在解析cmdline时,我们经常会使用到__setup宏,用来处理kernel的cmdline。
#define __setup(str, fn) \
__setup_param(str, fn, fn, 0)
#define __setup_param(str, unique_id, fn, early) \
static char __setup_str_##unique_id[] __initdata __aligned(1) = str; \
static struct obs_kernel_param __setup_##unique_id \
__used __section(.init.setup) \
__attribute__((aligned((sizeof(long))))) \
= { __setup_str_##unique_id, fn, early }
现在我是用到读取屏id。
__setup("lcd_id=", lcd_id_get);
将上面宏展开就是:
__setup_param(“lcd_id=”, lcd_id_get, lcd_id_get, 0)
static char __setup_str_lcd_id_get[] __initdata __aligned(1) = “lcd_id=”;
static struct obs_kernel_param __setup_lcd_id_get
__used __section(.init.setup)
attribute((aligned((sizeof(long)))))
= { __setup_str_lcd_id_get, lcd_id_get, 0 }
把宏里面的修饰拿掉就是:
static char __setup_str_lcd_id_get[] = "lcd_id=";
static struct obs_kernel_param __setup_lcd_id_get = { __setup_str_lcd_id_get, lcd_id_get, 0 }
这里定义了两个变量:一个字符串__setup_str_lcd_id_get,一个obs_kernel_param结构,其结构的原型为:
@/kernel/include/linux/init.h
struct obs_kernel_param {
const char *str;
int (*setup_func)(char *);
int early;
};
str元素为__setup_str_lcd_id_get, setup_func函数指针为lcd_id_get, early为0.
宏里面的__section(.init.setup)修饰,执行将__setup_lcd_id_get结构放到对应section。
#define INIT_SETUP(initsetup_align) \
. = ALIGN(initsetup_align); \
VMLINUX_SYMBOL(__setup_start) = .; \
*(.init.setup) \
VMLINUX_SYMBOL(__setup_end) = .;
INIT_SETUP宏将被vmlinux连接脚本使用。
/kernel/arch/arm/kernel/vmlinux.lds.S
.init.data : {
INIT_DATA
INIT_SETUP(16)
INIT_CALLS
CON_INITCALL
SECURITY_INITCALL
INIT_RAM_FS
}
在编译是,__setup()宏将编译进vmlinux中的.init.setup段中,这个段从__setup_start符号开始,在__setup_end,在内核启动是如何取出呢。
在start_kernel可以看到:
start_kernel()
setup_arch()
parse_early_param() // @/kernel/init
strlcpy(tmp_cmdline, boot_command_line, COMMAND_LINE_SIZE); // 赋值cmdline
parse_early_options(tmp_cmdline);
parse_args("early options", cmdline, NULL, 0, 0, 0, do_early_param);
parse_one(param, val, doing, params, num, min_level, max_level,unknown);
其中parse_one用来解析指令,然后do_early_param
/* Check for early params. */
static int __init do_early_param(char *param, char *val, const char *unused)
{
const struct obs_kernel_param *p;
for (p = __setup_start; p < __setup_end; p++) {
if ((p->early && parameq(param, p->str)) ||
(strcmp(param, "console") == 0 &&
strcmp(p->str, "earlycon") == 0)
) {
if (p->setup_func(val) != 0)
pr_warn("Malformed early option '%s'\n", param);
}
}
/* We accept everything at this stage. */
return 0;
}
do_early_param将取出符号 __setup_start到__setup_end之间的obs_kernel_param结构的变量,调用obs_kernel_param中设置的函数指针。
总结:
__setup宏用来指导建立obs_kernel_param结构,并编译到内核特定段中。在内核启动时,将取出obs_kernel_param结构并执行其中的函数。