驱动程序之_1_字符设备_5_原子操作

有的程序可能为了避免错误，不能被线程调度机制所打断，这时候就用到了原子操作，在进行原子操作的过程中，不会被任何事情所打断
例如，我们想要让驱动设备只能被打开一次，通常的想法就是
1、设置全局标志位
2、在open函数中对其运算，再判断是否是某个值，从而决定是否打开设备
3、在close函数中再对其进行与入口函数中相反的运算
这种做法不严谨，在系统中，应用程序只占据一个进程，系统每一定的时间会切换进程执行，如果在标志位运算已经开始，但未结束时，又打开一次设备，这时系统可能会跳到这个进程，最终导致两次打开设备都成功（运算在汇编指令中并不是一次完成的，而是进行了取值、运算、存值等过程，中间存在极小的时间间隔），对此即可采用原子操作，使得他在完成标志位运算前，不能跳到其他进程

定义原子变量（初始化为1）

static atomic_t canopen = ATOMIC_INIT(1);

自减，并判断是否为0，为0返回true

atomic_dec_and_test(&canopen)

自增

atomic_inc(&canopen);

附上驱动程序代码，测试用的应用程序与fasync中所用的相同

驱动程序

#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>
#include <linux/device.h>
#include <linux/irq.h>

static struct class *my_irq_class;
static struct class_device *my_irq_class_device;

static unsigned char key_val;

struct fasync_struct *button_fasync;

static atomic_t canopen = ATOMIC_INIT(1);

static DECLARE_WAIT_QUEUE_HEAD(button_waitq);

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

struct pin_desc key_pins_desc[4]={
	{S3C2410_GPF0,0x01},
	{S3C2410_GPF2,0x02},
	{S3C2410_GPG3,0x03},
	{S3C2410_GPG11,0x04},
};

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
	{
		key_val = pindesc->key_val;
	}
	
	kill_fasync(&button_fasync, SIGIO, POLL_IN);


	return IRQ_HANDLED;
}

static int my_irq_drv_open(struct inode *inode, struct file *file)
{
	if(!atomic_dec_and_test(&canopen))
	{
		atomic_inc(&canopen);
		return -EBUSY;
	}
	
	request_irq(IRQ_EINT0,buttons_irq,IRQT_BOTHEDGE,"S2",&key_pins_desc[0]);
	request_irq(IRQ_EINT2,buttons_irq,IRQT_BOTHEDGE,"S3",&key_pins_desc[1]);
	request_irq(IRQ_EINT11,buttons_irq,IRQT_BOTHEDGE,"S4",&key_pins_desc[2]);
	request_irq(IRQ_EINT19,buttons_irq,IRQT_BOTHEDGE,"S5",&key_pins_desc[3]);	
	printk(“requestedr\n");	
	
	return 0;
}

int my_irq_drv_close(struct inode *inode,struct file *file)
{
	atomic_inc(&canopen);
	
	free_irq(IRQ_EINT0,&key_pins_desc[0]);
	free_irq(IRQ_EINT2,&key_pins_desc[1]);
	free_irq(IRQ_EINT11,&key_pins_desc[2]);
	free_irq(IRQ_EINT19,&key_pins_desc[3]);
	printk("freed\r\n");	
		
	return 0;
}


ssize_t my_irq_drv_read(struct file *file, char __user *buf, size_t size, loff_t * ppos)
{
	if(size != 1)
		return -EINVAL;
	
	copy_to_user(buf,&key_val,1);

	return 0;
}

static int my_irq_drv_fasync(int fd,struct file *filp,int on)
{
	return fasync_helper(fd,filp,on,&button_fasync);
}

static struct file_operations irq_drv_fops = {
	.owner = THIS_MODULE,
	.open = my_irq_drv_open,
	.read = my_irq_drv_read,
	.release = my_irq_drv_close,
	.fasync = my_irq_drv_fasync,
};

static int major;

static int my_irq_drv_init(void)
{
	major = register_chrdev(0,"my_irq_drv",&irq_drv_fops);
	my_irq_class = class_create(THIS_MODULE,"my_irq_drv");
	my_irq_class_device = class_device_create(my_irq_class,NULL,MKDEV(major,0),NULL,"key_irq_drv");

	return 0;
}

static void my_irq_drv_exit(void)
{
	unregister_chrdev(major,"my_irq_drv");
	class_device_unregister(my_irq_class_device);
	class_destroy(my_irq_class);
}

module_init(my_irq_drv_init);
module_exit(my_irq_drv_exit);
MODULE_LICENSE("GPL");