1 | /* SPDX-License-Identifier: GPL-2.0-only */ |
2 | /* |
3 | * vlock.S - simple voting lock implementation for ARM |
4 | * |
5 | * Created by: Dave Martin, 2012-08-16 |
6 | * Copyright: (C) 2012-2013 Linaro Limited |
7 | * |
8 | * This algorithm is described in more detail in |
9 | * Documentation/arch/arm/vlocks.rst. |
10 | */ |
11 | |
12 | #include <linux/linkage.h> |
13 | #include "vlock.h" |
14 | |
15 | .arch armv7-a |
16 | |
17 | /* Select different code if voting flags can fit in a single word. */ |
18 | #if VLOCK_VOTING_SIZE > 4 |
19 | #define FEW(x...) |
20 | #define MANY(x...) x |
21 | #else |
22 | #define FEW(x...) x |
23 | #define MANY(x...) |
24 | #endif |
25 | |
26 | @ voting lock for first-man coordination |
27 | |
28 | .macro voting_begin rbase:req, rcpu:req, rscratch:req |
29 | mov \rscratch, #1 |
30 | strb \rscratch, [\rbase, \rcpu] |
31 | dmb |
32 | .endm |
33 | |
34 | .macro voting_end rbase:req, rcpu:req, rscratch:req |
35 | dmb |
36 | mov \rscratch, #0 |
37 | strb \rscratch, [\rbase, \rcpu] |
38 | dsb st |
39 | sev |
40 | .endm |
41 | |
42 | /* |
43 | * The vlock structure must reside in Strongly-Ordered or Device memory. |
44 | * This implementation deliberately eliminates most of the barriers which |
45 | * would be required for other memory types, and assumes that independent |
46 | * writes to neighbouring locations within a cacheline do not interfere |
47 | * with one another. |
48 | */ |
49 | |
50 | @ r0: lock structure base |
51 | @ r1: CPU ID (0-based index within cluster) |
52 | ENTRY(vlock_trylock) |
53 | add r1, r1, #VLOCK_VOTING_OFFSET |
54 | |
55 | voting_begin r0, r1, r2 |
56 | |
57 | ldrb r2, [r0, #VLOCK_OWNER_OFFSET] @ check whether lock is held |
58 | cmp r2, #VLOCK_OWNER_NONE |
59 | bne trylock_fail @ fail if so |
60 | |
61 | @ Control dependency implies strb not observable before previous ldrb. |
62 | |
63 | strb r1, [r0, #VLOCK_OWNER_OFFSET] @ submit my vote |
64 | |
65 | voting_end r0, r1, r2 @ implies DMB |
66 | |
67 | @ Wait for the current round of voting to finish: |
68 | |
69 | MANY( mov r3, #VLOCK_VOTING_OFFSET ) |
70 | 0: |
71 | MANY( ldr r2, [r0, r3] ) |
72 | FEW( ldr r2, [r0, #VLOCK_VOTING_OFFSET] ) |
73 | cmp r2, #0 |
74 | wfene |
75 | bne 0b |
76 | MANY( add r3, r3, #4 ) |
77 | MANY( cmp r3, #VLOCK_VOTING_OFFSET + VLOCK_VOTING_SIZE ) |
78 | MANY( bne 0b ) |
79 | |
80 | @ Check who won: |
81 | |
82 | dmb |
83 | ldrb r2, [r0, #VLOCK_OWNER_OFFSET] |
84 | eor r0, r1, r2 @ zero if I won, else nonzero |
85 | bx lr |
86 | |
87 | trylock_fail: |
88 | voting_end r0, r1, r2 |
89 | mov r0, #1 @ nonzero indicates that I lost |
90 | bx lr |
91 | ENDPROC(vlock_trylock) |
92 | |
93 | @ r0: lock structure base |
94 | ENTRY(vlock_unlock) |
95 | dmb |
96 | mov r1, #VLOCK_OWNER_NONE |
97 | strb r1, [r0, #VLOCK_OWNER_OFFSET] |
98 | dsb st |
99 | sev |
100 | bx lr |
101 | ENDPROC(vlock_unlock) |
102 | |