Article take you understand interrupt the key driver of polling mechanism

• This section continuesTake you to play with the interrupt mode key driver(详解)improvement,添加poll机制.
• 那么我们为什么还需要poll机制呢.之前的测试程序是这样：

while (1)
{
read(fd, &key_val, 1);
printf("key_val = 0x%x\n", key_val);
}

• 在没有poll机制的情况下,Most of the time the program is therereadthe position where it is dormant.If we don't want the program to stop at this position,Rather want when there is a key press,我们再去read,因此我们编写poll函数,测试程序调用pollfunction according to the return value,来决定是否执行read函数.
• poll机制作用:相当于定时器,Set a certain time to make the process wait for the resource,If the time is up, the interrupt is still in sleep state(等待队列),pollThe mechanism will wake up the interrupt,Get a resource once

1.pollMechanism Kernel Framework

• 如下图所示,在用户层上,使用poll或select函数时,和open、readThose functions are the same,Also into the kernelsys_poll函数里,接下来我们分析sys_poll函数来了解poll机制(位于/fs/select.c)

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1.1 sys_poll代码如下:

asmlinkage long sys_poll(struct pollfd __user *ufds, unsigned int nfds,long timeout_msecs)
{
        if (timeout_msecs > 0)    //参数timeout>0
　　　　{
　　       timeout_jiffies = msecs_to_jiffies(timeout_msecs);  //Calculated by frequencytimeoutHow many count values ​​are required for the time
　　　　}
　　　　else
　　　　{
          timeout_jiffies = timeout_msecs;    //如果timeout时间为0,直接赋值
       }
  return do_sys_poll(ufds, nfds, &timeout_jiffies);   //调用do_sys_poll.
}

1.2 然后进入do_sys_poll(位于fs/select.c):

int do_sys_poll(struct pollfd __user *ufds, unsigned int nfds, s64 *timeout)
{
　　... ...
　　/*初始化一个poll_wqueues变量table*/
　　poll_initwait(&table);
　　... ...
　　fdcount = do_poll(nfds, head, &table, timeout);
　　... ... 
}

1.3进入poll_initwait函数,Discovery mainly implements the following sentence,This will be analyzed later:

table ->pt-> qproc=__pollwait;    //__pollwait将在驱动的poll函数里的poll_wait函数用到

1.4然后进入do_poll函数, (位于fs/select.c):

static int do_poll(unsigned int nfds,  struct poll_list *list, struct poll_wqueues *wait,  s64 *timeout)
{
　　……
       for (;;)
　　　{
　　　　……
　　　　set_current_state(TASK_INTERRUPTIBLE);       //Set to wait queue state
　　　　......
    　　 for (; pfd != pfd_end; pfd++) {             //forRun multiple loopspoll机制
             　　　　　　/*将pfd和ptThe parameters are substituted into those registered in our driverpoll函数*/
                        if (do_pollfd(pfd, pt))     //若返回非0,count++,back and exit
　　　　　　　　　　　　　　{  count++;
                           pt = NULL; } }

　　　　……

　　　　/*count非0(.poll函数返回非0),timeoutTimeout counted up0,A signal is waiting*/

     　　if (count || !*timeout || signal_pending(current))
                         　　 break;
　　　　……
　　
　　　　/*进入休眠状态,只有当timeoutTimeout counted up0,Or exit after being woken up by an interrupt,*/
    　　　　 __timeout = schedule_timeout(__timeout);

　　　　……

　　 }

__set_current_state(TASK_RUNNING);  //开始运行
return count;

}

1.4.1上面do_pollfdHow exactly does the function workpfd和ptparameters are substituted?代码如下(位于fs/select.c):

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);
　　　　　　……

return mask;
}

• 上面file->f_op It's what we drivefile_oprations结构体,如下图所示:

• 所以do_pollfd(pfd, pt)Just execute our driver.poll(pfd, pt)函数(第2Section begins the analysis.poll函数)

1.4.2当poll进入休眠状态后,Who will wake it up?This is to analyze our driver.poll函数(第2Section begins the analysis.poll函数)

2.写驱动程序.poll函数,并分析.poll函数：

• Add the following code to the driver in the previous section:

#include <linux/poll.h>                //添加头文件
  
 /*   .poll驱动函数:  third_poll        */
static unsigned int third_poll(struct file *fp, poll_table * wait)  //fp:文件  wait:
{
         unsigned int mask =0; 
         poll_wait(fp, &button_wait, wait);
         if(even_press)              //中断事件标志, 1:Exit hibernation     0:进入休眠状态 
         mask |= POLLIN | POLLRDNORM ;
         return mask;     //当超时,It is returned to the application layer as0 ,Return when awakenedPOLLIN | POLLRDNORM ;

}

 

static struct file_operations third_drv_fops={
         .owner = THIS_MODULE,
         .open = third_drv_open,
         .read = third_drv_read,
    　　 .release=third_drv_class,   
    　　 .poll = third_poll,           //创建.poll函数
};

2.1 在我们1.4小节do_pollThe function has the following piece of code:

if (do_pollfd(pfd, pt))     //若返回非0,count++,back and exit
{      
　　　　 count++;
        pt = NULL;
         }

• 且在1.4.1分析出: do_pollfd(pfd, pt)It is pointing to the driverthird_poll()函数,
• So when we have keys pressed, 驱动函数third_poll()就会返回mask非0值,Then in the kernel functiondo_poll里的count就++,pollmechanism and exit sleep.

2.2Analysis is in the kernelpollHow the mechanism is woken up by an interrupt in the driver

• in the drive functionthird_poll()There is the following sentence:

poll_wait(fp, &button_wait, wait);

• 如上图所示,代入参数,poll_wait()就是执行了: p->qproc(filp, button_wait, p);
• It just corresponds to us1.3小节的:

table ->pt-> qproc=__pollwait;

• 所以poll_wait()函数就是调用了: __pollwait(filp, button_wait, p);
• Then we analyze__pollwait函数,pollwait的代码如下：

static void __pollwait(struct file  *filp, wait_queue_head_t  *wait_address,poll_table  *p)
{
 　　... ...
　　 //把currentThe process is mounted to&entry->wait下
　　 init_waitqueue_entry(&entry->wait, current);

　　 //再&entry->waitadd tobutton_wait中断下
　　 add_wait_queue(wait_address, &entry->wait);

}

• 它是将pollprocess was added tobutton_waitin the interrupt queue,这样,When a button is pressed,It will wake up in the interrupt service functionbutton_wait中断,同样也会唤醒poll机制,使pollMechanism re-process sleep count

2.3 驱动程序.poll函数返回值介绍

When interrupting the sleep state,返回mask为0
Returns when run：mask |= POLLIN | POLLRDNORM
The meaning of the parameters is as follows:

• 所以POLLIN | POLLRDNORM:普通数据可读|优先级带数据可读
• maskback to the application layerpoll函数,

3.Improve testing proceduresthird_poll_text.c(添加poll函数)

• 在linux中可以通过man poll 来查看poll函数如何使用
• poll函数原型如下（#include <poll.h>）:

int poll(struct pollfd *fds, nfds_t nfds, int timeout);

参数介绍:

1) *fds:是一个pollAn array of descriptor structures(可以处理多个poll),结构体pollfd如下:

struct pollfd {
               int   fd;         /* file descriptor 文件描述符*/
               short events;     /* requested events 请求的事件*/
               short revents;    /* returned events 返回的事件(函数返回值)*/
           };

• 其中events和reventsThe value parameters are as follows:

2) nfds:表示多少个poll,如果1个,就填入1

3) timeout:定时多少ms

返回值介绍:

1. 返回值为0:Indicates timeout orfdThe file descriptor could not be opened
2. 返回值为 -1:表示错误
3. 返回值为>0时 :are the following constants

• The final improved test code is as follows:

#include <sys/types.h>    
#include <sys/stat.h>    
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <poll.h>                 //添加poll头文件


/*useg:    thirdtext   */
int main(int argc,char **argv)
{
  int fd,ret;
  unsigned int val=0;
  struct pollfd fds;                          //定义poll文件描述结构体               
  fd=open("/dev/buttons",O_RDWR);          

 if(fd<0)
        {printf("can't open!!!\n");
        return -1;}

  fds.fd=fd;                   
  fds.events= POLLIN;           //请求类型是 普通或优先级带数据可读

 while(1)
 {

       ret=poll(&fds,1,5000) ;           //一个poll, 定时5000ms,进入休眠状态
                  if(ret==0)                        //超时
                   {
                             printf("time out \r\n");
                   }
                   else if(ret>0)                   //pollThe mechanism is awakened,表示有数据可读
                   {
                            read(fd,&val,1);         //读取一个值
                            printf("key_val=0X%x\r\n",val);
                   }  
 }
 return 0;
}

效果如下: