spinlock_common.h

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/*
 * 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(loops);
}