linux/include/linux/cnt32_to_63.h
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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 */
  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
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