include/linux/cnt32_to_63.h

   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 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