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  • 中断方式按键驱动中引入poll机制

    学习目的:


    上一篇实现了中断方式的按键驱动程序,当应用程序去读取按键值时,如果此时无按键按下或松开,应用程序将进入休眠。休眠过程虽然不占用CPU的资源,但如果无按键按下或松开情况发生,应用程序将永远不会被唤醒,那么程序中其他的事情就不能被处理,很显然这样是不合理的。引入poll机制,便能解决这种情况。当应用程序中调用poll函数,可以设置等待超时时间,如果等待超过了设置超时时间,程序会立即返回,继续执行其他操作。

    1、内核中poll机制实现

    我们知道应用程序通过调用C库中实现的open、read等系统调用进入内核,调用内核中对应的sys_open、sys_read函数。与此类似,poll最终也会调用到内核中的sys_poll,分析时先从sys_poll开始,从上往下看,层层递进,找出其他关系。

    1.1 sys_poll函数

    asmlinkage long sys_poll(struct pollfd __user *ufds, unsigned int nfds,
                long timeout_msecs)
    {
        s64 timeout_jiffies;
    
        if (timeout_msecs > 0) {
    #if HZ > 1000
            /* We can only overflow if HZ > 1000 */
            if (timeout_msecs / 1000 > (s64)0x7fffffffffffffffULL / (s64)HZ)
                timeout_jiffies = -1;
            else
    #endif
                timeout_jiffies = msecs_to_jiffies(timeout_msecs);
        } else {
            /* Infinite (< 0) or no (0) timeout */
            timeout_jiffies = timeout_msecs;
        }
    
        return do_sys_poll(ufds, nfds, &timeout_jiffies);
    }

    sys_poll位于fs/select.c文件中,它对超时时间进行处理,最后调用do_sys_poll

    1.2 do_sys_poll函数

    int do_sys_poll(struct pollfd __user *ufds, unsigned int nfds, s64 *timeout)
    {
        ...
        poll_initwait(&table);
        ...
        fdcount = do_poll(nfds, head, &table, timeout);
        ...
    }

    poll_initwait函数初始化一个poll_wqueues类型结构体,

    poll_initwait>>init_poll_funcptr(&pwq->pt, __pollwait)>>pt->qproc = qproc; 即table.pt.proc = __pollwait

    1.3 do_poll函数

    static int do_poll(unsigned int nfds,  struct poll_list *list,
               struct poll_wqueues *wait, s64 *timeout)
    {
        ...
        for (;;) {----------------------------------------------------------->①
    
                    ...
                    if (do_pollfd(pfd, pt)) {-------------------------------->②
                        count++;
                        pt = NULL;
                    }
                }
    
            ...
            if (count || !*timeout || signal_pending(current))
                break;
            count = wait->error;
            if (count)
                break;
    
            if (*timeout < 0) {
                /* Wait indefinitely */
                __timeout = MAX_SCHEDULE_TIMEOUT;
            } else if (unlikely(*timeout >= (s64)MAX_SCHEDULE_TIMEOUT-1)) {
    
                __timeout = MAX_SCHEDULE_TIMEOUT - 1;
                *timeout -= __timeout;
            } else {
                __timeout = *timeout;
                *timeout = 0;
            }
    
            __timeout = schedule_timeout(__timeout);------------------------>③
            if (*timeout >= 0)
                *timeout += __timeout;
        }
        __set_current_state(TASK_RUNNING);
        return count;
    }

     ①处可看出该函数内部是一个循环,他的退出条件为:

            a. do_pollfd返回非0,count非零、等待超时、有信号等待处理

            b. 等待出现错误

    ②处do_pollfd是该函数中的核心,后面再分析

    ③处让本进程进入休眠,应用程序执行poll调用后,如果①②的条件不满足,进程就会进入休眠。那么,谁唤醒呢?除了休眠到指定时间被系统唤醒外,还可以被驱动程序唤醒──记住这点,这就是为什么驱动的poll里要调用poll_wait的原因,后面分析。

     1.4 do_pollfd函数

    static inline unsigned int do_pollfd(struct pollfd *pollfd, poll_table *pwait)
    {
    ...
                if (file->f_op && file->f_op->poll)
                    mask = file->f_op->poll(file, pwait);
    
    ...
    }

    调用驱动程序中file_operations结构体中填充的poll函数

    1.4 驱动程序

    驱动程序里与poll相关的地方有两处:一是构造file_operation结构时,要定义自己的poll函数。二是通过poll_wait来调用上面说到的__pollwait函数,pollwait的代码如下:

    static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
    {
        if (p && wait_address)
            p->qproc(filp, wait_address, p);
    }

    p->qproc(filp, wait_address, p),即table.pt.proc = __pollwait,最终调用到__pollwait函数

    static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
                    poll_table *p)
    {
        struct poll_table_entry *entry = poll_get_entry(p);
        if (!entry)
            return;
        get_file(filp);
        entry->filp = filp;
        entry->wait_address = wait_address;
        init_waitqueue_entry(&entry->wait, current);
        add_wait_queue(wait_address, &entry->wait);
    }

    __pollwait将当前的应用程序的进程挂到应用程序中定义的等待列中

    执行到驱动程序的poll_wait函数时,进程并没有休眠,我们的驱动程序里实现的poll函数是不会引起休眠的。让进程进入休眠,是前面分析的do_sys_poll函数的③“__timeout = schedule_timeout(__timeout)”。

    poll_wait只是把本进程挂入某个队列,应用程序调用poll > sys_poll > do_sys_poll > poll_initwait,do_poll > do_pollfd > 我们自己写的poll函数后,再调用schedule_timeout进入休眠。如果我们的驱动程序发现情况就绪,可以把这个队列上挂着的进程唤醒。可见,poll_wait的作用,只是为了让驱动程序能找到要唤醒的进程。即使不用poll_wait,我们的程序也有机会被唤醒:chedule_timeout(__timeout),只是要休眠__time_out这段时间。

    现在来总结一下poll机制:

    1. poll > sys_poll > do_sys_poll > poll_initwait,poll_initwait函数注册一下回调函数__pollwait,它就是我们的驱动程序执行poll_wait时,真正被调用的函数。

    2. 接下来执行file->f_op->poll,即我们驱动程序里自己实现的poll函数

       它会调用poll_wait把自己挂入某个队列,这个队列也是我们的驱动自己定义的;

       它还判断一下设备是否就绪。

    3. 如果设备未就绪,do_sys_poll里会让进程休眠一定时间

    4. 进程被唤醒的条件有2:一是上面说的“一定时间”到了,二是被驱动程序唤醒。驱动程序发现条件就绪时,就把“某个队列”上挂着的进程唤醒,这个队列,就是前面通过poll_wait把本进程挂过去的队列。

    5. 如果驱动程序没有去唤醒进程,那么chedule_timeout(__timeou)超时后,会重复2、3动作,直到应用程序的poll调用传入的时间到达。

    2、引入poll中断驱动程序实现

    在file_operations中填充button_drv_poll函数,调用poll_wait函数,将当前进程挂入到button_waittq队列中,当按键按下触发中断服务函数唤醒button_waitp队列中poll中引发的休眠进程,返回POLLIN,应用程序有数据可读。

    测试应用程序使用poll函数,等待3s,如果3s内有按键按下,poll函数返回有数据可读,调用read函数读取按键值。若3s时间内没有按键按下,poll函数超时返回,应用程序中打因time out...提示

    static unsigned button_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 (event_trig)
            mask |= POLLIN | POLLRDNORM;
    
        return mask;
    }

    完整驱动代码

    #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 <plat/gpio-fns.h>
    #include <mach/gpio-nrs.h>
    #include <linux/interrupt.h>
    #include <linux/wait.h>
    #include <linux/sched.h>
    #include <linux/device.h>
    #include <linux/gpio.h>
    #include <linux/poll.h>
    
    #define BUTTON_NUMS    4
    #define IRQT_BOTHEDGE IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING
    
    static int major;
    static int event_trig = 0;
    
    static unsigned char key_status;
    
    static volatile unsigned long *gpfcon = NULL;
    static volatile unsigned long *gpgcon = NULL;
    static volatile unsigned long *gpfdat = NULL;
    static volatile unsigned long *gpgdat = NULL;
    
    static struct class *button_drv_class;
    static struct class_device    *button_drv_class_dev;
    
    static DECLARE_WAIT_QUEUE_HEAD(button_waitq);
    
    struct button_desc
    {
        int pin;
        int irq_type;
        unsigned long flags;
        char *name;
        int key_val;
    };
    
    static struct button_desc btn_desc[BUTTON_NUMS] = {
        {S3C2410_GPF(0),  IRQ_EINT0,  IRQT_BOTHEDGE, "S2", 1},
        {S3C2410_GPF(2),  IRQ_EINT2,  IRQT_BOTHEDGE, "S3", 2},
        {S3C2410_GPG(3),  IRQ_EINT11, IRQT_BOTHEDGE, "S4", 3},
        {S3C2410_GPG(11), IRQ_EINT19, IRQT_BOTHEDGE, "S5", 4},
    };
    
    static int button_drv_open(struct inode *inode, struct file *file);
    static ssize_t button_drv_read(struct file *file, char __user *buf, size_t count, loff_t *ppos);
    static ssize_t button_drv_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos);
    static int button_drv_close(struct inode *inode, struct file *file);
    static unsigned button_drv_poll(struct file *file, poll_table *wait);
    
    struct file_operations button_drv_fileop = {
        .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   =   button_drv_open,
        .read   =   button_drv_read,
        .write  =   button_drv_write,
        .release =  button_drv_close,
        .poll = button_drv_poll,
    };
    
    static irqreturn_t button_irq_handle(int irq, void *dev_id)
    {    
        struct button_desc *pdesc = NULL;
        unsigned char pin_val;
        
        pdesc = (struct button_desc *)dev_id;
        
        pin_val = gpio_get_value(pdesc->pin);    
    
        if(pin_val == 1)
        {
            key_status = pdesc->key_val | 0x80;
        }
        else
        {
            key_status = pdesc->key_val;
        }
        
        event_trig = 1;
        wake_up_interruptible(&button_waitq); 
        
        return IRQ_RETVAL(IRQ_HANDLED);
    }
    
    static int button_drv_open(struct inode *inode, struct file *file)
    {
        int i;
        
        *gpfcon &= ~((0x3<<(0*2)) | (0x3<<(2*2)));
        *gpgcon &= ~((0x3<<(3*2)) | (0x3<<(11*2)));
        
        /* 注册中断处理函数 */
        for(i = 0; i < BUTTON_NUMS; i++)
            request_irq(btn_desc[i].irq_type, button_irq_handle, btn_desc[i].flags, btn_desc[i].name, &btn_desc[i]);
        
        return 0;
    }
    
    static ssize_t button_drv_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
    {
        if(count != 1)
            return EINVAL;
        
        wait_event_interruptible(button_waitq, event_trig);
        
        if(copy_to_user(buf, &key_status, count))
            return EFAULT;
        
        event_trig = 0;
        return 0;
    }
    
    static ssize_t button_drv_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
    {
        printk("button_drv_write
    ");
        
        return 0;
    }
    
    static int button_drv_close(struct inode *inode, struct file *file)
    {
        int i;
        
        for(i = 0; i < BUTTON_NUMS; i++)
            free_irq(btn_desc[i].irq_type, &btn_desc[i]);
        
        return 0;
    }
    
    static unsigned button_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 (event_trig)
            mask |= POLLIN | POLLRDNORM;
    
        return mask;
    }
            
    static int button_drv_init(void)
    {
        major = register_chrdev(0, "button_light", &button_drv_fileop);
        
        button_drv_class = class_create(THIS_MODULE, "button_drv");
        //button_drv_class_dev = class_device_create(button_drv_class, NULL, MKDEV(major, 0), NULL, "button"); /* /dev/button */
        button_drv_class_dev = device_create(button_drv_class, NULL, MKDEV(major, 0), NULL, "button"); /* /dev/button */
        
        gpfcon = (volatile unsigned long *)ioremap(0x56000050, 16);
        gpgcon = (volatile unsigned long *)ioremap(0x56000060, 16);
        gpfdat = gpfcon + 1;
        gpgdat = gpgcon + 1;
        
        return 0;
    }
    
    static void button_drv_exit(void)
    {
        unregister_chrdev(major, "button_drv");
        
        //class_device_unregister(button_drv_class_dev);
        device_unregister(button_drv_class_dev);
        class_destroy(button_drv_class);
        
        iounmap(gpfcon);
        iounmap(gpgcon);
    }
    
    module_init(button_drv_init);
    module_exit(button_drv_exit);
    
    MODULE_LICENSE("GPL");
    irq_button_drv

    完成测试程序代码

    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <sys/fcntl.h>
    #include <stdlib.h>
    #include <stdio.h>
    #include <poll.h>
    
    
    int main(int argc, char **argv)
    {
        int fd, ret;
        unsigned char key_buf;
        struct pollfd fds;
        
        fd = open("/dev/button", O_RDWR);
        if(fd == -1)
        {
            printf("can't open...
    ");
            exit(EXIT_FAILURE);
        }
        
        fds.fd = fd;
        fds.events = POLLIN;
        
        while(1)
        {
            ret = poll(&fds, 1, 3000);
            if(ret > 0)
            {
                ret = read(fd, &key_buf, 1);
                if(ret < 0)
                {
                    printf("read err...
    ");
                    continue;
                }
                /* 判断有按键按下,打印按键信息 */
                printf("key_val=0x%x
    ", key_buf);
            }
            else if(ret == 0)
            {
                printf("time out...
    ");
            }
            else if(ret == -1)
            {
                printf("error...
    ");
            }
            
        }
    
        exit(EXIT_SUCCESS);
    }
    irq_button_test

    测试效果

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  • 原文地址:https://www.cnblogs.com/053179hu/p/13442759.html
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