Commit a7ea3bbf authored Jul 27, 2012 by Catalin Marinas Committed by Ingo Molnar Jul 31, 2012

time/jiffies: Allow CLOCK_TICK_RATE to be undefined



CLOCK_TICK_RATE is a legacy constant that defines the timer
device's granularity. On hardware with particularly coarse
granularity, this constant is used to reduce accumulated
time error when using jiffies as a clocksource, by calculating
the hardware's actual tick length rather then just assuming
it is 1sec/HZ.

However, for the most part this is unnecessary, as most modern
systems don't use jiffies for their clocksource, and their
tick device is sufficiently fine grained to avoid major error.

Thus, this patch allows an architecture to not define
CLOCK_TICK_RATE, in which case ACTHZ defaults to (HZ << 8).

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
[ Commit log & intention tweaks ]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/1343414893-45779-2-git-send-email-john.stultz@linaro.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 1f815fae

include/linux/jiffies.h

+8 −4

Original line number	Diff line number	Diff line
		@@ -39,9 +39,6 @@
		# error Invalid value of HZ.
		#endif

		/* LATCH is used in the interval timer and ftape setup. */
		#define LATCH ((CLOCK_TICK_RATE + HZ/2) / HZ) /* For divider */

		/* Suppose we want to divide two numbers NOM and DEN: NOM/DEN, then we can
		* improve accuracy by shifting LSH bits, hence calculating:
		* (NOM << LSH) / DEN
		@@ -54,8 +51,15 @@
		#define SH_DIV(NOM,DEN,LSH) ( (((NOM) / (DEN)) << (LSH)) \
		+ ((((NOM) % (DEN)) << (LSH)) + (DEN) / 2) / (DEN))

		#ifdef CLOCK_TICK_RATE
		/* LATCH is used in the interval timer and ftape setup. */
		# define LATCH ((CLOCK_TICK_RATE + HZ/2) / HZ) /* For divider */

		/* HZ is the requested value. ACTHZ is actual HZ ("<< 8" is for accuracy) */
		# define ACTHZ (SH_DIV(CLOCK_TICK_RATE, LATCH, 8))
		#else
		# define ACTHZ (HZ << 8)
		#endif

		/* TICK_NSEC is the time between ticks in nsec assuming real ACTHZ */
		#define TICK_NSEC (SH_DIV (1000000UL * 1000, ACTHZ, 8))

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