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/* SPDX-License-Identifier: GPL-2.0-only */ /* * vlock.S - simple voting lock implementation for ARM * * Created by: Dave Martin, 2012-08-16 * Copyright: (C) 2012-2013 Linaro Limited * * This algorithm is described in more detail in * Documentation/arm/vlocks.rst. */ #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 st 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 st sev bx lr ENDPROC(vlock_unlock) |