Github: atomic_cmpxchg_relaxed
Email: BuddyZhang1 buddy.zhang@aliyun.com
Architecture: ARMv7 Cortex A9-MP
目录
源码分析
static inline int atomic_cmpxchg_relaxed(atomic_t *ptr, int old, int new)
{
int oldval;
unsigned long res;
prefetchw(&ptr->counter);
do {
__asm__ __volatile__("@ atomic_cmpxchg\n"
"ldrex %1, [%3]\n"
"mov %0, #0\n"
"teq %1, %4\n"
"strexeq %0, %5, [%3]\n"
: "=&r" (res), "=&r" (oldval), "+Qo" (ptr->counter)
: "r" (&ptr->counter), "Ir" (old), "r" (new)
: "cc");
} while (res);
return oldval;
}atomic_cmpxchg_relaxed() 函数用于对 atmoic 变量的值进行对比，如果与内存中的值 相等，那么就替换成新值。参数 ptr 指向一个 atomic_t 变量；参数 old 代表与内存中对比 的值；new 代表替换的值。函数首先调用 prefetchw() 函数将 ptr->counter 值从内存预取 到 cache 里面，然后调用一个内嵌汇编。在内嵌汇编中，首先低啊用 ldrex 指令设置 独占标志，并将 ptr->counter 的值从内存读取到 oldval 变量里，接着将 res 变量清零。 接着调用 teq 指令对比 ptr->counter 的值是否和 old 参数相同，如果不相同，则直接 返回；如果相同，那么调用 strexeq 指令，如果此时独占标志还存在，那么 strexeq 指令就 将 new 参数的值写入到 ptr->counter 对应的内存中，并将 res 设置为 0；如果此时 独占标志被清零，那么 strexeq 不会将 new 值写入 ptr->counter 对应的内存，而是 仅仅将 res 设置为 1。由于 res 的值为 1，那么 while 循环继续循环重复之前的操作。
实践
驱动源码
/*
* atomic
*
* (C) 2019.05.05 <buddy.zhang@aliyun.com>
*
* 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.
*/
/* Memory access
*
*
* +----------+
* | |
* | Register | +--------+
* | | | |
* +----------+ | |
* A | |
* | | |
* +-----+ | +----------+ +----------+ | |
* | |<---o | | | | | |
* | CPU |<--------->| L1 Cache |<------>| L2 Cache |<------>| Memory |
* | |<---o | | | | | |
* +-----+ | +----------+ +----------+ | |
* | | |
* o--------------------------------------------->| |
* volatile/atomic | |
* | |
* +--------+
*/
/*
* atomic_cmpxchg_* (ARMv7 Cotex-A9MP)
*
* static inline int atomic_cmpxchg_relaxed(atomic_t *ptr, int old, int new)
* {
* int oldval;
* unsigned long res;
*
* prefetchw(&ptr->counter);
*
* do {
* __asm__ __volatile__("@ atomic_cmpxchg\n"
* "ldrex %1, [%3]\n"
* "mov %0, #0\n"
* "teq %1, %4\n"
* "strexeq %0, %5, [%3]\n"
* : "=&r" (res), "=&r" (oldval), "+Qo" (ptr->counter)
* : "r" (&ptr->counter), "Ir" (old), "r" (new)
* : "cc");
* } while (res);
*
* return oldval;
* }
*/
#include <linux/kernel.h>
#include <linux/init.h>
static atomic_t BiscuitOS_counter = ATOMIC_INIT(8);
/* atomic_* */
static __init int atomic_demo_init(void)
{
int val;
/* Atomic cmpxchg: Old == original */
val = atomic_cmpxchg_relaxed(&BiscuitOS_counter, 8, 9);
printk("[0]Atomic: oiginal-> %d new-> %d\n", val,
atomic_read(&BiscuitOS_counter));
/* Atomic cmpxchg: Old != original */
val = atomic_cmpxchg_relaxed(&BiscuitOS_counter, 1, 5);
printk("[1]Atomic: original-> %d new-> %d\n", val,
atomic_read(&BiscuitOS_counter));
return 0;
}
device_initcall(atomic_demo_init);驱动安装
驱动的安装很简单，首先将驱动放到 drivers/BiscuitOS/ 目录下，命名为 atomic.c， 然后修改 Kconfig 文件，添加内容参考如下：
diff --git a/drivers/BiscuitOS/Kconfig b/drivers/BiscuitOS/Kconfig
index 4edc5a5..1a9abee 100644
--- a/drivers/BiscuitOS/Kconfig
+++ b/drivers/BiscuitOS/Kconfig
@@ -6,4 +6,14 @@ if BISCUITOS_DRV
config BISCUITOS_MISC
bool "BiscuitOS misc driver"
+config BISCUITOS_ATOMIC
+ bool "atomic"
+
+if BISCUITOS_ATOMIC
+
+config DEBUG_BISCUITOS_ATOMIC
+ bool "atomic_cmpxchg_relaxed"
+
+endif # BISCUITOS_ATOMIC
+
endif # BISCUITOS_DRV接着修改 Makefile，请参考如下修改：
diff --git a/drivers/BiscuitOS/Makefile b/drivers/BiscuitOS/Makefile
index 82004c9..9909149 100644
--- a/drivers/BiscuitOS/Makefile
+++ b/drivers/BiscuitOS/Makefile
@@ -1 +1,2 @@
obj-$(CONFIG_BISCUITOS_MISC) += BiscuitOS_drv.o
+obj-$(CONFIG_BISCUITOS_ATOMIC) += atomic.o
--驱动配置
驱动配置请参考下面文章中关于驱动配置一节。在配置中，勾选如下选项，如下：
Device Driver--->
[*]BiscuitOS Driver--->
[*]atomic
[*]atomic_cmpxchg_relaxed()具体过程请参考：
驱动编译
驱动编译也请参考下面文章关于驱动编译一节：
驱动运行
驱动的运行，请参考下面文章中关于驱动运行一节：
启动内核，并打印如下信息：
usbcore: registered new interface driver usbhid
usbhid: USB HID core driver
[0]Atomic: original-> 8 new-> 9
[1]Atomic: original-> 9 new-> 9
aaci-pl041 10004000.aaci: ARM AC'97 Interface PL041 rev0 at 0x10004000, irq 24
aaci-pl041 10004000.aaci: FIFO 512 entries
oprofile: using arm/armv7-ca9驱动分析
在需要交换 atomic_t 变量值的地方可以使用 atomic_cmpxchg_relaxed() 函数。
附录
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