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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)
52ENTRY(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 )
700:
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
87trylock_fail:
88 voting_end r0, r1, r2
89 mov r0, #1 @ nonzero indicates that I lost
90 bx lr
91ENDPROC(vlock_trylock)
92
93@ r0: lock structure base
94ENTRY(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
101ENDPROC(vlock_unlock)