linux-kernel - Re: [PATCH 4/4] timekeeping: clocksource_cyc2ns: Document intended range limitation

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Date:   Mon, 21 Nov 2016 14:06:27 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Chris Metcalf <cmetcalf@...lanox.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>,
        Prarit Bhargava <prarit@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 4/4] timekeeping: clocksource_cyc2ns: Document intended
 range limitation

On Mon, Nov 21, 2016 at 7:28 AM, Chris Metcalf <cmetcalf@...lanox.com> wrote:
> On 11/21/2016 3:54 AM, Ingo Molnar wrote:
>>
>> * John Stultz <john.stultz@...aro.org> wrote:
>>
>>> From: Chris Metcalf <cmetcalf@...lanox.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@...il.com>
>>> Cc: Ingo Molnar <mingo@...nel.org>
>>> Cc: Prarit Bhargava <prarit@...hat.com>
>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>> Signed-off-by: Chris Metcalf <cmetcalf@...lanox.com>
>>> [jstultz: Fixed up to merge against HEAD & commit message tweaks]
>>> Signed-off-by: John Stultz <john.stultz@...aro.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?

Sure. Reworded to take Ingo's suggestions.

thanks
-john