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 <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 *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_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    =  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 <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;
}