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SubjectProblems implementing poll call
Date

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

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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