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David Howellsb4f151f2008-09-24 17:48:26 +01001/*
2 * 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 __LINUX_CNT32_TO_63_H__
14#define __LINUX_CNT32_TO_63_H__
15
16#include <linux/compiler.h>
17#include <linux/types.h>
18#include <asm/byteorder.h>
Nicolas Pitre058e3732008-11-09 00:27:53 -050019#include <asm/system.h>
David Howellsb4f151f2008-09-24 17:48:26 +010020
21/* this is used only to give gcc a clue about good code generation */
22union cnt32_to_63 {
23 struct {
24#if defined(__LITTLE_ENDIAN)
25 u32 lo, hi;
26#elif defined(__BIG_ENDIAN)
27 u32 hi, lo;
28#endif
29 };
30 u64 val;
31};
32
33
34/**
35 * cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
36 * @cnt_lo: The low part of the counter
37 *
38 * Many hardware clock counters are only 32 bits wide and therefore have
39 * a relatively short period making wrap-arounds rather frequent. This
40 * is a problem when implementing sched_clock() for example, where a 64-bit
41 * non-wrapping monotonic value is expected to be returned.
42 *
43 * To overcome that limitation, let's extend a 32-bit counter to 63 bits
44 * in a completely lock free fashion. Bits 0 to 31 of the clock are provided
45 * by the hardware while bits 32 to 62 are stored in memory. The top bit in
46 * memory is used to synchronize with the hardware clock half-period. When
47 * the top bit of both counters (hardware and in memory) differ then the
48 * memory is updated with a new value, incrementing it when the hardware
49 * counter wraps around.
50 *
51 * Because a word store in memory is atomic then the incremented value will
52 * always be in synch with the top bit indicating to any potential concurrent
53 * reader if the value in memory is up to date or not with regards to the
54 * needed increment. And any race in updating the value in memory is harmless
55 * as the same value would simply be stored more than once.
56 *
Nicolas Pitre058e3732008-11-09 00:27:53 -050057 * The restrictions for the algorithm to work properly are:
58 *
59 * 1) this code must be called at least once per each half period of the
60 * 32-bit counter;
61 *
62 * 2) this code must not be preempted for a duration longer than the
63 * 32-bit counter half period minus the longest period between two
Nicolas Pitreb8da46d2010-12-20 00:29:32 -050064 * calls to this code;
Nicolas Pitre058e3732008-11-09 00:27:53 -050065 *
66 * Those requirements ensure proper update to the state bit in memory.
67 * This is usually not a problem in practice, but if it is then a kernel
68 * timer should be scheduled to manage for this code to be executed often
69 * enough.
David Howellsb4f151f2008-09-24 17:48:26 +010070 *
Nicolas Pitreb8da46d2010-12-20 00:29:32 -050071 * And finally:
72 *
73 * 3) the cnt_lo argument must be seen as a globally incrementing value,
74 * meaning that it should be a direct reference to the counter data which
75 * can be evaluated according to a specific ordering within the macro,
76 * and not the result of a previous evaluation stored in a variable.
77 *
78 * For example, this is wrong:
79 *
80 * u32 partial = get_hw_count();
81 * u64 full = cnt32_to_63(partial);
82 * return full;
83 *
84 * This is fine:
85 *
86 * u64 full = cnt32_to_63(get_hw_count());
87 * return full;
88 *
David Howellsb4f151f2008-09-24 17:48:26 +010089 * Note that the top bit (bit 63) in the returned value should be considered
90 * as garbage. It is not cleared here because callers are likely to use a
91 * multiplier on the returned value which can get rid of the top bit
92 * implicitly by making the multiplier even, therefore saving on a runtime
93 * clear-bit instruction. Otherwise caller must remember to clear the top
94 * bit explicitly.
95 */
96#define cnt32_to_63(cnt_lo) \
97({ \
Nicolas Pitre058e3732008-11-09 00:27:53 -050098 static u32 __m_cnt_hi; \
David Howellsb4f151f2008-09-24 17:48:26 +010099 union cnt32_to_63 __x; \
100 __x.hi = __m_cnt_hi; \
Nicolas Pitre058e3732008-11-09 00:27:53 -0500101 smp_rmb(); \
David Howellsb4f151f2008-09-24 17:48:26 +0100102 __x.lo = (cnt_lo); \
103 if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
104 __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
105 __x.val; \
106})
107
108#endif