linux-kernel - Re: [PATCH 19/21] clocksource: Improve comment explaining clocks_calc_max

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Date:	Thu, 2 Apr 2015 10:50:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Prarit Bhargava <prarit@...hat.com>,
	Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH 19/21] clocksource: Improve comment explaining
 clocks_calc_max_nsecs()'s 50% safety margin

On Wed, Apr 01, 2015 at 08:34:39PM -0700, John Stultz wrote:
> 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@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Prarit Bhargava <prarit@...hat.com>
> Cc: Richard Cochran <richardcochran@...il.com>
> Signed-off-by: John Stultz <john.stultz@...aro.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)
>  {

Should we make a further note that the tk_fast things rely on this
slack since they're not strongly serialized against this? That is, they
can end up using an older cycle_last value and therefore end up with a
larger delta than other code.
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