blob: 8b7df283fedf8e16361f9834349e5f2934fba92e [file] [log] [blame]
Dave Martin9762f122012-08-17 16:07:01 +01001/*
2 * vlock.S - simple voting lock implementation for ARM
3 *
4 * Created by: Dave Martin, 2012-08-16
5 * Copyright: (C) 2012-2013 Linaro Limited
6 *
7 * This program is free software; you can redistribute it and/or modify
8 * it under the terms of the GNU General Public License version 2 as
9 * published by the Free Software Foundation.
10 *
11 * This program is distributed in the hope that it will be useful,
12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 * GNU General Public License for more details.
15 *
16 *
17 * This algorithm is described in more detail in
18 * Documentation/arm/vlocks.txt.
19 */
20
21#include <linux/linkage.h>
22#include "vlock.h"
23
24/* Select different code if voting flags can fit in a single word. */
25#if VLOCK_VOTING_SIZE > 4
26#define FEW(x...)
27#define MANY(x...) x
28#else
29#define FEW(x...) x
30#define MANY(x...)
31#endif
32
33@ voting lock for first-man coordination
34
35.macro voting_begin rbase:req, rcpu:req, rscratch:req
36 mov \rscratch, #1
37 strb \rscratch, [\rbase, \rcpu]
38 dmb
39.endm
40
41.macro voting_end rbase:req, rcpu:req, rscratch:req
42 dmb
43 mov \rscratch, #0
44 strb \rscratch, [\rbase, \rcpu]
Will Deacon40a5c0b2013-05-14 10:08:07 +010045 dsb st
Dave Martin9762f122012-08-17 16:07:01 +010046 sev
47.endm
48
49/*
50 * The vlock structure must reside in Strongly-Ordered or Device memory.
51 * This implementation deliberately eliminates most of the barriers which
52 * would be required for other memory types, and assumes that independent
53 * writes to neighbouring locations within a cacheline do not interfere
54 * with one another.
55 */
56
57@ r0: lock structure base
58@ r1: CPU ID (0-based index within cluster)
59ENTRY(vlock_trylock)
60 add r1, r1, #VLOCK_VOTING_OFFSET
61
62 voting_begin r0, r1, r2
63
64 ldrb r2, [r0, #VLOCK_OWNER_OFFSET] @ check whether lock is held
65 cmp r2, #VLOCK_OWNER_NONE
66 bne trylock_fail @ fail if so
67
68 @ Control dependency implies strb not observable before previous ldrb.
69
70 strb r1, [r0, #VLOCK_OWNER_OFFSET] @ submit my vote
71
72 voting_end r0, r1, r2 @ implies DMB
73
74 @ Wait for the current round of voting to finish:
75
76 MANY( mov r3, #VLOCK_VOTING_OFFSET )
770:
78 MANY( ldr r2, [r0, r3] )
79 FEW( ldr r2, [r0, #VLOCK_VOTING_OFFSET] )
80 cmp r2, #0
81 wfene
82 bne 0b
83 MANY( add r3, r3, #4 )
84 MANY( cmp r3, #VLOCK_VOTING_OFFSET + VLOCK_VOTING_SIZE )
85 MANY( bne 0b )
86
87 @ Check who won:
88
89 dmb
90 ldrb r2, [r0, #VLOCK_OWNER_OFFSET]
91 eor r0, r1, r2 @ zero if I won, else nonzero
92 bx lr
93
94trylock_fail:
95 voting_end r0, r1, r2
96 mov r0, #1 @ nonzero indicates that I lost
97 bx lr
98ENDPROC(vlock_trylock)
99
100@ r0: lock structure base
101ENTRY(vlock_unlock)
102 dmb
103 mov r1, #VLOCK_OWNER_NONE
104 strb r1, [r0, #VLOCK_OWNER_OFFSET]
Will Deacon40a5c0b2013-05-14 10:08:07 +0100105 dsb st
Dave Martin9762f122012-08-17 16:07:01 +0100106 sev
107 bx lr
108ENDPROC(vlock_unlock)