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1 | /* |
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] | |
40a5c0b4 | 45 | dsb st |
9762f12d DM |
46 | 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) | |
59 | ENTRY(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 ) | |
77 | 0: | |
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 | ||
94 | trylock_fail: | |
95 | voting_end r0, r1, r2 | |
96 | mov r0, #1 @ nonzero indicates that I lost | |
97 | bx lr | |
98 | ENDPROC(vlock_trylock) | |
99 | ||
100 | @ r0: lock structure base | |
101 | ENTRY(vlock_unlock) | |
102 | dmb | |
103 | mov r1, #VLOCK_OWNER_NONE | |
104 | strb r1, [r0, #VLOCK_OWNER_OFFSET] | |
40a5c0b4 | 105 | dsb st |
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106 | sev |
107 | bx lr | |
108 | ENDPROC(vlock_unlock) |