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) 52 ENTRY(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 ) 70 0: 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 87 trylock_fail: 88 voting_end r0, r1, r2 89 mov r0, #1 @ nonzero indicates that I lost 90 bx lr 91 ENDPROC(vlock_trylock) 92 93 @ r0: lock structure base 94 ENTRY(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 101 ENDPROC(vlock_unlock) 102