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1 | /* |
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 | #include <asm/system.h> |
20 | |
21 | /* this is used only to give gcc a clue about good code generation */ |
22 | union 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 | * |
57 | * 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 |
64 | * calls to this code. |
65 | * |
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. |
70 | * |
71 | * Note that the top bit (bit 63) in the returned value should be considered |
72 | * as garbage. It is not cleared here because callers are likely to use a |
73 | * multiplier on the returned value which can get rid of the top bit |
74 | * implicitly by making the multiplier even, therefore saving on a runtime |
75 | * clear-bit instruction. Otherwise caller must remember to clear the top |
76 | * bit explicitly. |
77 | */ |
78 | #define cnt32_to_63(cnt_lo) \ |
79 | ({ \ |
80 | static u32 __m_cnt_hi; \ |
81 | union cnt32_to_63 __x; \ |
82 | __x.hi = __m_cnt_hi; \ |
83 | smp_rmb(); \ |
84 | __x.lo = (cnt_lo); \ |
85 | if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \ |
86 | __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \ |
87 | __x.val; \ |
88 | }) |
89 | |
90 | #endif |
91 |
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