arch/tile/lib/spinlock_common.h

   1 /*
   2  * Copyright 2010 Tilera Corporation. All Rights Reserved.
   3  *
   4  *   This program is free software; you can redistribute it and/or
   5  *   modify it under the terms of the GNU General Public License
   6  *   as published by the Free Software Foundation, version 2.
   7  *
   8  *   This program is distributed in the hope that it will be useful, but
   9  *   WITHOUT ANY WARRANTY; without even the implied warranty of
  10  *   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
  11  *   NON INFRINGEMENT.  See the GNU General Public License for
  12  *   more details.
  13  * This file is included into spinlock_32.c or _64.c.
  14  */
  15
  16 /*
  17  * The mfspr in __spinlock_relax() is 5 or 6 cycles plus 2 for loop
  18  * overhead.
  19  */
  20 #ifdef __tilegx__
  21 #define CYCLES_PER_RELAX_LOOP 7
  22 #else
  23 #define CYCLES_PER_RELAX_LOOP 8
  24 #endif
  25
  26 /*
  27  * Idle the core for CYCLES_PER_RELAX_LOOP * iterations cycles.
  28  */
  29 static inline void
  30 relax(int iterations)
  31 {
  32         for (/*above*/; iterations > 0; iterations--)
  33                 __insn_mfspr(SPR_PASS);
  34         barrier();
  35 }
  36
  37 /* Perform bounded exponential backoff.*/
  38 static void delay_backoff(int iterations)
  39 {
  40         u32 exponent, loops;
  41
  42         /*
  43          * 2^exponent is how many times we go around the loop,
  44          * which takes 8 cycles.  We want to start with a 16- to 31-cycle
  45          * loop, so we need to go around minimum 2 = 2^1 times, so we
  46          * bias the original value up by 1.
  47          */
  48         exponent = iterations + 1;
  49
  50         /*
  51          * Don't allow exponent to exceed 7, so we have 128 loops,
  52          * or 1,024 (to 2,047) cycles, as our maximum.
  53          */
  54         if (exponent > 8)
  55                 exponent = 8;
  56
  57         loops = 1 << exponent;
  58
  59         /* Add a randomness factor so two cpus never get in lock step. */
  60         loops += __insn_crc32_32(stack_pointer, get_cycles_low()) &
  61                 (loops - 1);
  62
  63         relax(loops);
  64 }