[PATCH] pps: fix poll support

[Denis OSTERLAND-HEIM <denis.osterland@diehl.com>]

[BUG]
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
      }
    }

[CAUSE]
pps_cdev_poll() returns unconditionally EPOLLIN.

[FIX]
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: eae9d2ba0cfc ("LinuxPPS: core support")
CC: stable@vger.kernel.org # 5.4+
---
 drivers/pps/pps.c          | 11 +++++++++--
 include/linux/pps_kernel.h |  1 +
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
index 6a02245ea35f..9463232af8d2 100644
--- a/drivers/pps/pps.c
+++ b/drivers/pps/pps.c
@@ -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;
diff --git a/include/linux/pps_kernel.h b/include/linux/pps_kernel.h
index c7abce28ed29..aab0aebb529e 100644
--- a/include/linux/pps_kernel.h
+++ b/include/linux/pps_kernel.h
@@ -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 */
--
2.47.2
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