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/*
* 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 __LINUX_CNT32_TO_63_H__
#define __LINUX_CNT32_TO_63_H__
#include <linux/compiler.h>
#include <linux/types.h>
#include <asm/byteorder.h>
/* this is used only to give gcc a clue about good code generation */
union cnt32_to_63 {
struct {
#if defined(__LITTLE_ENDIAN)
u32 lo, hi;
#elif defined(__BIG_ENDIAN)
u32 hi, lo;
#endif
};
u64 val;
};
/**
* cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
* @cnt_lo: The low part of the counter
*
* 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 restrictions for the algorithm to work properly are:
*
* 1) this code must be called at least once per each half period of the
* 32-bit counter;
*
* 2) this code must not be preempted for a duration longer than the
* 32-bit counter half period minus the longest period between two
* calls to this code;
*
* Those requirements ensure proper update to the state bit in memory.
* This is usually not a problem in practice, but if it is then a kernel
* timer should be scheduled to manage for this code to be executed often
* enough.
*
* And finally:
*
* 3) the cnt_lo argument must be seen as a globally incrementing value,
* meaning that it should be a direct reference to the counter data which
* can be evaluated according to a specific ordering within the macro,
* and not the result of a previous evaluation stored in a variable.
*
* For example, this is wrong:
*
* u32 partial = get_hw_count();
* u64 full = cnt32_to_63(partial);
* return full;
*
* This is fine:
*
* u64 full = cnt32_to_63(get_hw_count());
* return full;
*
* 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.
*/
#define cnt32_to_63(cnt_lo) \
({ \
static u32 __m_cnt_hi; \
union cnt32_to_63 __x; \
__x.hi = __m_cnt_hi; \
smp_rmb(); \
__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
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