pps: fix poll support

Because pps_cdev_poll() returns unconditionally EPOLLIN,
a user space program that calls select/poll get always an immediate data
ready-to-read response. As a result the intended use to wait until next
data becomes ready does not work.

User space snippet:

struct pollfd pollfd = {
.fd = open("/dev/pps0", O_RDONLY),
.events = POLLIN|POLLERR,
.revents = 0 };
while(1) {
poll(&pollfd, 1, 2000/*ms*/); // returns immediate, but should wait
if(revents & EPOLLIN) { // always true
struct pps_fdata fdata;
memset(&fdata, 0, sizeof(memdata));
ioctl(PPS_FETCH, &fdata); // currently fetches data at max speed
}
}

Lets remember the last fetch event counter and compare this value
in pps_cdev_poll() with most recent event counter
and return 0 if they are equal.

Signed-off-by: Denis OSTERLAND-HEIM <[e-mail]>
Co-developed-by: Rodolfo Giometti <[e-mail]>
Signed-off-by: Rodolfo Giometti <[e-mail]>
Fixes: eae9d2ba0cfc ("LinuxPPS: core support")
Link: https://lore.kernel.org/all/[e-mail]/
Acked-by: Rodolfo Giometti <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/pps/pps.c

···
 
 	poll_wait(file, &pps->queue, wait);
 
+	if (pps->last_fetched_ev == pps->last_ev)
+		return 0;
+
 	return EPOLLIN | EPOLLRDNORM;
 }
 
···
 		if (err)
 			return err;
 
-		/* Return the fetched timestamp */
+		/* Return the fetched timestamp and save last fetched event  */
 		spin_lock_irq(&pps->lock);
+
+		pps->last_fetched_ev = pps->last_ev;
 
 		fdata.info.assert_sequence = pps->assert_sequence;
 		fdata.info.clear_sequence = pps->clear_sequence;
···
 		if (err)
 			return err;
 
-		/* Return the fetched timestamp */
+		/* Return the fetched timestamp and save last fetched event  */
 		spin_lock_irq(&pps->lock);
+
+		pps->last_fetched_ev = pps->last_ev;
 
 		compat.info.assert_sequence = pps->assert_sequence;
 		compat.info.clear_sequence = pps->clear_sequence;

include/linux/pps_kernel.h

···
 	int current_mode;			/* PPS mode at event time */
 
 	unsigned int last_ev;			/* last PPS event id */
+	unsigned int last_fetched_ev;		/* last fetched PPS event id */
 	wait_queue_head_t queue;		/* PPS event queue */
 
 	unsigned int id;			/* PPS source unique ID */