From ec4101e8903e318b9fd4e3bbf72b1eaba53c64e1 Mon Sep 17 00:00:00 2001 From: Chris Metcalf Date: Mon, 28 Nov 2016 14:35:20 -0800 Subject: [PATCH] timekeeping/clocksource_cyc2ns: Document intended range limitation 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. [jstultz: Fixed up to merge against HEAD & commit message tweaks, also included rewording suggestion by Ingo] Signed-off-by: Chris Metcalf Signed-off-by: John Stultz Cc: Prarit Bhargava Cc: Richard Cochran Link: http://lkml.kernel.org/r/1480372524-15181-4-git-send-email-john.stultz@linaro.org Signed-off-by: Thomas Gleixner --- 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 08398182f56e..65602d395a52 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 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. * * XXX - This could use some mult_lxl_ll() asm optimization */ -- 2.20.1