{
unsigned int mask = 0;

poll_wait(file, &button_waitq, wait);

if (ev_press)
mask |= POLLIN | POLLRDNORM;
return mask;
}

static int sixth_drv_fasync (int fd, struct file *filp, int on)
{
printk("driver: sixth_drv_fasync\n");
return fasync_helper (fd, filp, on, &button_async);
}

static struct file_operations sixth_drv_fops = {
.owner   = THIS_MODULE, /* 这是一个宏，推向编译模块时自动创建的__this_module变量 */
.open    = sixth_drv_open,
.read    = sixth_drv_read,
.release = sixth_drv_close,
.poll    = sixth_drv_poll,
.fasync  = sixth_drv_fasync,
};

int major;

static int sixth_drv_init(void)
{
major = register_chrdev(0,"sixth_drv",&sixth_drv_fops);
sixthdrv_class = class_create(THIS_MODULE,"sixthdrv");
sixthdrv_class_dev = class_device_create(sixthdrv_class,NULL,MKDEV(major,0),NULL,"buttons");
gpfcon = (volatile unsigned long *)ioremap(0x56000050,16);
gpfdat = gpfcon + 1;
gpgcon = (volatile unsigned long *)ioremap(0x56000060,16);
gpgdat = gpgcon + 1;
return 0;
}

static int sixth_drv_exit(void)
{
unregister_chrdev(major,"sixth_drv");
class_device_unregister(sixthdrv_class_dev);
class_destroy(sixthdrv_class);
iounmap(gpfcon);
iounmap(gpgcon);
return 0;
}

module_init(sixth_drv_init);
module_exit(sixth_drv_exit);
MODULE_LICENSE("GPL");

驱动测试函数：sixthdrvtest.c

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <poll.h>
#include <signal.h>
#include <sys/types.h>
#include <unistd.h>

/*
* sixthdrvtest
*/
int fd;

void czg_signal_handler(int signum)
{
unsigned char key_val = 0;
read(fd,&key_val,1);
printf("key_val: 0x%x\n",key_val);
}

int main(int argc, char **argv)
{
int ret;
int oflags;
signal(SIGIO,czg_signal_handler);
fd = open("/dev/buttons",O_RDWR);
if(fd < 0)
{
printf("can't open!\n");
return -1;
}
fcntl(fd,F_SETOWN,getpid()); // 告诉内核，发给谁
oflags = fcntl(fd,F_GETFL);
fcntl(fd,F_SETFL,oflags | FASYNC); // 改变fasync标记，
//最终会调用到驱动的faync > fasync_helper：初始化/释放fasync_struct
//然后当按键按下时候，在irqreturn_t buttons_irq中断处理中调用kill_fasync
while(1)
{
sleep(1000);
}
return 0;
}

测试：

make
arm-linux-gcc -o sixthdrvtest sixthdrvtest.c
cp sixthdrvtest sixth_drv.ko /work/nfs_root/czg
insmod sixth_drv.ko
./sixthdrvtest &

二、信号量：

信号量（semaphore）是用于保护临界区的一种常用方法，只有得到信号量的进程才能执行临界区代码。当获取不到信号量时，进程进入休眠等待状态。

//定义信号量
struct semaphore sem;
//初始化信号量
void sema_init (struct semaphore *sem, int val);
void init_MUTEX(struct semaphore *sem);//初始化为0
static DECLARE_MUTEX(button_lock);     //定义互斥锁
//获得信号量
void down(struct semaphore * sem);
int down_interruptible(struct semaphore * sem);
int down_trylock(struct semaphore * sem);
//释放信号量
void up(struct semaphore * sem);

代码只需要在上面的驱动程序sixth_drv.c稍做修改

static DECLARE_MUTEX(button_lock);     //定义互斥锁
/* 获 取 信 号 量 */
down(&button_lock);
/* 释放信号量 */
up(&button_lock);

三、阻塞：

阻塞操作：

是指在执行设备操作时若不能获得资源则挂起进程，直到满足可操作的条件后再进行操作。被挂起的进程进入休眠状态，被从调度器的运行队列移走，直到等待的条件被满足。

非阻塞操作：

进程在不能进行设备操作时并不挂起，它或者放弃，或者不停地查询，直至可以进行操作为止。



代码在上面的驱动程序sixth_drv.c稍做修改

static int sixth_drv_open(struct inode *inode,struct file *file)
{
if (file->f_flags &  O_NONBLOCK)
{
if (down_trylock(&button_lock))
return -EBUSY;
}
else
{
/* 获 取 信 号 量 */
down(&button_lock);
}
}
/*************************************************/
static ssize_t sixth_drv_read (struct file *file, char __user *buf, size_t count, loff_t *ppos)
{
//看用户需要读取的空间，和这里的是否相同
if(count != 1)
return -EINVAL;
if(file->f_flags & O_NONBLOCK)
{
if(!ev_press)
return -EAGAIN;
}
else
{
/* 如果无按键动作发生，则进行休眠状态 */
/* 如果ev_press等于0，休眠 */
wait_event_interruptible(button_waitq,ev_press);
}
}

阻塞：

驱动测试代码：sixthdrvtest.c

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <poll.h>
#include <signal.h>
#include <sys/types.h>
#include <unistd.h>

/*
* sixthdrvtest
*/
int fd;

void czg_signal_handler(int signum)
{
unsigned char key_val = 0;
read(fd,&key_val,1);
printf("key_val: 0x%x\n",key_val);
}

int main(int argc, char **argv)
{
int ret;
int oflags;
unsigned char key_val = 0;
//signal(SIGIO,czg_signal_handler);
fd = open("/dev/buttons",O_RDWR);
if(fd < 0)
{
printf("can't open!\n");
return -1;
}
//fcntl(fd,F_SETOWN,getpid()); // 告诉内核，发给谁
//oflags = fcntl(fd,F_GETFL);
//fcntl(fd,F_SETFL,oflags | FASYNC); // 改变fasync标记，
//最终会调用到驱动的faync > fasync_helper：初始化/释放fasync_struct
//然后当按键按下时候，在irqreturn_t buttons_irq中断处理中调用kill_fasync
while(1)
{
read(fd,&key_val,1);
printf("key_val: 0x%x\n",key_val);
//sleep(1000);
}
return 0;
}

非阻塞：

驱动测试代码：sixthdrvtest.c

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <poll.h>
#include <signal.h>
#include <sys/types.h>
#include <unistd.h>

/*
* sixthdrvtest
*/
int fd;

void czg_signal_handler(int signum)
{
unsigned char key_val = 0;
read(fd,&key_val,1);
printf("key_val: 0x%x\n",key_val);
}

int main(int argc, char **argv)
{
int ret;
int oflags;
unsigned char key_val = 0;
//signal(SIGIO,czg_signal_handler);
fd = open("/dev/buttons",O_RDWR | O_NONBLOCK);
if(fd < 0)
{
printf("can't open!\n");
return -1;
}
//fcntl(fd,F_SETOWN,getpid()); // 告诉内核，发给谁
//oflags = fcntl(fd,F_GETFL);
//fcntl(fd,F_SETFL,oflags | FASYNC); // 改变fasync标记，
//最终会调用到驱动的faync > fasync_helper：初始化/释放fasync_struct
//然后当按键按下时候，在irqreturn_t buttons_irq中断处理中调用kill_fasync
while(1)
{
ret = read(fd,&key_val,1);
printf("key_val: 0x%x,ret = %d\n",key_val,ret);
sleep(4);
}
return 0;
}