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ARM: 8619/1: udelay: document the various constants

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Explain where the value for UDELAY_MULT and UDELAY_SHIFT come from.
Also fix/clarify some comments pertaining to their usage in the
assembly code.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
This commit is contained in:
Nicolas Pitre
2016-10-07 05:38:35 +01:00
committed by Russell King
parent 1001354ca3
commit 207b1150c0
2 changed files with 34 additions and 8 deletions
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27
arch/arm/include/asm/delay.h
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@@ -9,6 +9,33 @@
#include <asm/memory.h>
#include <asm/param.h> /* HZ */
/*
* Loop (or tick) based delay:
*
* loops = loops_per_jiffy * jiffies_per_sec * delay_us / us_per_sec
*
* where:
*
* jiffies_per_sec = HZ
* us_per_sec = 1000000
*
* Therefore the constant part is HZ / 1000000 which is a small
* fractional number. To make this usable with integer math, we
* scale up this constant by 2^31, perform the actual multiplication,
* and scale the result back down by 2^31 with a simple shift:
*
* loops = (loops_per_jiffy * delay_us * UDELAY_MULT) >> 31
*
* where:
*
* UDELAY_MULT = 2^31 * HZ / 1000000
* = (2^31 / 1000000) * HZ
* = 2147.483648 * HZ
* = 2147 * HZ + 483648 * HZ / 1000000
*
* 31 is the biggest scale shift value that won't overflow 32 bits for
* delay_us * UDELAY_MULT assuming HZ <= 1000 and delay_us <= 2000.
*/
#define MAX_UDELAY_MS 2
#define UDELAY_MULT UL(2147 * HZ + 483648 * HZ / 1000000)
#define UDELAY_SHIFT 31