我們想來達到一個目的：同一時刻，只能有一個應用程序打開/dev/buttons

一般的，我們想達到目的會想到下面這種方法：

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++;
}

假設程序A來調(diào)用，那么進入open函數(shù)，canopen = 0,if條件不成立；如果A沒有釋放，程序B來調(diào)用時候,canopen = -1,if條件成立，return -EBUSY;

原則上是可以實現(xiàn)，但是我們進行–canopen操作實際上轉(zhuǎn)化程匯編使用三條指令(讀->改->寫)執(zhí)行的，由于Linux是多任務編程的。當我們程序A進行–canopen的讀取后，程序B剛好進行那么就有可能發(fā)生程序AB都能調(diào)用驅(qū)動！看下圖分析：

下面來介紹三種解決上面bug的方法 一、原子操作：

原子操作指的是在執(zhí)行過程中不會被別的代碼路徑所中斷的操作。
?常用原子操作函數(shù)舉例：

atomic_t?v?=?ATOMIC_INIT(0);?????//定義原子變量v并初始化為0
atomic_read(atomic_t?*v);????????//返回原子變量的值
void?atomic_inc(atomic_t?*v);????//原子變量增加1
void?atomic_dec(atomic_t?*v);????//原子變量減少1
int?atomic_dec_and_test(atomic_t?*v);?//自減操作后測試其是否為0，為0則返回true，否則返回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

驅(qū)動函數(shù):sixth_drv.c

#include#include#include#include#include#include#include#include#include#include#include#includestatic?struct?fasync_struct?*button_async;
static?struct?class?*sixthdrv_class;
static?struct?class_device?*sixthdrv_class_dev;

static?DECLARE_WAIT_QUEUE_HEAD(button_waitq);

/*?中斷事件標志,?中斷服務程序?qū)⑺?，s3c24xx_sixth_read將它清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;
};

/*?鍵值：?按下時，0x01、0x02、0x03?*/
/*?鍵值：?松開時，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);//定義原子變量canopen并初始化為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)
????{
????????/*?松開?*/
????????key_val?=?0x80?|?(pindesc->key_val);
????????*gpfdat?|=?((1<<4)?|?(1<<5)?|?(1<key_val;
????????*gpfdat?&=?~((1<<4)?|?(1<<5)?|?(1<<6));
????}
????ev_press?=?1;????????????????/*?表示中斷發(fā)生了?*/
????wake_up_interruptible(&button_waitq);???/*?喚醒休眠的進程?*/
????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為中斷引腳?*/
????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為輸入引腳?*/
????*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)
{
????//看用戶需要讀取的空間，和這里的是否相同
????if(count?!=?1)
????????return?-EINVAL;
????/*?如果無按鍵動作發(fā)生，則進行休眠狀態(tài)?*/
????/*?如果ev_press等于0，休眠?*/
????wait_event_interruptible(button_waitq,ev_press);

????/*?如果有按鍵動作發(fā)生，則返回按鍵的值?*/
????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_fasyncn");
????return?fasync_helper?(fd,?filp,?on,?&button_async);
}

static?struct?file_operations?sixth_drv_fops?=?{
????.owner???=?THIS_MODULE,?/*?這是一個宏，推向編譯模塊時自動創(chuàng)建的__this_module變量?*/
????.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");

驅(qū)動測試函數(shù)：sixthdrvtest.c

#include#include#include#include#include#include#include#include/*?
?*?sixthdrvtest
?*/
int?fd;

void?czg_signal_handler(int?signum)
{
????unsigned?char?key_val?=?0;
????read(fd,&key_val,1);
????printf("key_val:?0x%xn",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());?//?告訴內(nèi)核，發(fā)給誰
????oflags?=?fcntl(fd,F_GETFL);
????fcntl(fd,F_SETFL,oflags?|?FASYNC);?//?改變fasync標記，
????//最終會調(diào)用到驅(qū)動的faync?>?fasync_helper：初始化/釋放fasync_struct
????//然后當按鍵按下時候，在irqreturn_t?buttons_irq中斷處理中調(diào)用kill_fasync
????while(1)
????{
????????sleep(1000);
????}
????return?0;
}

測試：

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

二、信號量：

信號量（semaphore）是用于保護臨界區(qū)的一種常用方法，只有得到信號量的進程才能執(zhí)行臨界區(qū)代碼。當獲取不到信號量時，進程進入休眠等待狀態(tài)。

//定義信號量
struct?semaphore?sem;
//初始化信號量
void?sema_init?(struct?semaphore?*sem,?int?val);
void?init_MUTEX(struct?semaphore?*sem);//初始化為0
static?DECLARE_MUTEX(button_lock);?????//定義互斥鎖
//獲得信號量
void?down(struct?semaphore?*?sem);
int?down_interruptible(struct?semaphore?*?sem);?
int?down_trylock(struct?semaphore?*?sem);
//釋放信號量
void?up(struct?semaphore?*?sem);

代碼只需要在上面的驅(qū)動程序sixth_drv.c稍做修改

static?DECLARE_MUTEX(button_lock);?????//定義互斥鎖
/*?獲?取?信?號?量?*/
down(&button_lock);
/*?釋放信號量?*/
up(&button_lock);

三、阻塞： 阻塞操作：

是指在執(zhí)行設備操作時若不能獲得資源則掛起進程，直到滿足可操作的條件后再進行操作。被掛起的進程進入休眠狀態(tài)，被從調(diào)度器的運行隊列移走，直到等待的條件被滿足。

非阻塞操作：

進程在不能進行設備操作時并不掛起，它或者放棄，或者不停地查詢，直至可以進行操作為止。

代碼在上面的驅(qū)動程序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
????{
????????/*?獲?取?信?號?量?*/
????????down(&button_lock);
????}
}
/*************************************************/
static?ssize_t?sixth_drv_read?(struct?file?*file,?char?__user?*buf,?size_t?count,?loff_t?*ppos)
{
????//看用戶需要讀取的空間，和這里的是否相同
????if(count?!=?1)
????????return?-EINVAL;
????if(file->f_flags?&?O_NONBLOCK)
????{
????????if(!ev_press)
????????????return?-EAGAIN;
????}
????else
????{
????/*?如果無按鍵動作發(fā)生，則進行休眠狀態(tài)?*/
????/*?如果ev_press等于0，休眠?*/
????wait_event_interruptible(button_waitq,ev_press);
????}
}

阻塞：

驅(qū)動測試代碼：sixthdrvtest.c

#include#include#include#include#include#include#include#include/*?
?*?sixthdrvtest
?*/
int?fd;

void?czg_signal_handler(int?signum)
{
????unsigned?char?key_val?=?0;
????read(fd,&key_val,1);
????printf("key_val:?0x%xn",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());?//?告訴內(nèi)核，發(fā)給誰
????//oflags?=?fcntl(fd,F_GETFL);
????//fcntl(fd,F_SETFL,oflags?|?FASYNC);?//?改變fasync標記，
????//最終會調(diào)用到驅(qū)動的faync?>?fasync_helper：初始化/釋放fasync_struct
????//然后當按鍵按下時候，在irqreturn_t?buttons_irq中斷處理中調(diào)用kill_fasync
????while(1)
????{
????????read(fd,&key_val,1);
????????printf("key_val:?0x%xn",key_val);
????????//sleep(1000);
????}
????return?0;
}

非阻塞：

驅(qū)動測試代碼：sixthdrvtest.c

#include#include#include#include#include#include#include#include/*?
?*?sixthdrvtest
?*/
int?fd;

void?czg_signal_handler(int?signum)
{
????unsigned?char?key_val?=?0;
????read(fd,&key_val,1);
????printf("key_val:?0x%xn",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());?//?告訴內(nèi)核，發(fā)給誰
????//oflags?=?fcntl(fd,F_GETFL);
????//fcntl(fd,F_SETFL,oflags?|?FASYNC);?//?改變fasync標記，
????//最終會調(diào)用到驅(qū)動的faync?>?fasync_helper：初始化/釋放fasync_struct
????//然后當按鍵按下時候，在irqreturn_t?buttons_irq中斷處理中調(diào)用kill_fasync
????while(1)
????{
????????ret?=?read(fd,&key_val,1);
????????printf("key_val:?0x%x,ret?=?%dn",key_val,ret);
????????sleep(4);
????}
????return?0;
}