[PATCH 19/21] clocksource: Improve comment explaining clocks_calc_max_nsecs()'s 50% safety margin

John Stultz
2015-04-02T03:36:42Z

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.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.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)
{
--
1.9.1