Email: BuddyZhang1 buddy.zhang@aliyun.com
Architecture: ARMv7 Cortex A9-MP
目录
源码分析
static __always_inline void __write_once_size(volatile void *p, void *res, int size)
{
switch (size) {
case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
default:
barrier();
__builtin_memcpy((void *)p, (const void *)res, size);
barrier();
}
}__write_once_size() 函数用于将数据写入到内存。参数 p 为要写入的内存地址；参数 res 缓存要写入的数据；size 表示要写入的字节。函数首先判断要写入数据的大小，如果 是 1,2,4,8 字节写入，那么直接将缓存区数据写入到内存；如果是多个数据写入内存， 那么需要调用 __builtin_memcpy() 函数写入，并且在 __builtin_memcpy() 前后添加 内存屏障指令。
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.
*/
#include <linux/kernel.h>
#include <linux/init.h>
/* READ_ONCE/WRITE_ONCE */
#include <linux/compiler.h>
volatile char ch;
static __init int atomic_demo_init(void)
{
char cb = 'A';
/* Write data into memory not cache/buffer/register */
__write_once_size(&ch, &cb, sizeof(ch));
/* Read from memory not cache nor register */
cb = READ_ONCE(ch);
printk("cb: %c\n", cb);
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 "__write_once_size"
+
+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
[*]__write_once_size()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
cb: A
input: AT Raw Set 2 keyboard as /devices/platform/smb@4000000/smb@4000000:motherboard/smb@4000000:motherboard:iofpga@7,00000000/10006000.kmi/serio0/input/input0驱动分析
__write_once_size() 函数可以将数据写入到内存。
附录
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