Email: BuddyZhang1 buddy.zhang@aliyun.com
Architecture: ARMv7 Cortex A9-MP
目录
源码分析
#define atomic_xchg(v, new) (xchg(&((v)->counter), new))atomic_xchg() 函数用于交换 atomic_t 变量的值。参数 v 指向 atomic_t 变量；参数 new 代表需要替换的值。函数直接调用 xchg 进行交换，xchg 的核心实现如下：
#define atomic_xchg(v, new) (xchg(&((v)->counter), new))xchg 是核心实现如下：
static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
int size)
{
unsigned long ret;
unsigned int tmp;
asm volatile("@ __xchg4\n"
"1: ldrex %0, [%3]\n"
" strex %1, %2, [%3]\n"
" teq %1, #0\n"
" bne 1b"
: "=&r" (ret), "=&r" (tmp)
: "r" (x), "r" (ptr)
: "memory", "cc");
return ret;
}参数 x 代表需要交换的数值；参数 ptr 指向一个 atomic_t 变量；函数直接调用内嵌汇编。 汇编中首先调用 ldrex 指令标记独占，并且从内存中读取 ptr 对应的内容到 ret 变量里， 然后调用 strex 指令，如果此时独占标志还存在的话，那么就将 x 参数的值写入到 ptr 对应的内存地址。并将 tmp 的值设置为 0；如果此时独占标志已经被清零，那么 strex 指令 仅仅设置 tmp 的值为 1,。接着调用 teq 指令，如果 tmp 的值为 1，那么跳转到 1 处 重新执行之前的动作；如果 tmp 为 0，那么直接返回 ret 的值。因此，可以确保数据被 原子写入到内存。
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_xchg (ARMv7 Cotex-A9MP)
*
* static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
* int size)
* {
* unsigned long ret;
* unsigned int tmp;
*
* asm volatile("@ __xchg4\n"
* "1: ldrex %0, [%3]\n"
* " strex %1, %2, [%3]\n"
* " teq %1, #0\n"
* " bne 1b"
* : "=&r" (ret), "=&r" (tmp)
* : "r" (x), "r" (ptr)
* : "memory", "cc");
*
* return ret;
* }
*/
#include <linux/kernel.h>
#include <linux/init.h>
static atomic_t BiscuitOS_counter = ATOMIC_INIT(8);
/* atomic_* */
static __init int atomic_demo_init(void)
{
/* Atomic value before exhange data */
printk("[0]Atomic: oiginal-> %d\n", atomic_read(&BiscuitOS_counter));
/* Atomic xhg */
atomic_xchg(&BiscuitOS_counter, 6);
/* Atomic value after exhange data */
printk("[1]Atomic: new-> %d\n", 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_xchg"
+
+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_xchg()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: oiginal-> 8
[1]Atomic: new-> 6
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
交换 atomic_t 变量的值，可以使用 atomic_xchg() 函数。
附录
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