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 */ 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 * 56 * 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 63 * calls to this code; 64 * 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. 69 * 70 * 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 * 88 * 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({ \ 97 static u32 __m_cnt_hi; \ 98 union cnt32_to_63 __x; \ 99 __x.hi = __m_cnt_hi; \ 100 smp_rmb(); \ 101 __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 108