/*
   * Copyright 2010 Tilera Corporation. All Rights Reserved.
   *
   *   This program is free software; you can redistribute it and/or
   *   modify it under the terms of the GNU General Public License
   *   as published by the Free Software Foundation, version 2.
   *
   *   This program is distributed in the hope that it will be useful, but
   *   WITHOUT ANY WARRANTY; without even the implied warranty of
   *   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
   *   NON INFRINGEMENT.  See the GNU General Public License for
   *   more details.
   * This file is included into spinlock_32.c or _64.c.
   */
  
  /*
   * The mfspr in __spinlock_relax() is 5 or 6 cycles plus 2 for loop
   * overhead.
   */
  #ifdef __tilegx__
  #define CYCLES_PER_RELAX_LOOP 7
  #else
  #define CYCLES_PER_RELAX_LOOP 8
  #endif
  
  /*
   * Idle the core for CYCLES_PER_RELAX_LOOP * iterations cycles.
   */
  static inline void
  relax(int iterations)
  {
  	for (/*above*/; iterations > 0; iterations--)
  		__insn_mfspr(SPR_PASS);
  	barrier();
  }
  
  /* Perform bounded exponential backoff.*/

  static void delay_backoff(int iterations)

  {
  	u32 exponent, loops;
  
  	/*
  	 * 2^exponent is how many times we go around the loop,
  	 * which takes 8 cycles.  We want to start with a 16- to 31-cycle
  	 * loop, so we need to go around minimum 2 = 2^1 times, so we
  	 * bias the original value up by 1.
  	 */
  	exponent = iterations + 1;
  
  	/*
  	 * Don't allow exponent to exceed 7, so we have 128 loops,
  	 * or 1,024 (to 2,047) cycles, as our maximum.
  	 */
  	if (exponent > 8)
  		exponent = 8;
  
  	loops = 1 << exponent;
  
  	/* Add a randomness factor so two cpus never get in lock step. */
  	loops += __insn_crc32_32(stack_pointer, get_cycles_low()) &
  		(loops - 1);
  
  	relax(1 << exponent);
  }