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SubjectRe: [PATCH 4/4] timekeeping: clocksource_cyc2ns: Document intended range limitation
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On 11/21/2016 3:54 AM, Ingo Molnar wrote:
> * John Stultz <john.stultz@linaro.org> wrote:
>
>> From: Chris Metcalf <cmetcalf@mellanox.com>
>>
>> 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@gmail.com>
>> Cc: Ingo Molnar <mingo@kernel.org>
>> Cc: Prarit Bhargava <prarit@redhat.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
>> [jstultz: Fixed up to merge against HEAD & commit message tweaks]
>> Signed-off-by: John Stultz <john.stultz@linaro.org>
>> ---
>> 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?

Yes, I think that's an improvement. Thanks!

John, I assume you can just fix this up?

--
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com

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