Problem implementing poll method

From: Constantine Gavrilov (
Date: Thu Oct 17 2002 - 03:28:11 EST

Constantine Gavrilov
Linux Leader
Optibase Ltd
7 Shenkar St, Herzliya 46120, Israel
Phone: (972-9)-970-9140
Fax:   (972-9)-958-6099


I have a problem implementing poll method.

I have written a driver for MPEG encoder card. The user-space SDK needs to be able to wait for a certain event that is reported by the interrupt handler. I have done it using ioctl method, like this:

u32 timeout=milliseconds * HZ / 1000;

set_bit(0, &dev->fintwait); if(test_bit(0, &dev->fintwait)) { interruptible_sleep_on_timeout(&dev->interrupt_queue,timeout);

if (signal_pending(current)) { printk(KERN_ERR "optenc: IntWait restarted by signal\n"); return -ERESTARTSYS; }

if(test_bit(0, &dev->fintwait)) { printk(KERN_ERR "optenc: intwait timeout\n"); ..//returns wait timeout } else { ...//returns wait OK } } //returns wait OK The interrupt handler wakes up the queue and updates dev->fintwait like this:

clear_bit(0, &dev->fintwait); wake_up_interruptible(&dev->interrupt_queue);

It worked very well for me. I wanted to implement the same wait using the poll method. So, my poll function looks like this:

unsigned int optenc_poll(struct file *filp, poll_table *wait_table) { unsigned int mask = 0; struct mydev *dev = filp->private_data; set_bit(0, &dev->fintwait); if(test_bit(0, &dev->fintwait)) { poll_wait(filp, &dev->interrupt_queue, wait_table); if(test_bit(0, &dev->fintwait)) return mask; else { mask |= POLLIN |POLLRDNORM; return mask; } } else { mask |= POLLIN |POLLRDNORM; return mask; } }

Seems straightforward and the same thing as above. But, I have the following problems:

a) I have a lot of calls with wait_table = NULL and poll_wait does not block. I always do select on one file descriptor only and I never use zero timeout, so I do not understand the reason for it.

b) Even when wait_table is not NULL, poll_wait returns before (!!) interrupt handler wakes up the queue. I have checked it with printk. It always like this:

set_bit poll_wait poll_wait returns and test_bit is true wake_up and clear_bit

It is like poll_wait does not seem to block and I have spurious calles with wait_table == NULL. Any ideas?

Just to verify, the user-space wait function looks like this :

BOOL Wait(int timeout) {

fd_set set; struct timeval tv; int retval;

FD_ZERO(&set); FD_SET(fd, &set); tv.tv_sec = timeout/1000; tv.tv_usec = (timeout%1000)*1000;

int rc=select(fd+1, &set, NULL, NULL, &tv); if(rc == -1) { PERROR("select"); return FALSE; } if(rc == 1) return TRUE; else return FALSE; }

I use 2.4.18-pre7ac1 and I have also checked stock RedHat's 2.4.9-34.

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