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pps: fix poll support

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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 <denis.osterland@diehl.com>
Co-developed-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Rodolfo Giometti <giometti@enneenne.com>
Fixes: eae9d2ba0c ("LinuxPPS: core support")
Link: https://lore.kernel.org/all/f6bed779-6d59-4f0f-8a59-b6312bd83b4e@enneenne.com/
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Link: https://lore.kernel.org/r/c3c50ad1eb19ef553eca8a57c17f4c006413ab70.camel@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Denis OSTERLAND-HEIM 2025-05-28 12:57:50 +02:00 • committed by Greg Kroah-Hartman
parent 6bca1e9558
commit 12c409aa1e
2 changed files with 10 additions and 2 deletions
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11
drivers/pps/pps.c
View file

@ -41,6 +41,9 @@ static __poll_t pps_cdev_poll(struct file *file, poll_table *wait)
poll_wait(file, &pps->queue, wait);
if (pps->last_fetched_ev == pps->last_ev)
return 0;
return EPOLLIN | EPOLLRDNORM;
}
@ -186,9 +189,11 @@ static long pps_cdev_ioctl(struct file *file,
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;
fdata.info.assert_tu = pps->assert_tu;
@ -272,9 +277,11 @@ static long pps_cdev_compat_ioctl(struct file *file,
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;
compat.info.current_mode = pps->current_mode;

1
include/linux/pps_kernel.h
View file

@ -52,6 +52,7 @@ struct pps_device {
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 */