Re: [PATCH 4/4] timekeeping: clocksource_cyc2ns: Document intended range limitation
From: Ingo Molnar
Date: Mon Nov 21 2016 - 03:54:11 EST
* John Stultz <john.stultz@xxxxxxxxxx> wrote:
> From: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
> The "cycles" argument should not be an absolute clocksource cycle
> value, as the implementation's arithmetic will overflow relatively
> easily with wide (64 bit) clocksource counters.
> For performance, the implementation is simple and fast, since the
> function is intended for only relatively small delta values of
> clocksource cycles.
> Cc: Richard Cochran <richardcochran@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
> [jstultz: Fixed up to merge against HEAD & commit message tweaks]
> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> include/linux/clocksource.h | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
> diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
> index 0839818..0881bca 100644
> --- a/include/linux/clocksource.h
> +++ b/include/linux/clocksource.h
> @@ -169,7 +169,10 @@ static inline u32 clocksource_hz2mult(u32 hz, u32 shift_constant)
> * @mult: cycle to nanosecond multiplier
> * @shift: cycle to nanosecond divisor (power of two)
> - * Converts cycles to nanoseconds, using the given mult and shift.
> + * Converts clocksource cycles to nanoseconds, using the given mult and shift.
> + * The code is optimized for performance and not intended to work
> + * with absolute clocksource cycles, as it will easily overflow,
> + * but just intended for relative (delta) clocksource cycles.
Had to read this explanation twice, how about:
* Converts clocksource cycles to nanoseconds, using the given @mult and @shift.
* The code is optimized for performance and is not intended to work
* with absolute clocksource cycles (as those will easily overflow),
* but is only intended to be used with relative (delta) clocksource cycles.
Did I get it right?