xref: /openbmc/linux/include/linux/cnt32_to_63.h (revision d2912cb1)
1 /* SPDX-License-Identifier: GPL-2.0-only */
2 /*
3  *  Extend a 32-bit counter to 63 bits
4  *
5  *  Author:	Nicolas Pitre
6  *  Created:	December 3, 2006
7  *  Copyright:	MontaVista Software, Inc.
8  */
9 
10 #ifndef __LINUX_CNT32_TO_63_H__
11 #define __LINUX_CNT32_TO_63_H__
12 
13 #include <linux/compiler.h>
14 #include <linux/types.h>
15 #include <asm/byteorder.h>
16 
17 /* this is used only to give gcc a clue about good code generation */
18 union cnt32_to_63 {
19 	struct {
20 #if defined(__LITTLE_ENDIAN)
21 		u32 lo, hi;
22 #elif defined(__BIG_ENDIAN)
23 		u32 hi, lo;
24 #endif
25 	};
26 	u64 val;
27 };
28 
29 
30 /**
31  * cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
32  * @cnt_lo: The low part of the counter
33  *
34  * Many hardware clock counters are only 32 bits wide and therefore have
35  * a relatively short period making wrap-arounds rather frequent.  This
36  * is a problem when implementing sched_clock() for example, where a 64-bit
37  * non-wrapping monotonic value is expected to be returned.
38  *
39  * To overcome that limitation, let's extend a 32-bit counter to 63 bits
40  * in a completely lock free fashion. Bits 0 to 31 of the clock are provided
41  * by the hardware while bits 32 to 62 are stored in memory.  The top bit in
42  * memory is used to synchronize with the hardware clock half-period.  When
43  * the top bit of both counters (hardware and in memory) differ then the
44  * memory is updated with a new value, incrementing it when the hardware
45  * counter wraps around.
46  *
47  * Because a word store in memory is atomic then the incremented value will
48  * always be in synch with the top bit indicating to any potential concurrent
49  * reader if the value in memory is up to date or not with regards to the
50  * needed increment.  And any race in updating the value in memory is harmless
51  * as the same value would simply be stored more than once.
52  *
53  * The restrictions for the algorithm to work properly are:
54  *
55  * 1) this code must be called at least once per each half period of the
56  *    32-bit counter;
57  *
58  * 2) this code must not be preempted for a duration longer than the
59  *    32-bit counter half period minus the longest period between two
60  *    calls to this code;
61  *
62  * Those requirements ensure proper update to the state bit in memory.
63  * This is usually not a problem in practice, but if it is then a kernel
64  * timer should be scheduled to manage for this code to be executed often
65  * enough.
66  *
67  * And finally:
68  *
69  * 3) the cnt_lo argument must be seen as a globally incrementing value,
70  *    meaning that it should be a direct reference to the counter data which
71  *    can be evaluated according to a specific ordering within the macro,
72  *    and not the result of a previous evaluation stored in a variable.
73  *
74  * For example, this is wrong:
75  *
76  *	u32 partial = get_hw_count();
77  *	u64 full = cnt32_to_63(partial);
78  *	return full;
79  *
80  * This is fine:
81  *
82  *	u64 full = cnt32_to_63(get_hw_count());
83  *	return full;
84  *
85  * Note that the top bit (bit 63) in the returned value should be considered
86  * as garbage.  It is not cleared here because callers are likely to use a
87  * multiplier on the returned value which can get rid of the top bit
88  * implicitly by making the multiplier even, therefore saving on a runtime
89  * clear-bit instruction. Otherwise caller must remember to clear the top
90  * bit explicitly.
91  */
92 #define cnt32_to_63(cnt_lo) \
93 ({ \
94 	static u32 __m_cnt_hi; \
95 	union cnt32_to_63 __x; \
96 	__x.hi = __m_cnt_hi; \
97  	smp_rmb(); \
98 	__x.lo = (cnt_lo); \
99 	if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
100 		__m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
101 	__x.val; \
102 })
103 
104 #endif
105