[ARM] 3978/1: macro to provide a 63-bit value from a 32-bit hardware counter

/*

 *  include/asm/cnt32_to_63.h -- extend a 32-bit counter to 63 bits

 *

 *  Author:	Nicolas Pitre

 *  Created:	December 3, 2006

 *  Copyright:	MontaVista Software, Inc.

 *

 * This program is free software; you can redistribute it and/or modify

 * it under the terms of the GNU General Public License version 2

 * as published by the Free Software Foundation.

 */

#ifndef __INCLUDE_CNT32_TO_63_H__

#define __INCLUDE_CNT32_TO_63_H__

#include <linux/compiler.h>

#include <asm/types.h>

#include <asm/byteorder.h>

/*

 * Prototype: u64 cnt32_to_63(u32 cnt)

 * Many hardware clock counters are only 32 bits wide and therefore have

 * a relatively short period making wrap-arounds rather frequent.  This

 * is a problem when implementing sched_clock() for example, where a 64-bit

 * non-wrapping monotonic value is expected to be returned.

 *

 * To overcome that limitation, let's extend a 32-bit counter to 63 bits

 * in a completely lock free fashion. Bits 0 to 31 of the clock are provided

 * by the hardware while bits 32 to 62 are stored in memory.  The top bit in

 * memory is used to synchronize with the hardware clock half-period.  When

 * the top bit of both counters (hardware and in memory) differ then the

 * memory is updated with a new value, incrementing it when the hardware

 * counter wraps around.

 *

 * Because a word store in memory is atomic then the incremented value will

 * always be in synch with the top bit indicating to any potential concurrent

 * reader if the value in memory is up to date or not with regards to the

 * needed increment.  And any race in updating the value in memory is harmless

 * as the same value would simply be stored more than once.

 *

 * The only restriction for the algorithm to work properly is that this

 * code must be executed at least once per each half period of the 32-bit

 * counter to properly update the state bit in memory. This is usually not a

 * problem in practice, but if it is then a kernel timer could be scheduled

 * to manage for this code to be executed often enough.

 *

 * Note that the top bit (bit 63) in the returned value should be considered

 * as garbage.  It is not cleared here because callers are likely to use a

 * multiplier on the returned value which can get rid of the top bit

 * implicitly by making the multiplier even, therefore saving on a runtime

 * clear-bit instruction. Otherwise caller must remember to clear the top

 * bit explicitly.

 */

/* this is used only to give gcc a clue about good code generation */

typedef union {

	struct {

#if defined(__LITTLE_ENDIAN)

		u32 lo, hi;

#elif defined(__BIG_ENDIAN)

		u32 hi, lo;

#endif

	};

	u64 val;

} cnt32_to_63_t;

#define cnt32_to_63(cnt_lo) \

({ \

	static volatile u32 __m_cnt_hi = 0; \

	cnt32_to_63_t __x; \

	__x.hi = __m_cnt_hi; \

	__x.lo = (cnt_lo); \

 	if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \

		__m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \

	__x.val; \

})

#endif