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Diffstat (limited to 'arch/arm/common/vlock.S')
| -rw-r--r-- | arch/arm/common/vlock.S | 108 |
1 files changed, 108 insertions, 0 deletions
diff --git a/arch/arm/common/vlock.S b/arch/arm/common/vlock.S new file mode 100644 index 00000000000..ff198583f68 --- /dev/null +++ b/arch/arm/common/vlock.S @@ -0,0 +1,108 @@ +/* + * vlock.S - simple voting lock implementation for ARM + * + * Created by: Dave Martin, 2012-08-16 + * Copyright: (C) 2012-2013 Linaro Limited + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License version 2 as + * published by the Free Software Foundation. + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + * + * + * This algorithm is described in more detail in + * Documentation/arm/vlocks.txt. + */ + +#include <linux/linkage.h> +#include "vlock.h" + +/* Select different code if voting flags can fit in a single word. */ +#if VLOCK_VOTING_SIZE > 4 +#define FEW(x...) +#define MANY(x...) x +#else +#define FEW(x...) x +#define MANY(x...) +#endif + +@ voting lock for first-man coordination + +.macro voting_begin rbase:req, rcpu:req, rscratch:req + mov \rscratch, #1 + strb \rscratch, [\rbase, \rcpu] + dmb +.endm + +.macro voting_end rbase:req, rcpu:req, rscratch:req + dmb + mov \rscratch, #0 + strb \rscratch, [\rbase, \rcpu] + dsb + sev +.endm + +/* + * The vlock structure must reside in Strongly-Ordered or Device memory. + * This implementation deliberately eliminates most of the barriers which + * would be required for other memory types, and assumes that independent + * writes to neighbouring locations within a cacheline do not interfere + * with one another. + */ + +@ r0: lock structure base +@ r1: CPU ID (0-based index within cluster) +ENTRY(vlock_trylock) + add r1, r1, #VLOCK_VOTING_OFFSET + + voting_begin r0, r1, r2 + + ldrb r2, [r0, #VLOCK_OWNER_OFFSET] @ check whether lock is held + cmp r2, #VLOCK_OWNER_NONE + bne trylock_fail @ fail if so + + @ Control dependency implies strb not observable before previous ldrb. + + strb r1, [r0, #VLOCK_OWNER_OFFSET] @ submit my vote + + voting_end r0, r1, r2 @ implies DMB + + @ Wait for the current round of voting to finish: + + MANY( mov r3, #VLOCK_VOTING_OFFSET ) +0: + MANY( ldr r2, [r0, r3] ) + FEW( ldr r2, [r0, #VLOCK_VOTING_OFFSET] ) + cmp r2, #0 + wfene + bne 0b + MANY( add r3, r3, #4 ) + MANY( cmp r3, #VLOCK_VOTING_OFFSET + VLOCK_VOTING_SIZE ) + MANY( bne 0b ) + + @ Check who won: + + dmb + ldrb r2, [r0, #VLOCK_OWNER_OFFSET] + eor r0, r1, r2 @ zero if I won, else nonzero + bx lr + +trylock_fail: + voting_end r0, r1, r2 + mov r0, #1 @ nonzero indicates that I lost + bx lr +ENDPROC(vlock_trylock) + +@ r0: lock structure base +ENTRY(vlock_unlock) + dmb + mov r1, #VLOCK_OWNER_NONE + strb r1, [r0, #VLOCK_OWNER_OFFSET] + dsb + sev + bx lr +ENDPROC(vlock_unlock) |