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Linux kernel mirror (for testing)
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1/*
2 * include/asm/cnt32_to_63.h -- extend a 32-bit counter to 63 bits
3 *
4 * Author: Nicolas Pitre
5 * Created: December 3, 2006
6 * Copyright: MontaVista Software, Inc.
7 *
8 * This program is free software; you can redistribute it and/or modify
9 * it under the terms of the GNU General Public License version 2
10 * as published by the Free Software Foundation.
11 */
12
13#ifndef __INCLUDE_CNT32_TO_63_H__
14#define __INCLUDE_CNT32_TO_63_H__
15
16#include <linux/compiler.h>
17#include <asm/types.h>
18#include <asm/byteorder.h>
19
20/*
21 * Prototype: u64 cnt32_to_63(u32 cnt)
22 * Many hardware clock counters are only 32 bits wide and therefore have
23 * a relatively short period making wrap-arounds rather frequent. This
24 * is a problem when implementing sched_clock() for example, where a 64-bit
25 * non-wrapping monotonic value is expected to be returned.
26 *
27 * To overcome that limitation, let's extend a 32-bit counter to 63 bits
28 * in a completely lock free fashion. Bits 0 to 31 of the clock are provided
29 * by the hardware while bits 32 to 62 are stored in memory. The top bit in
30 * memory is used to synchronize with the hardware clock half-period. When
31 * the top bit of both counters (hardware and in memory) differ then the
32 * memory is updated with a new value, incrementing it when the hardware
33 * counter wraps around.
34 *
35 * Because a word store in memory is atomic then the incremented value will
36 * always be in synch with the top bit indicating to any potential concurrent
37 * reader if the value in memory is up to date or not with regards to the
38 * needed increment. And any race in updating the value in memory is harmless
39 * as the same value would simply be stored more than once.
40 *
41 * The only restriction for the algorithm to work properly is that this
42 * code must be executed at least once per each half period of the 32-bit
43 * counter to properly update the state bit in memory. This is usually not a
44 * problem in practice, but if it is then a kernel timer could be scheduled
45 * to manage for this code to be executed often enough.
46 *
47 * Note that the top bit (bit 63) in the returned value should be considered
48 * as garbage. It is not cleared here because callers are likely to use a
49 * multiplier on the returned value which can get rid of the top bit
50 * implicitly by making the multiplier even, therefore saving on a runtime
51 * clear-bit instruction. Otherwise caller must remember to clear the top
52 * bit explicitly.
53 */
54
55/* this is used only to give gcc a clue about good code generation */
56typedef union {
57 struct {
58#if defined(__LITTLE_ENDIAN)
59 u32 lo, hi;
60#elif defined(__BIG_ENDIAN)
61 u32 hi, lo;
62#endif
63 };
64 u64 val;
65} cnt32_to_63_t;
66
67#define cnt32_to_63(cnt_lo) \
68({ \
69 static volatile u32 __m_cnt_hi = 0; \
70 cnt32_to_63_t __x; \
71 __x.hi = __m_cnt_hi; \
72 __x.lo = (cnt_lo); \
73 if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
74 __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
75 __x.val; \
76})
77
78#endif