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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.*/
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);
}
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