Github: atomic_dec_unless_positive
Email: BuddyZhang1 buddy.zhang@aliyun.com
Architecture: ARMv7 Cortex A9-MP
目录
源码分析
#ifndef atomic_dec_unless_positive
static inline bool atomic_dec_unless_positive(atomic_t *v)
{
int c = atomic_read(v);
do {
if (unlikely(c > 0))
return false;
} while (!atomic_try_cmpxchg(v, &c, c - 1));
return true;
}
#endifatomic_dec_unless_positive() 函数用于对负数执行减一操作。函数首先判断 atomic_t 变量的值是否为正数，如果是，则直接返回；如果不是，则调用 atomic_try_cmpxchg() 函数进行减一操作。
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 | |
* | |
* +--------+
*/
#include <linux/kernel.h>
#include <linux/init.h>
static atomic_t BiscuitOS_counter = ATOMIC_INIT(2);
/* atomic_* */
static __init int atomic_demo_init(void)
{
/* Legacy data */
printk("[0]Atomic: %d\n", atomic_read(&BiscuitOS_counter));
atomic_dec_unless_positive(&BiscuitOS_counter);
/* New data */
printk("[1]Atomic: %d\n", atomic_read(&BiscuitOS_counter));
/* add > 0 */
atomic_sub(3, &BiscuitOS_counter);
printk("[2]Atomic: %d\n", atomic_read(&BiscuitOS_counter));
atomic_dec_unless_positive(&BiscuitOS_counter);
/* New data */
printk("[3]Atomic: %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_dec_unless_positive"
+
+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_dec_unless_positive()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: 2
[1]Atomic: 2
[2]Atomic: -1
[3]Atomic: -2
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
对负数减一操作，可以直接使用这个函数。
附录
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