mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-09-04 01:49:36 +00:00
![Chris Metcalf](/assets/img/avatar_default.png)
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
/*
|
|
* 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);
|
|
}
|