linux驱动开发学习二：创建一个阻塞型的字符设备

Posted 2020-11-22 一张红枫叶

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在Linux 驱动程序中，可以使用等待队列来实现阻塞进程的唤醒。等待队列的头部定义如下，是一个双向列表。

struct list_head {

struct list_head *next, *prev;

};

struct __wait_queue_head {

spinlock_t lock;

struct list_head task_list;

};

typedef struct __wait_queue_head wait_queue_head_t;

Linux提供了如下关于等待队列的操作：

1 __init_waitqueue_head(&my_queue)：进行等待队列的初始化。

2 DECLARE_WAITQUEUE: 定义等待队列元素。代码如下

#define __WAITQUEUE_INITIALIZER(name, tsk) { \

.private = tsk, \

.func = default_wake_function, \

.task_list = { NULL, NULL } }

}

#define DECLARE_WAITQUEUE(name, tsk) \

wait_queue_t name = __WAITQUEUE_INITIALIZER(name, tsk)

其实就是两个宏定义，这个两个宏定义扩展开来其实就是

wait_queue_t name={

.private = tsk, #private代表当前进程指针

.func = default_wake_function, #唤醒的回调函数

.task_list = { NULL, NULL } } #等待队列

}

那么回到代码中的调用DECLARE_WAITQUEUE(wait, current)就是定义了一个等待队列元素wait。等待队列的private就等于当前进程指针。

3 add_wait_queue/remove_wait_queue：添加移除等待队列

4 等待事件

wait_event(queue,condition)

wait_event_interrupt(queue,condition)

wait_event_timeout(queue,condition,timeout)

wait_event_interruptible_timeout(queue,condition,timeout)

等待第1个参数queue作为等待队列头部的队列被唤醒，而且第2个参数必须满足，否则继续阻塞，wait_event和wait_event_interrupt的区别在于后者可以被信号中断打断。加上timeout后的宏意味着阻塞等待的超时时间，在第三个参数timeout到达时，不论condition是否满足，均返回。

来看下代码的实现，首先是wait_event。先判断条件，如果条件，则立即退出，否则进入__wait_event

#define wait_event(wq, condition) \

do { \

if (condition) \

break; \

__wait_event(wq, condition); \

} while (0)

#define __wait_event(wq, condition) \

do { \

DEFINE_WAIT(__wait); \

for (;;) { \

prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE); \

if (condition) \

break; \

schedule(); \

} \

finish_wait(&wq, &__wait); \

} while (0)

(1) DEFINE_WAIT(__wait) 中申明一个当前进展的等待队列

#define DEFINE_WAIT(name) DEFINE_WAIT_FUNC(name, autoremove_wake_function)

#define DEFINE_WAIT_FUNC(name, function) \

wait_queue_t name = { \

.private = current, \

.func = function, \

.task_list = LIST_HEAD_INIT((name).task_list), \

}

(2) 进入一个无限循环。prepare_to_wait中将申明的等待任务加入到等待队列中去。并设置任务状态为TASK_UNINTERRUPTIBLE。如果condition满足，则退出循环。如果不满足则进行一次任务调度

(3) 满足条件退出循环后调用finish_wait从等待任务队列里面删除并设置进程状态为TASK_RUNNING

wait_event_timeout的代码也一样，不过是加了个schedule_timeout，如果超时则不管条件是否满足都直接退出

#define __wait_event_timeout(wq, condition, ret) \

do { \

DEFINE_WAIT(__wait); \

for (;;) { \

prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE); \

if (condition) \

break; \

ret = schedule_timeout(ret); \

if (!ret) \

break; \

} \

finish_wait(&wq, &__wait); \

} while (0)

5 wake_up(wait_queue_head_t *queue) wake_up会唤醒作为等待队列头部的队列中的所有进程。代码按如下，遍历队列，然后依次执行唤醒回调函数

static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,

int nr_exclusive, int wake_flags, void *key)

{

wait_queue_t *curr, *next;

list_for_each_entry_safe(curr, next, &q->task_list, task_list) {

unsigned flags = curr->flags;

if (curr->func(curr, mode, wake_flags, key) &&

(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)

break;

}

回调函数在生成队列wait的时候赋值为default_wake_function, 里面执行的就是try_to_wake_up。

6 sleep_on在等待队列上睡眠。将当前进程置成TASK_UNINTERRUPTIBLE，然后将当前进程加入到等待队列中去，待睡眠时间超时后，再从等待队列中删除。代码如下

sleep_on_common(wait_queue_head_t *q, int state, long timeout)

{

unsigned long flags;

wait_queue_t wait;

init_waitqueue_entry(&wait, current);

__set_current_state(state);

spin_lock_irqsave(&q->lock, flags);

__add_wait_queue(q, &wait);

spin_unlock(&q->lock);

timeout = schedule_timeout(timeout);

spin_lock_irq(&q->lock);

__remove_wait_queue(q, &wait);

spin_unlock_irqrestore(&q->lock, flags);

return timeout;

}

现在将前面的globalmem改成阻塞型的，代码如下：

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/cdev.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
#include <linux/delay.h>
#include <linux/wait.h>
#include <linux/sched/signal.h>

#define GLOBALMEM_SIZE 0X1000
#define MEM_CLEAR 0X1
#define GLOBALMEM_MAJOR 230

static int globalmem_major=GLOBALMEM_MAJOR;
module_param(globalmem_major,int,S_IRUGO);

struct globalmem_dev{
    struct cdev cdev;
    unsigned char mem[GLOBALMEM_SIZE];
    struct mutex mutex;
    unsigned int current_len;
    wait_queue_head_t r_wait;
    wait_queue_head_t w_wait;
};

struct globalmem_dev *globalmem_devp;

static int globalmem_open(struct inode *inode,struct file *filp)
{
    filp->private_data=globalmem_devp;
    return 0;
}


static int globalmem_release(struct inode *inode,struct file *filp)
{
    return 0;
}

static long globalmem_ioctl(struct file *filp,unsigned int cmd,unsigned long arg)
{
    struct globalmem_dev *dev=filp->private_data;
    switch(cmd)
    {
        case MEM_CLEAR:
            mutex_lock(&dev->mutex);
            memset(dev->mem,0,GLOBALMEM_SIZE);
            printk(KERN_INFO "globalmem is set to zero\n");
            mutex_unlock(&dev->mutex);
        default:
            return -EINVAL;
    }
    return 0;
}


static ssize_t globalmem_read_queue(struct file *filp,char __user *buf,size_t size,loff_t *ppos)
{
    int ret;
    struct globalmem_dev *dev=filp->private_data;
    DECLARE_WAITQUEUE(wait,current);
    mutex_lock(&dev->mutex);
    add_wait_queue(&dev->r_wait,&wait);
    while(dev->current_len == 0){
        if(filp->f_flags & O_NONBLOCK){
            ret=-EAGAIN;
            goto out;
        }
        __set_current_state(TASK_INTERRUPTIBLE);
        mutex_unlock(&dev->mutex);
        schedule();
        if(signal_pending(current)){
            ret=-ERESTARTSYS;
            goto out2;
        }
        mutex_lock(&dev->mutex);
    }
    if(size > dev->current_len)
        size=dev->current_len;
    if(copy_to_user(buf,dev->mem,size)){
        printk("copy_to_user_fail\n");
        ret=-EFAULT;
        goto out;
    }else{
        memcpy(dev->mem,dev->mem+size,dev->current_len-size);
        dev->current_len-=size;
        wake_up_interruptible(&dev->w_wait);
        ret=size;
    }
    out:
        mutex_unlock(&dev->mutex);
    out2:
        remove_wait_queue(&dev->w_wait,&wait);
        set_current_state(TASK_RUNNING);

    return ret;
}


static ssize_t globalmem_write_queue(struct file *filp,const char __user *buf,size_t size, loff_t *ppos)
{
    int ret;
    struct globalmem_dev *dev=filp->private_data;
    DECLARE_WAITQUEUE(wait,current);
    mutex_lock(&dev->mutex);
    add_wait_queue(&dev->w_wait,&wait);
    while(dev->current_len == GLOBALMEM_SIZE){
        if(filp->f_flags & O_NONBLOCK){
            ret=-EAGAIN;
            goto out;
        }
        __set_current_state(TASK_INTERRUPTIBLE);
        mutex_unlock(&dev->mutex);
        schedule();
        if(signal_pending(current)){
            ret=-ERESTARTSYS;
            goto out2;
        }
        mutex_lock(&dev->mutex);
    } 
    if(size > GLOBALMEM_SIZE- dev->current_len)
        size=GLOBALMEM_SIZE - dev->current_len;
    if(copy_from_user(dev->mem+dev->current_len,buf,size)){
        ret=-EFAULT;
        goto out;
    }else{
        dev->current_len+=size;
        wake_up_interruptible(&dev->r_wait);
        ret=size;
    }
    out:
        mutex_unlock(&dev->mutex);
    out2:
        remove_wait_queue(&dev->w_wait,&wait);
        set_current_state(TASK_RUNNING);
    return ret;
}


static loff_t globalmem_llseek(struct file *filp,loff_t offset,int orig)
{
    loff_t ret=0;
    switch(orig){
        case 0:
            if (offset < 0)
                ret=-EFAULT;
                break;
            if ((unsigned int)offset > GLOBALMEM_SIZE){
                ret=-EFAULT;
                break;
            }
            filp->f_pos=(unsigned int)offset;
            ret=filp->f_pos;
            break;
        case 1:
            if((filp->f_pos+offset) > GLOBALMEM_SIZE){
                ret=-EFAULT;
                break;
            }
            if((filp->f_pos+offset) < 0){
                ret=-EFAULT;
                break;
            }
            filp->f_pos+=offset;
            ret=filp->f_pos;
            break;
    }
    return ret;
}

static const struct file_operations globalmem_fops={
    .owner=THIS_MODULE,
    .llseek=globalmem_llseek,
    .read=globalmem_read_queue,
    .write=globalmem_write_queue,
    .unlocked_ioctl=globalmem_ioctl,
    .open=globalmem_open,
    .release=globalmem_release,
};


static void globalmem_setup_dev(struct globalmem_dev *dev,int index)
{
    int err,devno=MKDEV(globalmem_major,index);
    cdev_init(&dev->cdev,&globalmem_fops);
    dev->cdev.owner=THIS_MODULE;
    err=cdev_add(&dev->cdev,devno,1);
    if(err)
        printk(KERN_NOTICE "Error %d adding globalmem%d",err,index);
}

static int __init globalmem_init(void)
{
    int ret;
    dev_t devno=MKDEV(globalmem_major,0);
    printk("devno=%d\n",devno);
    if(globalmem_major)
        ret=register_chrdev_region(devno,1,"globalmem_tmp");
    else{
        ret=alloc_chrdev_region(&devno,0,1,"globalmem_tmp");
        globalmem_major=MAJOR(devno);
    }
    if(ret < 0)
        return ret;
    globalmem_devp=kzalloc(sizeof(struct globalmem_dev),GFP_KERNEL);
    if(!globalmem_devp){
        ret=-EFAULT;
        goto fail_malloc;
    }

    mutex_init(&globalmem_devp->mutex);
    globalmem_setup_dev(globalmem_devp,0);
    printk("globalmem init success\n");
    init_waitqueue_head(&globalmem_devp->r_wait);
    init_waitqueue_head(&globalmem_devp->w_wait);
    return 0;
    fail_malloc:
        unregister_chrdev_region(devno,1);
        return ret;
}


module_init(globalmem_init);

static void __exit globalmem_exit(void)
{
    cdev_del(&globalmem_devp->cdev);
    kfree(globalmem_devp);
    unregister_chrdev_region(MKDEV(globalmem_major,0),1);
    printk("global_mem exited\n");
}

module_exit(globalmem_exit);

MODULE_AUTHOR("zhf");
MODULE_LICENSE("GPL");

代码主要做了如下几点改动：

1 在globalmem_dev中增加读和写的队列r_wait以及w_wait

2 在globalmem_init中调用init_waitqueue_head初始化写和读队列

3 在globalmem_read_queue中将当前进程加入读的等待队列，只有当读队列完成copy_to_user的操作后，才唤醒写队列的进程

4 在globalmem_write_queue中将当前进程加入写的等待队列，只有当写队列完成copy_from_user的操作后，才唤醒读队列的进程