字符设备驱动程序之同步互斥阻塞

1. 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。

2. 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);

3. 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。

fd = open("...", O_RDWR | O_NONBLOCK);

例子：按键驱动程序

fifth_drv.c

#include

static struct class *sixthdrv_class;

static struct class_device *sixthdrv_class_dev;

volatile unsigned long *gpfcon;

volatile unsigned long *gpfdat;

volatile unsigned long *gpgcon;

volatile unsigned long *gpgdat;

static DECLARE_WAIT_QUEUE_HEAD(button_waitq);

/* 中断事件标志, 中断服务程序将它置1，sixth_drv_read将它清0 */

static volatile int ev_press = 0;

static struct fasync_struct *button_async;

struct pin_desc{

unsigned int pin;

unsigned int key_val;

};

/* 键值: 按下时, 0x01, 0x02, 0x03, 0x04 */

/* 键值: 松开时, 0x81, 0x82, 0x83, 0x84 */

static unsigned char key_val;

struct pin_desc pins_desc[4] = {

{S3C2410_GPF0, 0x01},

{S3C2410_GPF2, 0x02},

{S3C2410_GPG3, 0x03},

{S3C2410_GPG11, 0x04},

};

//static atomic_t canopen = ATOMIC_INIT(1); //定义原子变量并初始化为1

static DECLARE_MUTEX(button_lock); //定义互斥锁

* 确定按键值

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;

}

else

{

/* 按下 */

key_val = pindesc->key_val;

}

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 0

if (!atomic_dec_and_test(&canopen))

{

atomic_inc(&canopen);

return -EBUSY;

}

#endif

if (file->f_flags & O_NONBLOCK)

{

if (down_trylock(&button_lock))

return -EBUSY;

}

else

{

/* 获取信号量 */

down(&button_lock);

}

/* 配置GPF0,2为输入引脚 */

/* 配置GPG3,11为输入引脚 */

request_irq(IRQ_EINT0, buttons_irq, IRQT_BOTHEDGE, "S2", &pins_desc[0]);

request_irq(IRQ_EINT2, buttons_irq, IRQT_BOTHEDGE, "S3", &pins_desc[1]);

request_irq(IRQ_EINT11, buttons_irq, IRQT_BOTHEDGE, "S4", &pins_desc[2]);

request_irq(IRQ_EINT19, buttons_irq, IRQT_BOTHEDGE, "S5", &pins_desc[3]);

return 0;

}

ssize_t sixth_drv_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)

{

if (size != 1)

return -EINVAL;

if (file->f_flags & O_NONBLOCK)

{

if (!ev_press)

return -EAGAIN;

}

else

{

/* 如果没有按键动作, 休眠 */

wait_event_interruptible(button_waitq, ev_press);

}

/* 如果有按键动作, 返回键值 */

copy_to_user(buf, &key_val, 1);

ev_press = 0;

return 1;

}

int sixth_drv_close(struct inode *inode, struct file *file)

{

//atomic_inc(&canopen);

free_irq(IRQ_EINT0, &pins_desc[0]);

free_irq(IRQ_EINT2, &pins_desc[1]);

free_irq(IRQ_EINT11, &pins_desc[2]);

free_irq(IRQ_EINT19, &pins_desc[3]);

up(&button_lock);

return 0;

}

static unsigned sixth_drv_poll(struct file *file, poll_table *wait)

{

unsigned int mask = 0;

poll_wait(file, &button_waitq, wait); // 不会立即休眠

if (ev_press)

mask |= POLLIN | POLLRDNORM;

return mask;

}

static int sixth_drv_fasync (int fd, struct file *filp, int on)

{

printk("driver: sixth_drv_fasyncn");

return fasync_helper (fd, filp, on, &button_async);

}

static struct file_operations sencod_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", &sencod_drv_fops);

sixthdrv_class = class_create(THIS_MODULE, "sixth_drv");

sixthdrv_class_dev = class_device_create(sixthdrv_class, NULL, MKDEV(major, 0), NULL, "buttons"); /* /dev/buttons */

gpfcon = (volatile unsigned long *)ioremap(0x56000050, 16);

gpfdat = gpfcon + 1;

gpgcon = (volatile unsigned long *)ioremap(0x56000060, 16);

gpgdat = gpgcon + 1;

return 0;

}

static void 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");

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

sixthdrvtest.c

#include

/* sixthdrvtest

int fd;

void my_signal_fun(int signum)

{

unsigned char key_val;

read(fd, &key_val, 1);

printf("key_val: 0x%xn", key_val);

}

int main(int argc, char **argv)

{

unsigned char key_val;

int ret;

int Oflags;

//signal(SIGIO, my_signal_fun);

fd = open("/dev/buttons", O_RDWR | O_NONBLOCK);

if (fd < 0)

{

printf("can't open!n");

return -1;

}

//fcntl(fd, F_SETOWN, getpid());

//Oflags = fcntl(fd, F_GETFL);

//fcntl(fd, F_SETFL, Oflags | FASYNC);

while (1)

{

ret = read(fd, &key_val, 1);

printf("key_val: 0x%x, ret = %dn", key_val, ret);

sleep(5);

}

return 0;

}