Implementation of Poll Mechanism in Char Device Driver - select

Question

Hello Dear participants of stackoverflow,
I'm new to kernel space development and still in the beginning of the road.
I developed a basic char device driver that can read open close etc . But couldn't find a proper source and how to tutorial for Poll/select mechanism sample.
I've written the sample code for poll function below:
static unsigned int dev_poll(struct file * file, poll_table *wait)
{
poll_wait(file,&dev_wait,wait);
if (size_of_message > 0 ){
printk(KERN_INFO "size_of_message > 0 returning POLLIN | POLLRDNORM\n");
return POLLIN | POLLRDNORM;
}
else {
printk(KERN_INFO "dev_poll return 0\n");
return 0;
}
}
It works fine but couldn't undestand a few things.
When I call select from user space program as
struct timeval time = {5,0 } ;
select(fd + 1 , &readfs,NULL,NULL,&time);
the dev_poll function in driver called once and return zero or POLLIN in order to buffer size . And then never called again. In user space , after 5 seconds the program continue if dev_poll returned 0.
What I couldn't understand is here , How the driver code will decide and let user space program if there is something in buffer that is readable withing this 5 seconds , if it's called once and returned immediately.
Is there anyway in kernel module to gather information of timeval parameter that comes from userspace ?
Thank you from now on.
Regards,

Answer 1

Call poll_wait() actually places some wait object into a waitqueue, specified as a second parameter. When wait object is fired (via waitqueue's wake_up or similar function), the poll function is evaluated again.
Kernel driver needn't to bother about timeouts: when time is out, the wait object will be removed from the waitqueue automatically.

Answer 2

Hello dear curious people like me about poll . I came up with a solution.
From another topic on stackowerflow a guy said that the poll_function is called multiple times if kernel need to last situation. So basically I implement that code .
when poll called call wait_poll(wait_queue_head);
when device have buffered data(this is usually in driver write function).
call wake_up macro with wait_queue_head paramater.
So after this step poll function of driver is called again .
So here you can return whatever you want to return. In this case POLLIN | POLLRDNORM..
Here is my sample code for write and poll in the driver.
static unsigned int dev_poll(struct file * file, poll_table *wait)
{
static int dev_poll_called_count = 0 ;
dev_poll_called_count ++;
poll_wait(file,&dev_wait,wait);
read_wait_queue_length++;
printk(KERN_INFO "Inside dev_poll called time is : %d read_wait_queue_length %d\n",dev_poll_called_count,read_wait_queue_length);
printk(KERN_INFO "After poll_wait wake_up called\n");
if (size_of_message > 0 ){
printk(KERN_INFO "size_of_message > 0 returning POLLIN | POLLRDNORM\n");
return POLLIN | POLLRDNORM;
}
else {
printk(KERN_INFO "dev_poll return 0\n");
return 0;
}
}
static ssize_t dev_write(struct file *filep, const char *buffer, size_t len, loff_t *offset){
printk(KERN_INFO "Inside write \n");;
int ret;
ret = copy_from_user(message, buffer, len);
size_of_message = len ;
printk(KERN_INFO "EBBChar: Received %zu characters from the user\n", size_of_message);
if (ret)
return -EFAULT;
message[len] = '\0';
printk(KERN_INFO "gelen string %s", message);
if (read_wait_queue_length)
{
wake_up(&dev_wait);
read_wait_queue_length = 0;
}
return len;
}