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Message-ID: <20230425183313.462051641@linutronix.de>
From:   Thomas Gleixner <tglx@linutronix.de>
To:     LKML <linux-kernel@vger.kernel.org>
Cc:     Frederic Weisbecker <frederic@kernel.org>,
        Anna-Maria Behnsen <anna-maria@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Sebastian Siewior <bigeasy@linutronix.de>,
        syzbot+5c54bd3eb218bb595aa9@syzkaller.appspotmail.com,
        Dmitry Vyukov <dvyukov@google.com>
Subject: [patch 12/20] posix-timers: Document sys_clock_getoverrun()
References: <20230425181827.219128101@linutronix.de>
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Date:   Tue, 25 Apr 2023 20:49:14 +0200 (CEST)
Precedence: bulk

Document the syscall in detail and with coherent sentences.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
---
 kernel/time/posix-timers.c |   25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -782,14 +782,23 @@ SYSCALL_DEFINE2(timer_gettime32, timer_t
 
 #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)
 {