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/arm/vlocks.rst.
10 */
11
12#include <linux/linkage.h>
13#include "vlock.h"
14
15/* Select different code if voting flags  can fit in a single word. */
16#if VLOCK_VOTING_SIZE > 4
17#define FEW(x...)
18#define MANY(x...) x
19#else
20#define FEW(x...) x
21#define MANY(x...)
22#endif
23
24@ voting lock for first-man coordination
25
26.macro voting_begin rbase:req, rcpu:req, rscratch:req
27	mov	\rscratch, #1
28	strb	\rscratch, [\rbase, \rcpu]
29	dmb
30.endm
31
32.macro voting_end rbase:req, rcpu:req, rscratch:req
33	dmb
34	mov	\rscratch, #0
35	strb	\rscratch, [\rbase, \rcpu]
36	dsb	st
37	sev
38.endm
39
40/*
41 * The vlock structure must reside in Strongly-Ordered or Device memory.
42 * This implementation deliberately eliminates most of the barriers which
43 * would be required for other memory types, and assumes that independent
44 * writes to neighbouring locations within a cacheline do not interfere
45 * with one another.
46 */
47
48@ r0: lock structure base
49@ r1: CPU ID (0-based index within cluster)
50ENTRY(vlock_trylock)
51	add	r1, r1, #VLOCK_VOTING_OFFSET
52
53	voting_begin	r0, r1, r2
54
55	ldrb	r2, [r0, #VLOCK_OWNER_OFFSET]	@ check whether lock is held
56	cmp	r2, #VLOCK_OWNER_NONE
57	bne	trylock_fail			@ fail if so
58
59	@ Control dependency implies strb not observable before previous ldrb.
60
61	strb	r1, [r0, #VLOCK_OWNER_OFFSET]	@ submit my vote
62
63	voting_end	r0, r1, r2		@ implies DMB
64
65	@ Wait for the current round of voting to finish:
66
67 MANY(	mov	r3, #VLOCK_VOTING_OFFSET			)
680:
69 MANY(	ldr	r2, [r0, r3]					)
70 FEW(	ldr	r2, [r0, #VLOCK_VOTING_OFFSET]			)
71	cmp	r2, #0
72	wfene
73	bne	0b
74 MANY(	add	r3, r3, #4					)
75 MANY(	cmp	r3, #VLOCK_VOTING_OFFSET + VLOCK_VOTING_SIZE	)
76 MANY(	bne	0b						)
77
78	@ Check who won:
79
80	dmb
81	ldrb	r2, [r0, #VLOCK_OWNER_OFFSET]
82	eor	r0, r1, r2			@ zero if I won, else nonzero
83	bx	lr
84
85trylock_fail:
86	voting_end	r0, r1, r2
87	mov	r0, #1				@ nonzero indicates that I lost
88	bx	lr
89ENDPROC(vlock_trylock)
90
91@ r0: lock structure base
92ENTRY(vlock_unlock)
93	dmb
94	mov	r1, #VLOCK_OWNER_NONE
95	strb	r1, [r0, #VLOCK_OWNER_OFFSET]
96	dsb	st
97	sev
98	bx	lr
99ENDPROC(vlock_unlock)
100