linux-kernel - [tip:timers/core] clocksource: Improve comment explaining clocks_calc_max

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Date:	Fri, 3 Apr 2015 01:20:49 -0700
From:	tip-bot for John Stultz <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, hpa@...or.com,
	richardcochran@...il.com, mingo@...nel.org, john.stultz@...aro.org,
	peterz@...radead.org, prarit@...hat.com
Subject: [tip:timers/core] clocksource:
  Improve comment explaining clocks_calc_max_nsecs()'s 50% safety margin

Commit-ID:  8e56f33f8439b2f8e7f4ae7f3d0bfe683ecc3b09
Gitweb:     http://git.kernel.org/tip/8e56f33f8439b2f8e7f4ae7f3d0bfe683ecc3b09
Author:     John Stultz <john.stultz@...aro.org>
AuthorDate: Wed, 1 Apr 2015 20:34:39 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 3 Apr 2015 08:18:35 +0200

clocksource: Improve comment explaining clocks_calc_max_nsecs()'s 50% safety margin

Ingo noted that the description of clocks_calc_max_nsecs()'s
50% safety margin was somewhat circular. So this patch tries
to improve the comment to better explain what we mean by the
50% safety margin and why we need it.

Signed-off-by: John Stultz <john.stultz@...aro.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Richard Cochran <richardcochran@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/1427945681-29972-20-git-send-email-john.stultz@linaro.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/time/clocksource.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index c3be3c7..15facb1 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -472,8 +472,11 @@ static u32 clocksource_max_adjustment(struct clocksource *cs)
  * @max_cyc:	maximum cycle value before potential overflow (does not include
  *		any safety margin)
  *
- * NOTE: This function includes a safety margin of 50%, so that bad clock values
- * can be detected.
+ * NOTE: This function includes a safety margin of 50%, in other words, we
+ * return half the number of nanoseconds the hardware counter can technically
+ * cover. This is done so that we can potentially detect problems caused by
+ * delayed timers or bad hardware, which might result in time intervals that
+ * are larger then what the math used can handle without overflows.
  */
 u64 clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask, u64 *max_cyc)
 {
--
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