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)
102