本文主要是介绍字符设备之poll机制,希望对大家解决编程问题提供一定的参考价值,需要的开发者们随着小编来一起学习吧!
poll机制作用:相当于一个定时器,时间到了还没有资源就唤醒进程。主要用途就是:进程设置一段时间用来等待资源,如果时间到了资源还没有到来,进程就立刻从睡眠状态唤醒不再等待。当然这只是使用于这段时间以后资源对于该进程已经无用的情况。
内核中poll机制的实现过程:
sys_poll函数在include/linux/syscalls.h中声明
//函数定义前加宏asmlinkage ,表示这些函数通过堆栈而不是通过寄存器传递参数。
asmlinkage long sys_poll(struct pollfd __user *ufds, unsigned int nfds,long timeout);
在系统调用表arch\arm\kernel\calls.S中调用
CALL(sys_poll) //系统调用跳转表的一项
关于系统调用表的初始化在arch/arm/kernel/entry-common.S中
.equ NR_syscalls,0 //将NR_syscalls初始化为0
#define CALL(x) .equ NR_syscalls,NR_syscalls+1 //将CALL(x) 定义为: NR_syscalls = NR_syscalls + 1
#include "calls.S"//将calls.S的内容包进来,CALL(x)上面已经有了定义,这里就相当于执行了多次NR_syscalls++,最后就统计了系统调用的个数,并对NR_syscalls进行了4的倍数对齐,这一招,特么好厉害!
#undef CALL //撤销CALL宏定义
#define CALL(x) .long x //对CALL重新进行宏定义,也是4字节对齐
arch/arm/kernel/entry-common.S中:
sys_syscall:bic scno, r0, #__NR_OABI_SYSCALL_BASEcmp scno, #__NR_syscall - __NR_SYSCALL_BASEcmpne scno, #NR_syscalls @ check rangestmloia sp, {r5, r6} @ shuffle argsmovlo r0, r1movlo r1, r2movlo r2, r3movlo r3, r4ldrlo pc, [tbl, scno, lsl #2]b sys_ni_syscall
最终sys_poll()函数,就相当于下面的函数:在fs/select.c文件中,SYSCALL_DEFINE3是有3个参数的系统调用的宏定义SYSCALL_DEFINE3(poll, struct pollfd __user *, ufds, unsigned int, nfds,long, timeout_msecs)
{......ret = do_sys_poll(ufds, nfds, to);//调用......
}
好,看看应用层调用poll函数时的底层驱动执行线路【app:】
poll();
【kernel: 】
sys_polldo_sys_poll(struct pollfd __user *ufds, unsigned int nfds,struct timespec *end_time)poll_initwait(&table);init_poll_funcptr(&table->pt, __pollwait);-->pt->qproc = __pollwait; //初始化qproc函数指针,让他指向__pollwait函数do_poll(nfds, head, &table, end_time);for(;;){for (; pfd != pfd_end; pfd++) //查询多个驱动程序{if (do_pollfd(pfd, pt)) -> mask = file->f_op->poll(file, pwait);return mask;{ //do_pollfd函数相当于调用驱动里面的xxx_poll函数,下面另外再进行分析,返回值mask非零,count++,记录等待事件发生的进程数count++;pt = NULL;}}if (count || timed_out) //若count不为0(有等待的事件发生了)或者timed_out不为0(有信号发生或超时),则推出休眠break;//上述条件不满足下面开始进入休眠,若有等待的事件发生了,超时或收到信号则唤醒poll_schedule_timeout(wait, TASK_INTERRUPTIBLE, to, slack)}
驱动里边的xxx_poll()函数分析xxx_poll(struct file *file, poll_table *wait)poll_wait(file, &xxxx_waitq, wait);
//
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); //调用之前poll_initwait()函数设置的函数qproc即__pollwait,__pollwait函数只是把当前进程挂到等待队列,只是add_wait_queue(wait_address, &entry->wait);不进入休眠
}
测试驱动程序:poll_dev.c
#include <linux/delay.h>
#include <linux/irq.h>
#include <asm/uaccess.h>
#include <asm/irq.h>
#include <asm/io.h>
#include <linux/module.h>
#include <linux/device.h> //class_create
#include <mach/regs-gpio.h> //S3C2440_GPF1
#include <mach/hardware.h>
#include <linux/interrupt.h> //wait_event_interruptible
#include <linux/fs.h>
#include <linux/poll.h> //poll/* 定义并初始化等待队列头 */
static DECLARE_WAIT_QUEUE_HEAD(button_waitq);static struct class *buttondev_class;
static struct device *buttons_device;static struct pin_desc{unsigned int pin;unsigned int key_val;
};static struct pin_desc pins_desc[4] = {{S3C2410_GPF1,0x01}, //S3C2410_GPF1是对GPF1引脚这种“设备”的编号dev_id{S3C2410_GPF4,0x02},{S3C2410_GPF2,0x03},{S3C2410_GPF0,0x04},
};
static int ev_press = 0;static unsigned char key_val;
int major;/* 中断处理函数 */
static irqreturn_t handle_irq(int irq, void *dev_id)
{struct pin_desc *irq_pindesc = (struct pin_desc *)dev_id;//unsigned int pinval;pinval = s3c2410_gpio_getpin(irq_pindesc->pin);//获取按键值:有按键按下返回按键值0/* 键值: 按下时, 0x01, 0x02, 0x03, 0x04 *//* 键值: 松开时, 0x81, 0x82, 0x83, 0x84 */if(pinval){/* 松开 */key_val = 0x80 | (irq_pindesc->key_val);}else{/* 按下 */key_val = irq_pindesc->key_val;}ev_press = 1; /* 表示中断已经发生 */wake_up_interruptible(&button_waitq); /* 唤醒休眠的进程 */return IRQ_HANDLED;
}static int buttons_dev_open(struct inode * inode, struct file * filp)
{/* K1-EINT1,K2-EINT4,K3-EINT2,K4-EINT0* 配置GPF1、GPF4、GPF2、GPF0为相应的外部中断引脚* IRQT_BOTHEDGE应该改为IRQ_TYPE_EDGE_BOTH*/request_irq(IRQ_EINT1, handle_irq, IRQ_TYPE_EDGE_FALLING, "K1",&pins_desc[0]);request_irq(IRQ_EINT4, handle_irq, IRQ_TYPE_EDGE_FALLING, "K2",&pins_desc[1]);request_irq(IRQ_EINT2, handle_irq, IRQ_TYPE_EDGE_FALLING, "K3",&pins_desc[2]);request_irq(IRQ_EINT0, handle_irq, IRQ_TYPE_EDGE_FALLING, "K4",&pins_desc[3]);return 0;
}static int buttons_dev_close(struct inode *inode, struct file *file)
{free_irq(IRQ_EINT1,&pins_desc[0]);free_irq(IRQ_EINT4,&pins_desc[1]);free_irq(IRQ_EINT2,&pins_desc[2]);free_irq(IRQ_EINT0,&pins_desc[3]);return 0;
}static ssize_t buttons_dev_read(struct file *file, char __user *user, size_t size,loff_t *ppos)
{if (size != 1)return -EINVAL;//wait_event_interruptible(button_waitq, ev_press);//使用poll机制,这里就不需要再判断要不要进入睡眠了。copy_to_user(user, &key_val, 1);/* 将ev_press清零 */ev_press = 0;return 1;
}//关键点///
static unsigned int buttons_dev_poll(struct file *file, poll_table *wait) //该函数一旦被调用就触发poll机制
{unsigned int mask = 0;/* 该函数,只是将进程挂在button_waitq队列上,而不是立即休眠 */poll_wait(file, &button_waitq, wait); /**** 假设进程还poll在上面这一函数里边,尚未超时,假设此时有中断到来,中断处理程序将ev_press置位,然后唤醒休眠队列上对应的进程***//* 进程唤醒之后,立马往下执行。唤醒的可能原因:超时/中断处理 */if(ev_press){mask |= POLLIN | POLLRDNORM; /* 有数据可读 */}/* 如果有按键按下时,mask |= POLLIN | POLLRDNORM,否则mask = 0 */return mask;
}
////* File operations struct for character device */
static const struct file_operations buttons_dev_fops = {.owner = THIS_MODULE,.open = buttons_dev_open,.read = buttons_dev_read,.release = buttons_dev_close,.poll = buttons_dev_poll,
};/* 驱动入口函数 */
static int buttons_dev_init(void)
{/* 主设备号设置为0表示由系统自动分配主设备号 */major = register_chrdev(0, "buttons_dev", &buttons_dev_fops);/* 创建buttondev类 */buttondev_class = class_create(THIS_MODULE, "buttondev");/* 在buttondev类下创建buttons设备,供应用程序打开设备*/buttons_device = device_create(buttondev_class, NULL, MKDEV(major, 0), NULL, "buttons");//return 0;
}/* 驱动出口函数 */
static void buttons_dev_exit(void)
{unregister_chrdev(major, "buttons_dev");device_unregister(buttons_device); //卸载类下的设备class_destroy(buttondev_class); //卸载类
}/* 模块加载和卸载函数的修饰 */
module_init(buttons_dev_init);
module_exit(buttons_dev_exit); MODULE_AUTHOR("CLBIAO");
MODULE_DESCRIPTION("Just for Demon");
MODULE_LICENSE("GPL"); //遵循GPL协议
测试应用程序:app_poll.c
/* 文件的编译指令是arm-linux-gcc -static -o app_poll app_poll.c */
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <poll.h>/* fourth_test*/
int main(int argc ,char *argv[]){int fd;unsigned char key_val;struct pollfd fds;int ret;fd = open("/dev/buttons",O_RDWR);if (fd < 0){printf("open error\n");}fds.fd = fd;//查询的文件fds.events = POLLIN; //期待收到poll_in值,表示有数据while(1){/* A value of 0 indicates that the call timed out and no file descriptors were ready* poll函数返回0时,表示5s时间到了,而这段时间里,没有事件发生"数据可读"*/ret = poll(&fds,1,5000);if(ret == 0){printf("time out\n");}else /* 如果没有超时,则读出按键值 */{read(fd,&key_val,1);printf("key_val = 0x%x\n",key_val);} }return 0;
}
测试结果:
小结:poll流程图
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