printk如何定向输出到UART的？

最新推荐文章于 2024-08-09 15:55:12 发布

skywalkzf

最新推荐文章于 2024-08-09 15:55:12 发布

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分类专栏： Linux学习文章标签： command initialization printing features null types

本文链接：https://blog.youkuaiyun.com/skywalkzf/article/details/6754879

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本文深入探讨了Linux系统中console配置的实现方式，包括如何通过修改CONFIG_CMDLINE设置控制台输出位置，以及内核初始化过程中console配置的具体处理流程。

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我们通常在config里面配置这么一句“CONFIG_CMDLINE="console=tty0 console=ttyMT3,921600n1 root=/dev/ram"”，然后修改console的赋值就可以改变输出的位置，代码中如何实现的呢？

这里紧接着上文中的printk.c文件中，有这么一个函数先看一下：
/*
* Set up a list of consoles. Called from init/main.c
*/
拉风的注释直接告诉了我们这个函数被init/main.c文件中某个函数调用。

static int __init console_setup(char *str)
{
	char buf[sizeof(console_cmdline[0].name) + 4]; /* 4 for index */
	char *s, *options, *brl_options = NULL;
	int idx;


#ifdef CONFIG_A11Y_BRAILLE_CONSOLE
	if (!memcmp(str, "brl,", 4)) {
		brl_options = "";
		str += 4;
	} else if (!memcmp(str, "brl=", 4)) {
		brl_options = str + 4;
		str = strchr(brl_options, ',');
		if (!str) {
			printk(KERN_ERR "need port name after brl=\n");
			return 1;
		}
		*(str++) = 0;
	}
#endif


	/*
	 * Decode str into name, index, options.
	 */
	if (str[0] >= '0' && str[0] <= '9') {
		strcpy(buf, "ttyS");
		strncpy(buf + 4, str, sizeof(buf) - 5);
	} else {
		strncpy(buf, str, sizeof(buf) - 1);
	}
	buf[sizeof(buf) - 1] = 0;
	if ((options = strchr(str, ',')) != NULL)
		*(options++) = 0;
#ifdef __sparc__
	if (!strcmp(str, "ttya"))
		strcpy(buf, "ttyS0");
	if (!strcmp(str, "ttyb"))
		strcpy(buf, "ttyS1");
#endif
	for (s = buf; *s; s++)
		if ((*s >= '0' && *s <= '9') || *s == ',')
			break;
	idx = simple_strtoul(s, NULL, 10);
	*s = 0;


	__add_preferred_console(buf, idx, options, brl_options);
	console_set_on_cmdline = 1;
	return 1;
}

可以看到这里面有对字符串的解析，获得tty的name，index，options。加入链表是由下面的宏来做的。

__setup("console=", console_setup);

好像是不是缺少了点东东，在main.c中调用的的链表呢？在这里register_console()。

/*
 * The console driver calls this routine during kernel initialization
 * to register the console printing procedure with printk() and to
 * print any messages that were printed by the kernel before the
 * console driver was initialized.
 *
 * This can happen pretty early during the boot process (because of
 * early_printk) - sometimes before setup_arch() completes - be careful
 * of what kernel features are used - they may not be initialised yet.
 *
 * There are two types of consoles - bootconsoles (early_printk) and
 * "real" consoles (everything which is not a bootconsole) which are
 * handled differently.
 *  - Any number of bootconsoles can be registered at any time.
 *  - As soon as a "real" console is registered, all bootconsoles
 *    will be unregistered automatically.
 *  - Once a "real" console is registered, any attempt to register a
 *    bootconsoles will be rejected
 */
void register_console(struct console *newcon)
{
	............................
	/*
	 *	See if this console matches one we selected on
	 *	the command line.
	 */
	 //看上面的注释我们也可以知道了command line中的定义在这里也起作用了
	for (i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0];
			i++) {
		if (strcmp(console_cmdline[i].name, newcon->name) != 0)
			continue;
		if (newcon->index >= 0 &&
		    newcon->index != console_cmdline[i].index)
			continue;
		if (newcon->index < 0)
			newcon->index = console_cmdline[i].index;
#ifdef CONFIG_A11Y_BRAILLE_CONSOLE
		if (console_cmdline[i].brl_options) {
			newcon->flags |= CON_BRL;
			braille_register_console(newcon,
					console_cmdline[i].index,
					console_cmdline[i].options,
					console_cmdline[i].brl_options);
			return;
		}
#endif
		if (newcon->setup &&
		    newcon->setup(newcon, console_cmdline[i].options) != 0)/////在这里可以看到setup的过程了
			break;
		newcon->flags |= CON_ENABLED;
		newcon->index = console_cmdline[i].index;
		if (i == selected_console) {
			newcon->flags |= CON_CONSDEV;
			preferred_console = selected_console;
		}
		break;
	}


	if (!(newcon->flags & CON_ENABLED))//其他的console driver屏蔽掉
		return;


........................................
}

当然了，上面的函数在源码中的注释很是详细，如果需要看的，读源码是最佳的选择。

到了这里，so command line的tty端口的指定就到了这里了。

Have Fun!