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Github: atomic_sub_and_test

Email: BuddyZhang1 buddy.zhang@aliyun.com

Architecture: ARMv7 Cortex A9-MP

目录


源码分析

/**
 * atomic_sub_and_test - subtract value from variable and test result
 * @i: integer value to subtract
 * @v: pointer of type atomic_t
 *
 * Atomically subtracts @i from @v and returns
 * true if the result is zero, or false for all
 * other cases.
 */
#ifndef atomic_sub_and_test
static inline bool atomic_sub_and_test(int i, atomic_t *v)
{
        return atomic_sub_return(i, v) == 0;
}
#endif

atomic_sub_and_test() 函数用于对 atomic_t 变量做减法,并检查结果是否为零。 函数直接调用 atomic_sub_return() 函数对 atomic_t 变量进行加法,然后检查返回 值。

atomic_sub_return

atomic_sub_return 源码解析


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(8);

/* atomic_* */
static __init int atomic_demo_init(void)
{
	/* Legacy data */
	printk("[0]Atomic: %d\n", atomic_read(&BiscuitOS_counter));

	if (atomic_sub_and_test(8, &BiscuitOS_counter))
		printk("Atomic is zero\n");
	else
		printk("Atomic is no zero\n");

	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_sub_and_test"
+
+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_sub_and_test()

具体过程请参考:

Linux 5.0 开发环境搭建 – 驱动配置

驱动编译

驱动编译也请参考下面文章关于驱动编译一节:

Linux 5.0 开发环境搭建 – 驱动编译

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:

Linux 5.0 开发环境搭建 – 驱动运行

启动内核,并打印如下信息:

usbcore: registered new interface driver usbhid
usbhid: USB HID core driver
[0]Atomic: 8
Atomic is zero
aaci-pl041 10004000.aaci: FIFO 512 entries
oprofile: using arm/armv7-ca9

驱动分析

在对某统计数进行减法操作并检查值是,atomic_sub_and_test() 再适合不过。


附录

BiscuitOS Home

BiscuitOS Driver

BiscuitOS Kernel Build

Linux Kernel

Bootlin: Elixir Cross Referencer

搭建高效的 Linux 开发环境

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