Commit b70eb300 authored by Paul Burton's avatar Paul Burton Committed by Ralf Baechle
Browse files

MIPS: cmpxchg: Implement 1 byte & 2 byte xchg() 



Implement 1 & 2 byte xchg() using read-modify-write atop a 4 byte
cmpxchg(). This allows us to support these atomic operations despite the
MIPS ISA only providing for 4 & 8 byte atomic operations.

This is required in order to support queued spinlocks (qspinlock) in a
later patch, since these make use of a 2 byte xchg() in their slow path.

Signed-off-by: default avatarPaul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16354/


Signed-off-by: default avatarRalf Baechle <ralf@linux-mips.org>
parent 8263db4d

arch/mips/include/asm/cmpxchg.h

+7 −2
Original line number Diff line number Diff line
@@ -70,9 +70,16 @@ extern unsigned long __xchg_called_with_bad_pointer(void) 
	__ret;								\

})



extern unsigned long __xchg_small(volatile void *ptr, unsigned long val,

				  unsigned int size);



static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int size)

{

	switch (size) {

	case 1:

	case 2:

		return __xchg_small(ptr, x, size);



	case 4:

		return __xchg_asm("ll", "sc", (volatile u32 *)ptr, x);
@@ -91,8 +98,6 @@ static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int siz 
({									\

	__typeof__(*(ptr)) __res;					\

									\

	BUILD_BUG_ON(sizeof(*(ptr)) & ~0xc);				\

									\

	smp_mb__before_llsc();						\

									\

	__res = (__typeof__(*(ptr)))					\

arch/mips/kernel/Makefile

+1 −1
Original line number Diff line number Diff line
@@ -4,7 +4,7 @@ 


extra-y		:= head.o vmlinux.lds



obj-y		+= cpu-probe.o branch.o elf.o entry.o genex.o idle.o irq.o \

obj-y		+= cmpxchg.o cpu-probe.o branch.o elf.o entry.o genex.o idle.o irq.o \

		   process.o prom.o ptrace.o reset.o setup.o signal.o \

		   syscall.o time.o topology.o traps.o unaligned.o watch.o \

		   vdso.o cacheinfo.o

arch/mips/kernel/cmpxchg.c

0 → 100644
+52 −0
Original line number Diff line number Diff line 
/*

 * Copyright (C) 2017 Imagination Technologies

 * Author: Paul Burton <paul.burton@imgtec.com>

 *

 * This program is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License as published by the

 * Free Software Foundation;  either version 2 of the  License, or (at your

 * option) any later version.

 */



#include <linux/bitops.h>

#include <asm/cmpxchg.h>



unsigned long __xchg_small(volatile void *ptr, unsigned long val, unsigned int size)

{

	u32 old32, new32, load32, mask;

	volatile u32 *ptr32;

	unsigned int shift;



	/* Check that ptr is naturally aligned */

	WARN_ON((unsigned long)ptr & (size - 1));



	/* Mask value to the correct size. */

	mask = GENMASK((size * BITS_PER_BYTE) - 1, 0);

	val &= mask;



	/*

	 * Calculate a shift & mask that correspond to the value we wish to

	 * exchange within the naturally aligned 4 byte integerthat includes

	 * it.

	 */

	shift = (unsigned long)ptr & 0x3;

	if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN))

		shift ^= sizeof(u32) - size;

	shift *= BITS_PER_BYTE;

	mask <<= shift;



	/*

	 * Calculate a pointer to the naturally aligned 4 byte integer that

	 * includes our byte of interest, and load its value.

	 */

	ptr32 = (volatile u32 *)((unsigned long)ptr & ~0x3);

	load32 = *ptr32;



	do {

		old32 = load32;

		new32 = (load32 & ~mask) | (val << shift);

		load32 = cmpxchg(ptr32, old32, new32);

	} while (load32 != old32);



	return (load32 & mask) >> shift;

}

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