目錄

• • • • （一）poll輪詢的作用
      • （二）poll輪詢相關(guān)的接口
      • （三）poll使用流程
      • （四）實(shí)例代碼

（一）poll輪詢的作用

以阻塞的方式打開文件，那么對(duì)多個(gè)文件讀寫時(shí)，若某個(gè)文件未準(zhǔn)備好，則系統(tǒng)會(huì)處于讀寫阻塞，并影響其他文件的讀寫，poll輪訓(xùn)就是實(shí)現(xiàn)既可使用輸入輸出流又不想阻塞在任何一個(gè)設(shè)備的讀寫操作
調(diào)用poll函數(shù)返回時(shí)，會(huì)返回一個(gè)文件是否可讀寫的標(biāo)志狀態(tài)，用戶程序根據(jù)不同的標(biāo)志狀態(tài)來(lái)讀寫相應(yīng)的文件，實(shí)現(xiàn)阻塞方式打開但是非阻塞方式讀寫的結(jié)果

（二）poll輪詢相關(guān)的接口

系統(tǒng)層接口：

int poll(struct pollfd *fds, nfds_t nfds, int timeout); struct pollfd *fds： 輪訓(xùn)描述符的集合數(shù)據(jù)結(jié)構(gòu) struct pollfd {int fd; /* file descriptor */輪訓(xùn)的文件描述符short events; /* requested events */對(duì)于輪訓(xùn)的文件描述所檢測(cè)的事件類型short revents; /* returned events */對(duì)于輪訓(xùn)的文件描述所返回的事件類型 }; nfds_t nfds：輪訓(xùn)描述符的個(gè)數(shù) int timeout：輪詢時(shí)間

請(qǐng)求事件和返回事件類型：

POLLIN There is data to read. POLLOUT Writing is now possible, though a write larger that the available spacein a socket or pipe will still block (unless O_NONBLOCK is set). POLLPRI： There is urgent data to read POLLERR： Error condition (only returned in revents; ignored in events).POLLHUP： Hang up (only returned in revents; ignored in events). POLLNVAL： Invalid request: fd not open (only returned in revents; ignored in events). POLLRDNORM Equivalent to POLLIN. POLLRDBAND Priority band data can be read (generally unused on Linux).POLLWRNOR Equivalent to POLLOUT.POLLWRBAND Priority data may be written. RETURN VALUE On success, a positive number is returned; this is the number of structures which havenonzero revents fields (in other words, those descriptors with events or errors reported).A value of 0 indicates that the call timed out and no file descriptors were ready. Onerror, -1 is returned, and errno is set appropriately.

內(nèi)核層接口:

unsigned int (*poll) (struct file *, struct poll_table_struct *);//file_operations結(jié)構(gòu)體里

設(shè)置輪訓(xùn)的等待隊(duì)列poll_wait：

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

喚醒等待隊(duì)列節(jié)點(diǎn)wake_up_interruptible或wake_up：

#define wake_up(x) __wake_up(x, TASK_NORMAL, 1, NULL) void __wake_up(wait_queue_head_t *q, unsigned int mode, int nr, void *key); /*** __wake_up - wake up threads blocked on a waitqueue.* @q: the waitqueue* @mode: which threads* @nr_exclusive: how many wake-one or wake-many threads to wake up* @key: is directly passed to the wakeup function** It may be assumed that this function implies a write memory barrier before* changing the task state if and only if any tasks are woken up.*/

（三）poll使用流程

1.內(nèi)核層編寫poll相關(guān)的函數(shù)

unsigned int cdev_poll (struct file * fp, struct poll_table_struct * table)

2.在合適cdev_poll函數(shù)中調(diào)用poll_wait

poll_wait(fp,&waithead,table);//不會(huì)進(jìn)行阻塞，添加等待隊(duì)列到輪詢列表中

3.在合適的地方調(diào)用wake_up_interruptible喚醒poll等待隊(duì)列

wake_up_interruptible(&waithead)

（四）實(shí)例代碼

假設(shè)要求創(chuàng)建多個(gè)設(shè)備節(jié)點(diǎn)，每個(gè)設(shè)備節(jié)點(diǎn)對(duì)應(yīng)一個(gè)設(shè)備，在系統(tǒng)調(diào)用多次read接口來(lái)讀取底層設(shè)備的狀態(tài)。例如4個(gè)按鍵 ----- 4個(gè)設(shè)備節(jié)點(diǎn) ----分別讀取4個(gè)設(shè)備節(jié)點(diǎn)文件的數(shù)據(jù)獲取按鍵的狀態(tài)-----不確定哪一個(gè)按鍵被按下
分析：
若采用阻塞方式操作：四個(gè)讀寫操作會(huì)相互阻塞，如按鍵1未被按下，則讀取不到數(shù)據(jù)，既是按鍵4被按下，也會(huì)被按鍵1的操作阻塞，獲取不到數(shù)據(jù)
若采用非阻塞操作：既是讀取不到數(shù)據(jù)，也會(huì)直接返回，對(duì)于監(jiān)測(cè)按鍵的狀態(tài)，按鍵隨時(shí)可能被按下，所以需要的是循環(huán)檢測(cè)，采用非阻塞就會(huì)導(dǎo)致一直在進(jìn)行無(wú)用功
采用多路io輪詢操作：檢測(cè)是文件描述符是否發(fā)生變化，變化就去操作對(duì)應(yīng)的文件，否則阻塞在指定的操作
chrdev.c

#include <linux/kernel.h> #include <linux/module.h> #include <linux/fs.h> #include <linux/cdev.h> #include <linux/device.h> #include <linux/slab.h> #include <linux/interrupt.h> #include <linux/gpio.h> #include <linux/init.h> #include <linux/wait.h> #include <linux/sched.h> #include <linux/uaccess.h> #include <linux/poll.h>#define CDEVCOUNT 5 #define CDEVNAME "cdevdevice" #define INODENAME "mycdev" int count=0; dev_t dev=0; int keyflag;//等待隊(duì)列第二個(gè)參數(shù) char keyvalue[4]={-1,-1,-1,-1}; struct cdev * cdev =NULL; struct class * cdevclass =NULL; wait_queue_head_t waithead;struct Key {unsigned int gpios;char * name;int num;unsigned int irq; };struct Key key[]={{EXYNOS4_GPX3(2),"K1",0},{EXYNOS4_GPX3(3),"K2",1},{EXYNOS4_GPX3(4),"K3",2},{EXYNOS4_GPX3(5),"K4",3}, };irqreturn_t key_handler(int irq, void * dev) {struct Key * tmp =(struct Key *)dev;int value[4];value[tmp->num]= gpio_get_value(tmp->gpios);gpio_set_value(EXYNOS4X12_GPM4(tmp->num),value[tmp->num]);keyflag = 1;keyvalue[tmp->num] = value[tmp->num];wake_up_interruptible(&waithead);return IRQ_HANDLED; }int cdev_open (struct inode *node, struct file *file) {printk("cdev_open is install\n");return 0; } ssize_t cdev_read (struct file *fp, char __user *buf, size_t size, loff_t *offset) {int ret =0;if((fp->f_flags & O_NONBLOCK)== O_NONBLOCK)//非阻塞{if(!keyflag)return -EAGAIN;}else//阻塞{wait_event_interruptible(waithead,keyflag);}keyflag =0;ret = copy_to_user(buf,keyvalue,4);if(ret <0)return -EFAULT;printk("cdev_read is install\n");return 0; } ssize_t cdev_write (struct file *fp, const char __user * buf, size_t size, loff_t *offset) {printk("cdev_write is install\n");return 0; } unsigned int cdev_poll (struct file * fp, struct poll_table_struct * table) {unsigned int bitmask =0;printk("this is poll_wait before\n");poll_wait(fp,&waithead,table);//不會(huì)進(jìn)行阻塞，添加等待隊(duì)列到輪詢列表中if(keyflag)//必須有判斷標(biāo)志bitmask = POLLIN;printk("this is poll_wait after\n");return bitmask; }int cdev_release (struct inode *node, struct file *fp) {printk("cdev_release is install\n");return 0; } struct file_operations fop={.open=cdev_open,.read=cdev_read,.write=cdev_write,.release=cdev_release,.poll = cdev_poll, };void mycdev_add(void) {//1.申請(qǐng)?jiān)O(shè)備號(hào)--動(dòng)態(tài)int ret =alloc_chrdev_region(&dev,0, CDEVCOUNT, CDEVNAME);if(ret)return ;//初始化cdev結(jié)構(gòu)體cdev = cdev_alloc();if(!cdev){goto out;}cdev_init(cdev,&fop);//添加字符設(shè)備到系統(tǒng)中ret =cdev_add(cdev,dev, CDEVCOUNT);if(ret){goto out1;}//創(chuàng)建設(shè)備類cdevclass = class_create(THIS_MODULE, INODENAME);if(IS_ERR(cdevclass)){goto out2;}for (count=0;count<CDEVCOUNT;count++)device_create(cdevclass, NULL, dev+count, NULL, "mydevice%d",count);out:unregister_chrdev_region(dev,CDEVCOUNT); return ; out1:unregister_chrdev_region(dev,CDEVCOUNT);kfree(cdev);return ; out2:cdev_del(cdev);unregister_chrdev_region(dev,CDEVCOUNT);kfree(cdev);return ; }static int __init dev_module_init(void) {int ret=0,i=0;unsigned long flags= IRQF_TRIGGER_RISING|IRQF_TRIGGER_FALLING|IRQF_SHARED;for(i=0;i<4;i++){key[i].irq = gpio_to_irq(key[i].gpios);ret =request_irq(key[i].irq, key_handler,flags,key[i].name,(void *) &key[i]);}init_waitqueue_head(&waithead);//創(chuàng)建等待隊(duì)列頭mycdev_add();printk("this is dev_module_init \n");return 0; }static void __exit dev_module_cleanup(void) {int i=0;for(i=0;i<4;i++){key[i].irq = gpio_to_irq(key[i].gpios);free_irq(key[i].irq,(void *)&key[i]);}for(count=0;count<CDEVCOUNT;count++){device_destroy(cdevclass, dev+count);}class_destroy(cdevclass);cdev_del(cdev);unregister_chrdev_region(dev, CDEVCOUNT);kfree(cdev);printk("this is dev_module_cleanup\n"); }module_init(dev_module_init); module_exit(dev_module_cleanup); MODULE_LICENSE("GPL");

chr_app.c

#include<stdio.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <poll.h>int ret=0; struct pollfd fds[1]; char status[]={-1,-1,-1,-1}; char key[]={-1,-1,-1,-1}; int main(int argc, char *argv[]) {int i=0,fd= open(argv[1],O_RDWR/*|O_NONBLOCK*/);if(fd== -1){perror("open");return -1;}while(1){fds[0].fd = fd;fds[0].events = POLLIN;ret = poll(fds,1,5000);//只有錯(cuò)誤、滿足條件、超時(shí)才會(huì)返回，不滿足條件在這死等if(ret == -1){perror("poll\n");return -1;}else if(ret == 0){printf("timeout\n");}else if(ret & POLLIN){printf("ret >0\n");if(read(fd,status,4)<0)/*疑問(wèn)：把read屏蔽掉打印出錯(cuò)*/{printf("按鍵狀態(tài)未改變\n");sleep(1);continue;}for(i=0;i<4;i++){if(status[i] != key[i] ){printf("key%d is %s\n",i,status[i]?"up":"down");status[i]=key[i];}}}}close(fd);return 0; }

Makefile

CFLAG =-C TARGET = chrdev TARGET1 = chr_app KERNEL = /mydriver/linux-3.5 obj-m += $(TARGET).oall:make $(CFLAG) $(KERNEL) M=$(PWD)arm-linux-gcc -o $(TARGET1) $(TARGET1).c clean:make $(CFLAG) $(KERNEL) M=$(PWD) clean

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