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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 | ||
20 | /* this is used only to give gcc a clue about good code generation */ | |
21 | union 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 | * | |
56 | * The only restriction for the algorithm to work properly is that this | |
57 | * code must be executed at least once per each half period of the 32-bit | |
58 | * counter to properly update the state bit in memory. This is usually not a | |
59 | * problem in practice, but if it is then a kernel timer could be scheduled | |
60 | * to manage for this code to be executed often enough. | |
61 | * | |
62 | * Note that the top bit (bit 63) in the returned value should be considered | |
63 | * as garbage. It is not cleared here because callers are likely to use a | |
64 | * multiplier on the returned value which can get rid of the top bit | |
65 | * implicitly by making the multiplier even, therefore saving on a runtime | |
66 | * clear-bit instruction. Otherwise caller must remember to clear the top | |
67 | * bit explicitly. | |
68 | */ | |
69 | #define cnt32_to_63(cnt_lo) \ | |
70 | ({ \ | |
71 | static volatile u32 __m_cnt_hi; \ | |
72 | union cnt32_to_63 __x; \ | |
73 | __x.hi = __m_cnt_hi; \ | |
74 | __x.lo = (cnt_lo); \ | |
75 | if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \ | |
76 | __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \ | |
77 | __x.val; \ | |
78 | }) | |
79 | ||
80 | #endif |