江苏龙网

  1. 主页 > 生活百科 > >

Linux select/poll机制原理分析( 二 )

Linux

  • struct poll_table_entry[]：存放不同设备的poll_table_entry，这些条目的增加是在驱动调用poll_wait->__pollwait()时进行初始化并完成添加的；
    • 2.4 驱动编写启示如果驱动中要支持select的接口调用，那么需要做哪些事情呢？如果理解了上文中的内容，你会毫不犹豫的大声说出以下几条：
    1. 定义一个等待队列头wait_queue_head_t，用于收留等待队列任务；
    2. struct file_operations结构体中的poll函数需要实现，比如xxx_poll()；
    3. xxx_poll()函数中，当然不要忘了poll_wait函数的调用了，此外，该函数的返回值mask需要注意是在条件满足时对应的值，比如EPOLLIN/EPOLL/EPOLLERR等，这个返回值是在do_select()函数中会去判断处理的；
    4. 条件满足的时候，wake_up_interruptible唤醒任务，当然也可以使用wake_up，区别是：wake_up_interruptible只能唤醒处于TASK_INTERRUPTIBLE状态的任务，而wake_up能唤醒处于TASK_INTERRUPTIBLE和TASK_UNINTERRUPTIBLE状态的任务；
    2.5 select/poll的差异
    • select与poll本质上基本类似，其中select是由BSD UNIX引入，poll由SystemV引入；
    • select与poll需要轮询文件描述符集合，并在用户态和内核态之间进行拷贝，在文件描述符很多的情况下开销会比较大，select默认支持的文件描述符数量是1024；
    • Linux提供了epoll机制，改进了select与poll在效率与资源上的缺点，未深入了解；
    3. 示例代码3.1 内核驱动示例代码中的逻辑：
    1. 驱动维护一个count值，当count值大于0时，表明条件满足，poll返回正常的mask值；
    2. poll函数每执行一次，count值就减去一次；
    3. count的值可以由用户通过ioctl来进行设置；
    #include <linux/init.h>#include <linux/module.h>#include <linux/poll.h>#include <linux/wait.h>#include <linux/cdev.h>#include <linux/mutex.h>#include <linux/slab.h>#include <asm/ioctl.h>#define POLL_DEV_NAME"poll"#define POLL_MAGIC'P'#define POLL_SET_COUNT(_IOW(POLL_MAGIC, 0, unsigned int))struct poll_dev { struct cdev cdev; struct class *class; struct device *device; wait_queue_head_t wq_head; struct mutex poll_mutex; unsigned int count; dev_t devno;};struct poll_dev *g_poll_dev = NULL;static int poll_open(struct inode *inode, struct file *filp){ filp->private_data = https://www.isolves.com/it/rj/czxt/linux/2020-10-10/g_poll_dev; return 0;}static int poll_close(struct inode *inode, struct file *filp){ return 0;}static unsigned int poll_poll(struct file *filp, struct poll_table_struct *wait){ unsigned int mask = 0; struct poll_dev *dev = filp->private_data; mutex_lock(&dev->poll_mutex); poll_wait(filp, &dev->wq_head, wait); if (dev->count > 0) {mask |= POLLIN | POLLRDNORM;/* decrease each time */dev->count--; } mutex_unlock(&dev->poll_mutex); return mask;}static long poll_ioctl(struct file *filp, unsigned int cmd,unsigned long arg){ struct poll_dev *dev = filp->private_data; unsigned int cnt; switch (cmd) {case POLL_SET_COUNT:mutex_lock(&dev->poll_mutex);if (copy_from_user(&cnt, (void __user *)arg, _IOC_SIZE(cmd))) {pr_err("copy_from_user fail:%dn", __LINE__);return -EFAULT;}if (dev->count == 0) {wake_up_interruptible(&dev->wq_head);}/* update count */dev->count += cnt;mutex_unlock(&dev->poll_mutex);break;default:return -EINVAL; } return 0;}static struct file_operations poll_fops = { .owner = THIS_MODULE, .open = poll_open, .release = poll_close, .poll = poll_poll, .unlocked_ioctl = poll_ioctl, .compat_ioctl = poll_ioctl,};static int __init poll_init(void){ int ret; if (g_poll_dev == NULL) {g_poll_dev = (struct poll_dev *)kzalloc(sizeof(struct poll_dev), GFP_KERNEL);if (g_poll_dev == NULL) {pr_err("struct poll_dev allocate failn");return -1;} } /* allocate device number */ ret = alloc_chrdev_region(&g_poll_dev->devno, 0, 1, POLL_DEV_NAME); if (ret < 0) {pr_err("alloc_chrdev_region fail:%dn", ret);goto alloc_chrdev_err; } /* set char-device */ cdev_init(&g_poll_dev->cdev, &poll_fops); g_poll_dev->cdev.owner = THIS_MODULE; ret = cdev_add(&g_poll_dev->cdev, g_poll_dev->devno, 1); if (ret < 0) {pr_err("cdev_add fail:%dn", ret);goto cdev_add_err; } /* create device */ g_poll_dev->class = class_create(THIS_MODULE, POLL_DEV_NAME); if (IS_ERR(g_poll_dev->class)) {pr_err("class_create failn");goto class_create_err; } g_poll_dev->device = device_create(g_poll_dev->class, NULL,g_poll_dev->devno, NULL, POLL_DEV_NAME); if (IS_ERR(g_poll_dev->device)) {pr_err("device_create failn");goto device_create_err; } mutex_init(&g_poll_dev->poll_mutex); init_waitqueue_head(&g_poll_dev->wq_head); return 0;device_create_err: class_destroy(g_poll_dev->class);class_create_err: cdev_del(&g_poll_dev->cdev);cdev_add_err: unregister_chrdev_region(g_poll_dev->devno, 1);alloc_chrdev_err: kfree(g_poll_dev); g_poll_dev = NULL; return -1;}static void __exit poll_exit(void){ cdev_del(&g_poll_dev->cdev); device_destroy(g_poll_dev->class, g_poll_dev->devno); unregister_chrdev_region(g_poll_dev->devno, 1); class_destroy(g_poll_dev->class); kfree(g_poll_dev); g_poll_dev = NULL;}module_init(poll_init);module_exit(poll_exit);MODULE_DESCRIPTION("select/poll test");MODULE_AUTHOR("LoyenWang");MODULE_LICENSE("GPL");


    推荐阅读


    上一篇：红楼梦中的平儿是个什么样的人 红楼梦中的平儿的性格特征

    下一篇：乾陵挖开了吗 乾陵地宫入口找到了吗