444eef1ba4
We were carefully computing a value to use for the number of loops to spin for, and then ignoring it. Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
64 lines
1.7 KiB
C
64 lines
1.7 KiB
C
/*
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* Copyright 2010 Tilera Corporation. All Rights Reserved.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation, version 2.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
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* NON INFRINGEMENT. See the GNU General Public License for
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* more details.
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* This file is included into spinlock_32.c or _64.c.
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*/
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/*
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* The mfspr in __spinlock_relax() is 5 or 6 cycles plus 2 for loop
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* overhead.
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*/
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#ifdef __tilegx__
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#define CYCLES_PER_RELAX_LOOP 7
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#else
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#define CYCLES_PER_RELAX_LOOP 8
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#endif
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/*
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* Idle the core for CYCLES_PER_RELAX_LOOP * iterations cycles.
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*/
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static inline void
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relax(int iterations)
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{
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for (/*above*/; iterations > 0; iterations--)
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__insn_mfspr(SPR_PASS);
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barrier();
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}
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/* Perform bounded exponential backoff.*/
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static void delay_backoff(int iterations)
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{
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u32 exponent, loops;
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/*
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* 2^exponent is how many times we go around the loop,
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* which takes 8 cycles. We want to start with a 16- to 31-cycle
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* loop, so we need to go around minimum 2 = 2^1 times, so we
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* bias the original value up by 1.
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*/
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exponent = iterations + 1;
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/*
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* Don't allow exponent to exceed 7, so we have 128 loops,
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* or 1,024 (to 2,047) cycles, as our maximum.
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*/
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if (exponent > 8)
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exponent = 8;
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loops = 1 << exponent;
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/* Add a randomness factor so two cpus never get in lock step. */
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loops += __insn_crc32_32(stack_pointer, get_cycles_low()) &
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(loops - 1);
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relax(loops);
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}
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