1/* 2 * vlock.S - simple voting lock implementation for ARM 3 * 4 * Created by: Dave Martin, 2012-08-16 5 * Copyright: (C) 2012-2013 Linaro Limited 6 * 7 * This program is free software; you can redistribute it and/or modify 8 * it under the terms of the GNU General Public License version 2 as 9 * published by the Free Software Foundation. 10 * 11 * This program is distributed in the hope that it will be useful, 12 * but WITHOUT ANY WARRANTY; without even the implied warranty of 13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 14 * GNU General Public License for more details. 15 * 16 * 17 * This algorithm is described in more detail in 18 * Documentation/arm/vlocks.txt. 19 */ 20 21#include <linux/linkage.h> 22#include "vlock.h" 23 24/* Select different code if voting flags can fit in a single word. */ 25#if VLOCK_VOTING_SIZE > 4 26#define FEW(x...) 27#define MANY(x...) x 28#else 29#define FEW(x...) x 30#define MANY(x...) 31#endif 32 33@ voting lock for first-man coordination 34 35.macro voting_begin rbase:req, rcpu:req, rscratch:req 36 mov \rscratch, #1 37 strb \rscratch, [\rbase, \rcpu] 38 dmb 39.endm 40 41.macro voting_end rbase:req, rcpu:req, rscratch:req 42 dmb 43 mov \rscratch, #0 44 strb \rscratch, [\rbase, \rcpu] 45 dsb st 46 sev 47.endm 48 49/* 50 * The vlock structure must reside in Strongly-Ordered or Device memory. 51 * This implementation deliberately eliminates most of the barriers which 52 * would be required for other memory types, and assumes that independent 53 * writes to neighbouring locations within a cacheline do not interfere 54 * with one another. 55 */ 56 57@ r0: lock structure base 58@ r1: CPU ID (0-based index within cluster) 59ENTRY(vlock_trylock) 60 add r1, r1, #VLOCK_VOTING_OFFSET 61 62 voting_begin r0, r1, r2 63 64 ldrb r2, [r0, #VLOCK_OWNER_OFFSET] @ check whether lock is held 65 cmp r2, #VLOCK_OWNER_NONE 66 bne trylock_fail @ fail if so 67 68 @ Control dependency implies strb not observable before previous ldrb. 69 70 strb r1, [r0, #VLOCK_OWNER_OFFSET] @ submit my vote 71 72 voting_end r0, r1, r2 @ implies DMB 73 74 @ Wait for the current round of voting to finish: 75 76 MANY( mov r3, #VLOCK_VOTING_OFFSET ) 770: 78 MANY( ldr r2, [r0, r3] ) 79 FEW( ldr r2, [r0, #VLOCK_VOTING_OFFSET] ) 80 cmp r2, #0 81 wfene 82 bne 0b 83 MANY( add r3, r3, #4 ) 84 MANY( cmp r3, #VLOCK_VOTING_OFFSET + VLOCK_VOTING_SIZE ) 85 MANY( bne 0b ) 86 87 @ Check who won: 88 89 dmb 90 ldrb r2, [r0, #VLOCK_OWNER_OFFSET] 91 eor r0, r1, r2 @ zero if I won, else nonzero 92 bx lr 93 94trylock_fail: 95 voting_end r0, r1, r2 96 mov r0, #1 @ nonzero indicates that I lost 97 bx lr 98ENDPROC(vlock_trylock) 99 100@ r0: lock structure base 101ENTRY(vlock_unlock) 102 dmb 103 mov r1, #VLOCK_OWNER_NONE 104 strb r1, [r0, #VLOCK_OWNER_OFFSET] 105 dsb st 106 sev 107 bx lr 108ENDPROC(vlock_unlock) 109