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| 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 |
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