Commit ec4101e8 authored Nov 28, 2016 by Chris Metcalf Committed by Thomas Gleixner Nov 29, 2016

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 <cmetcalf@mellanox.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Link: http://lkml.kernel.org/r/1480372524-15181-4-git-send-email-john.stultz@linaro.org


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent ba58d102

include/linux/clocksource.h

+4 −1

Original line number	Diff line number	Diff line
		@@ -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
		*/