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Commit 207b1150 authored by Nicolas Pitre's avatar Nicolas Pitre Committed by Russell King
Browse files

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



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: default avatarNicolas Pitre <nico@linaro.org>
Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent 1001354c

arch/arm/include/asm/delay.h

+27 −0
Original line number Diff line number Diff line
@@ -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

arch/arm/lib/delay-loop.S

+7 −8
Original line number Diff line number Diff line
@@ -17,24 +17,23 @@ 
.LC1:		.word	UDELAY_MULT



/*

 * loops = r0 * HZ * loops_per_jiffy / 1000000

 *

 * r0  <= 2000

 * HZ  <= 1000

 */



ENTRY(__loop_udelay)

		ldr	r2, .LC1

		mul	r0, r2, r0

ENTRY(__loop_const_udelay)			@ 0 <= r0 <= 0x7fffff06

		mul	r0, r2, r0		@ r0 = delay_us * UDELAY_MULT

ENTRY(__loop_const_udelay)			@ 0 <= r0 <= 0xfffffaf0

		ldr	r2, .LC0

		ldr	r2, [r2]

		umull	r1, r0, r2, r0

		adds	r1, r1, #0xffffffff

		adcs	r0, r0, r0

		umull	r1, r0, r2, r0		@ r0-r1 = r0 * loops_per_jiffy

		adds	r1, r1, #0xffffffff	@ rounding up ...

		adcs	r0, r0, r0		@ and right shift by 31

		reteq	lr



/*

 * loops = r0 * HZ * loops_per_jiffy / 1000000

 */

		.align 3



@ Delay routine