如何在内核模块代码中添加轮询功能？

Question

据我所知，要从内核空间通知用户空间，一种方法是使用轮询。这意味着内核驱动程序应首先提供轮询方法。下面的代码是从互联网上找到的，它确实有效！

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include <linux/string.h>
#include <linux/vmalloc.h>
#include <asm/uaccess.h>

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Fortune Cookie Kernel Module");
MODULE_AUTHOR("M. Tim Jones");

#define MAX_COOKIE_LENGTH       PAGE_SIZE

static struct proc_dir_entry *proc_entry;
static char *cookie_buf;  // Space for fortune strings
static int write_index;   // Index to write next fortune
static int read_index;    // Index to read next fortune

ssize_t fortune_write( struct file *filp, const char __user *buff,
                        unsigned long len, void *data )
// Refer to: ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
{
  int space_available = (MAX_COOKIE_LENGTH-write_index);

  if (len > space_available) {
    printk(KERN_INFO "fortune: cookie buffer is full!\n");
    return -ENOSPC;
  }

  if (copy_from_user( &cookie_buf[write_index], buff, len )) {
    return -EFAULT;
  }

  write_index += len;
  cookie_buf[write_index-1] = 0;

  return len;
}

ssize_t fortune_read(struct file *file, char *buf, size_t count, loff_t *f_pos){
// Refer to: ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
    int len;

    //there's no fortune or a fortune has already been read
    //the *f_pos > 0 hack is needed because `cat /proc/fortune` would otherwise
    //display every thing in the cookie_buf
    if(write_index == 0 || *f_pos > 0){
        return 0;
    }

    // cicle through fortunes
    if(read_index >= write_index){
        read_index = 0;
    }

    len = sprintf(buf, "%s\n", &cookie_buf[read_index]);

    read_index += len;
    *f_pos += len;

    return len;
}

static const struct file_operations proc_test_fops = {
   .owner        = THIS_MODULE,
//    .open        = led_proc_open,
   .read        = fortune_read,
//    .llseek        = seq_lseek,
//    .release    = single_release,
   .write        = fortune_write,
//    unsigned int (*poll) (struct file *, struct poll_table_struct *);
//    int (*fasync) (int, struct file *, int);
};

int __init init_fortune_module( void )
{
    int ret = 0;
    cookie_buf = (char *)vmalloc( MAX_COOKIE_LENGTH );
    if (!cookie_buf) {
        ret = -ENOMEM;
    } else {
        memset( cookie_buf, 0, MAX_COOKIE_LENGTH );
//        proc_entry = create_proc_entry( "fortune", 0644, NULL );
        proc_entry = proc_create( "fortune", 0644, NULL, &proc_test_fops );

        if (proc_entry == NULL) {
            ret = -ENOMEM;
            vfree(cookie_buf);
            printk(KERN_INFO "fortune: Couldn't create proc entry\n");
        } else {
            write_index = 0;
            read_index = 0;
            printk(KERN_INFO "fortune: Module loaded.\n");
        }
    }

    return ret;
}

void __exit exit_fortune_module( void )
{
//    remove_proc_entry("fortune", &proc_entry);
    proc_remove(proc_entry);
    vfree(cookie_buf);
    printk(KERN_INFO "fortune: Module unloaded.\n");
}

module_init( init_fortune_module );
module_exit( exit_fortune_module );

我可以这样使它起作用：

echo "hello" > /proc/fortune

然后

cat /proc/fortune

看结果。

但是如何添加轮询方法呢？我试了几次，但还是失败了。有人可以帮忙吗？谢谢！

Answer 1

您可以在内核本身中找到一些很好的示例。看下一个文件：

• 驱动程序/char/rtc.c
• fs / proc / kmsg.c
• drivers / char / random.c

要将poll()功能添加到代码中，请遵循以下步骤。

1. 包括所需的标题：

#include <linux/wait.h>
#include <linux/poll.h>

1. 声明waitqueue变量：

static DECLARE_WAIT_QUEUE_HEAD(fortune_wait);

1. 添加fortune_poll()函数并将其（作为.poll回调）添加到文件操作结构中：

static unsigned int fortune_poll(struct file *file, poll_table *wait)
{
poll_wait(file, &fortune_wait, wait);
if (new-data-is-ready)
    return POLLIN | POLLRDNORM;
return 0;
}

static const struct file_operations proc_test_fops = {
....
.poll = fortune_poll,
};

请注意，POLLIN | POLLRDNORM如果您有一些新数据要读取，并且0没有新数据要读取（poll()呼叫超时），则应返回。有关详细信息，请参见man2民意调查。

1. 有了新数据后，通知您的等待队列：

   wake_up_interruptible(&fortune_wait);
   

这是实现poll()操作的基本内容。根据您的任务，可能需要在函数中使用一些waitqueue
API
.read（例如wait_event_interruptible()）。