/*
   *  Extend a 32-bit counter to 63 bits
   *
   *  Author:	Nicolas Pitre
   *  Created:	December 3, 2006
   *  Copyright:	MontaVista Software, Inc.
   *
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License version 2
   * as published by the Free Software Foundation.
   */
  
  #ifndef __LINUX_CNT32_TO_63_H__
  #define __LINUX_CNT32_TO_63_H__
  
  #include <linux/compiler.h>
  #include <linux/types.h>
  #include <asm/byteorder.h>

  #include <asm/system.h>

  
  /* this is used only to give gcc a clue about good code generation */
  union cnt32_to_63 {
  	struct {
  #if defined(__LITTLE_ENDIAN)
  		u32 lo, hi;
  #elif defined(__BIG_ENDIAN)
  		u32 hi, lo;
  #endif
  	};
  	u64 val;
  };
  
  
  /**
   * cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
   * @cnt_lo: The low part of the counter
   *
   * Many hardware clock counters are only 32 bits wide and therefore have
   * a relatively short period making wrap-arounds rather frequent.  This
   * is a problem when implementing sched_clock() for example, where a 64-bit
   * non-wrapping monotonic value is expected to be returned.
   *
   * To overcome that limitation, let's extend a 32-bit counter to 63 bits
   * in a completely lock free fashion. Bits 0 to 31 of the clock are provided
   * by the hardware while bits 32 to 62 are stored in memory.  The top bit in
   * memory is used to synchronize with the hardware clock half-period.  When
   * the top bit of both counters (hardware and in memory) differ then the
   * memory is updated with a new value, incrementing it when the hardware
   * counter wraps around.
   *
   * Because a word store in memory is atomic then the incremented value will
   * always be in synch with the top bit indicating to any potential concurrent
   * reader if the value in memory is up to date or not with regards to the
   * needed increment.  And any race in updating the value in memory is harmless
   * as the same value would simply be stored more than once.
   *

   * The restrictions for the algorithm to work properly are:
   *
   * 1) this code must be called at least once per each half period of the
   *    32-bit counter;
   *
   * 2) this code must not be preempted for a duration longer than the
   *    32-bit counter half period minus the longest period between two

   *    calls to this code;

   *
   * Those requirements ensure proper update to the state bit in memory.
   * This is usually not a problem in practice, but if it is then a kernel
   * timer should be scheduled to manage for this code to be executed often
   * enough.

   *

   * And finally:
   *
   * 3) the cnt_lo argument must be seen as a globally incrementing value,
   *    meaning that it should be a direct reference to the counter data which
   *    can be evaluated according to a specific ordering within the macro,
   *    and not the result of a previous evaluation stored in a variable.
   *
   * For example, this is wrong:
   *
   *	u32 partial = get_hw_count();
   *	u64 full = cnt32_to_63(partial);
   *	return full;
   *
   * This is fine:
   *
   *	u64 full = cnt32_to_63(get_hw_count());
   *	return full;
   *

   * Note that the top bit (bit 63) in the returned value should be considered
   * as garbage.  It is not cleared here because callers are likely to use a
   * multiplier on the returned value which can get rid of the top bit
   * implicitly by making the multiplier even, therefore saving on a runtime
   * clear-bit instruction. Otherwise caller must remember to clear the top
   * bit explicitly.
   */
  #define cnt32_to_63(cnt_lo) \
  ({ \

  	static u32 __m_cnt_hi; \

  	union cnt32_to_63 __x; \
  	__x.hi = __m_cnt_hi; \

   	smp_rmb(); \

  	__x.lo = (cnt_lo); \
  	if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
  		__m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
  	__x.val; \
  })
  
  #endif