linux - 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该首先提供 poll 方法。下面的代码是从网上找来的，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 );

我可以这样做让它工作:

echo "hello" > /proc/fortune

然后

cat /proc/fortune

查看结果。

但是如何给它添加poll方法呢？我尝试了几次，但还是失败了。

Answer 1

你可以在内核中找到一些很好的例子。查看下一个文件:

• drivers/rtc/dev.c , drivers/rtc/interface.c
• kernel/printk/printk.c
• drivers/char/random.c

要将 poll() 函数添加到您的代码中，请按照以下步骤操作。

1. 包括所需的 header :

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

2. 声明等待队列变量:

    static DECLARE_WAIT_QUEUE_HEAD(fortune_wait);
   

3. 添加 fortune_poll() 函数并将其(作为 .poll 回调)添加到您的文件操作结构中:

    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，您应该返回 POLLIN | POLLRDNORM 如果您有一些新数据要读取，0 如果没有新数据要读取(poll() 调用超时)。参见 man 2 poll了解详情。

4. 一旦有新数据就通知您的等待队列:

    wake_up_interruptible(&fortune_wait);
   

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

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另见相关问题:Implementing poll in a Linux kernel module .