[tip: timers/core] posix-timers: Document sys_clock_getoverrun()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     1adc8de86737c3d134c2da5ed8e56c2de4729923
Gitweb:        https://git.kernel.org/tip/1adc8de86737c3d134c2da5ed8e56c2de4729923
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Tue, 25 Apr 2023 20:49:14 +02:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Mon, 05 Jun 2023 17:03:38 +02:00

posix-timers: Document sys_clock_getoverrun()

Document the syscall in detail and with coherent sentences.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230425183313.462051641@xxxxxxxxxxxxx

---
 kernel/time/posix-timers.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 663d3c6..67c1d4d 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -783,14 +783,23 @@ SYSCALL_DEFINE2(timer_gettime32, timer_t, timer_id,
 
 #endif
 
-/*
- * Get the number of overruns of a POSIX.1b interval timer.  This is to
- * be the overrun of the timer last delivered.  At the same time we are
- * accumulating overruns on the next timer.  The overrun is frozen when
- * the signal is delivered, either at the notify time (if the info block
- * is not queued) or at the actual delivery time (as we are informed by
- * the call back to posixtimer_rearm().  So all we need to do is
- * to pick up the frozen overrun.
+/**
+ * sys_timer_getoverrun - Get the number of overruns of a POSIX.1b interval timer
+ * @timer_id:	The timer ID which identifies the timer
+ *
+ * The "overrun count" of a timer is one plus the number of expiration
+ * intervals which have elapsed between the first expiry, which queues the
+ * signal and the actual signal delivery. On signal delivery the "overrun
+ * count" is calculated and cached, so it can be returned directly here.
+ *
+ * As this is relative to the last queued signal the returned overrun count
+ * is meaningless outside of the signal delivery path and even there it
+ * does not accurately reflect the current state when user space evaluates
+ * it.
+ *
+ * Returns:
+ *	-EINVAL		@timer_id is invalid
+ *	1..INT_MAX	The number of overruns related to the last delivered signal
  */
 SYSCALL_DEFINE1(timer_getoverrun, timer_t, timer_id)
 {