/* 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/arch/arm/vlocks.rst.
 */

#include <linux/linkage.h>
#include "vlock.h"

.arch armv7-a

/* 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)