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
relax(int iterations)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.*/
delay_backoff(int iterations)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 }
65