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.txt. 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