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+/*
+ * 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)