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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>
19
20/* this is used only to give gcc a clue about good code generation */
21union cnt32_to_63 {
22 struct {
23#if defined(__LITTLE_ENDIAN)
24 u32 lo, hi;
25#elif defined(__BIG_ENDIAN)
26 u32 hi, lo;
27#endif
28 };
29 u64 val;
30};
31
32
33/**
34 * cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
35 * @cnt_lo: The low part of the counter
36 *
37 * Many hardware clock counters are only 32 bits wide and therefore have
38 * a relatively short period making wrap-arounds rather frequent. This
39 * is a problem when implementing sched_clock() for example, where a 64-bit
40 * non-wrapping monotonic value is expected to be returned.
41 *
42 * To overcome that limitation, let's extend a 32-bit counter to 63 bits
43 * in a completely lock free fashion. Bits 0 to 31 of the clock are provided
44 * by the hardware while bits 32 to 62 are stored in memory. The top bit in
45 * memory is used to synchronize with the hardware clock half-period. When
46 * the top bit of both counters (hardware and in memory) differ then the
47 * memory is updated with a new value, incrementing it when the hardware
48 * counter wraps around.
49 *
50 * Because a word store in memory is atomic then the incremented value will
51 * always be in synch with the top bit indicating to any potential concurrent
52 * reader if the value in memory is up to date or not with regards to the
53 * needed increment. And any race in updating the value in memory is harmless
54 * as the same value would simply be stored more than once.
55 *
Nicolas Pitre058e3732008-11-09 00:27:53 -050056 * The restrictions for the algorithm to work properly are:
57 *
58 * 1) this code must be called at least once per each half period of the
59 * 32-bit counter;
60 *
61 * 2) this code must not be preempted for a duration longer than the
62 * 32-bit counter half period minus the longest period between two
Nicolas Pitreb8da46d2010-12-20 00:29:32 -050063 * calls to this code;
Nicolas Pitre058e3732008-11-09 00:27:53 -050064 *
65 * Those requirements ensure proper update to the state bit in memory.
66 * This is usually not a problem in practice, but if it is then a kernel
67 * timer should be scheduled to manage for this code to be executed often
68 * enough.
David Howellsb4f151f2008-09-24 17:48:26 +010069 *
Nicolas Pitreb8da46d2010-12-20 00:29:32 -050070 * And finally:
71 *
72 * 3) the cnt_lo argument must be seen as a globally incrementing value,
73 * meaning that it should be a direct reference to the counter data which
74 * can be evaluated according to a specific ordering within the macro,
75 * and not the result of a previous evaluation stored in a variable.
76 *
77 * For example, this is wrong:
78 *
79 * u32 partial = get_hw_count();
80 * u64 full = cnt32_to_63(partial);
81 * return full;
82 *
83 * This is fine:
84 *
85 * u64 full = cnt32_to_63(get_hw_count());
86 * return full;
87 *
David Howellsb4f151f2008-09-24 17:48:26 +010088 * Note that the top bit (bit 63) in the returned value should be considered
89 * as garbage. It is not cleared here because callers are likely to use a
90 * multiplier on the returned value which can get rid of the top bit
91 * implicitly by making the multiplier even, therefore saving on a runtime
92 * clear-bit instruction. Otherwise caller must remember to clear the top
93 * bit explicitly.
94 */
95#define cnt32_to_63(cnt_lo) \
96({ \
Nicolas Pitre058e3732008-11-09 00:27:53 -050097 static u32 __m_cnt_hi; \
David Howellsb4f151f2008-09-24 17:48:26 +010098 union cnt32_to_63 __x; \
99 __x.hi = __m_cnt_hi; \
Nicolas Pitre058e3732008-11-09 00:27:53 -0500100 smp_rmb(); \
David Howellsb4f151f2008-09-24 17:48:26 +0100101 __x.lo = (cnt_lo); \
102 if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
103 __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
104 __x.val; \
105})
106
107#endif