一、异步通知概念：

　　异步通知是指：一旦设备就绪，则主动通知应用程序，应用程序根本就不需要查询设备状态，类似于中断的概念，一个进程收到一个信号与处理器收到一个中断请求可以说是一样的。信号是异步的，一个进程不必通过任何操作来等待信号的到达。下面我们就看一下在linux中机制的实现方式。

　　在linux中，异步通知是使用信号来实现的，而在linux，大概有30种信号，比如大家熟悉的ctrl+c的SIGINT信号，进程能够忽略或者捕获除过SIGSTOP和SIGKILL的全部信号，当信号背捕获以后，有相应的signal()函数来捕获信号，函数原型：sighandler_t signal(int signum, sighandler_t handler); 第 一个参数就是指定的信号的值，而第二个参数便是此信号的信号处理函数，当为SIG_IGN,表示信号被忽略，当为SIG_DFL时，表示采用系统的默认方 式来处理该信号。当然，信号处理函数也可以自己定义。当signal()调用成功后，返回处理函数handler值，调用失败后返回SIG_ERR。

二、信号处理要点：

①、注册信号处理函数：应用注册
②、发送者：驱动drv
③、接受者：应用app
④、发送方法：kill_fasync (&button_async, SIGIO, POLL_IN);

三、 原子操作：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：
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);  //自减操作后测试结果，0返回真，否则返回假 

四、信号量
1.定义：struct semaphore sem:
2.初始化：void sema_init(struct semaphore *sem, int val);
　　　　　void init_MUTEX(stuct semaphore *sem); //初始化为0
     static DECLARE_MUTEX(button_lock); //定义互斥锁

3.获得信号量: void down(struct semaphore *sem);
　　　　　　   int down_interruptible(struct semaphore *sem);
　　　　　　　int down_trylock(struct semaphore *sem);
4.释放信号量：

　　　　　　 void up(struct semaphore *sem);

五、阻塞：执行设备操作时若不能获得资源，则挂起进入休眠状态，被从调度器的运行队列移走，直到条件满足。

　　非阻塞：执行设备操作时若不能获得资源，释放或查询等待条件满足。　：　fd = open("/dev/buttons", O_RDWR | O_NONBLOCK);

六、示例代码：
1.驱动代码: signal_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 class *signaldrv_class;
static struct class_device *signaldrv_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，signal_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 signal_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 signal_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 signal_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 signal_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 signal_drv_fasync (int fd, struct file *filp, int on)
{
printk("driver: signal_drv_fasync\n");
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 = signal_drv_open,
.read = signal_drv_read,
.release = signal_drv_close,
.poll = signal_drv_poll,
.fasync = signal_drv_fasync,
};

int major;
static int signal_drv_init(void)
{
major = register_chrdev(0, "signal_drv", &sencod_drv_fops);

signaldrv_class = class_create(THIS_MODULE, "signal_drv");

signaldrv_class_dev = class_device_create(signaldrv_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 signal_drv_exit(void)
{
unregister_chrdev(major, "signal_drv");
class_device_unregister(signaldrv_class_dev);
class_destroy(signaldrv_class);
iounmap(gpfcon);
iounmap(gpgcon);
return 0;
}

module_init(signal_drv_init);

module_exit(signal_drv_exit);

MODULE_LICENSE("GPL");

2.测试代码：signaltest.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>
#include <fcntl.h>

/* sixthdrvtest
*/
int fd;

void my_signal_fun(int signum)
{
unsigned char key_val;
read(fd, &key_val, 1);
printf("key_val: 0x%x\n", 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 = %d\n", key_val, ret);
sleep(5);
}

return 0;
}

3.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 += signal_drv.o

rm -rf modules.order

obj-m+= signal_drv.o