linux printk

printk如何调用底层驱动？

printk->vprintk_func->vprintk_default(在未定义CONFIG_PRINTK_NMI)->vprintk_emit->console_unlock->call_console_drivers（通过遍历console_drivers（for_each_console））->(con->write)

asmlinkage __visible int printk(const char *fmt, ...)
{
	va_list args;
	int r;

	va_start(args, fmt);
	r = vprintk_func(fmt, args);
	va_end(args);

	return r;
}

static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
{
	return vprintk_default(fmt, args);
}

int vprintk_default(const char *fmt, va_list args)
{
	int r;
	r = vprintk_emit(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args);
	return r;
}

asmlinkage int vprintk_emit(int facility, int level,
			    const char *dict, size_t dictlen,
			    const char *fmt, va_list args) {
	console_unlock();
}

static void call_console_drivers(int level,
				 const char *ext_text, size_t ext_len,
				 const char *text, size_t len)
{
	struct console *con;

	trace_console_rcuidle(text, len);

	if (!console_drivers)
		return;
	//for (con = console_drivers; con != NULL; con = con->next)
	for_each_console(con) {
		if (exclusive_console && con != exclusive_console)
			continue;
		if (!(con->flags & CON_ENABLED))
			continue;
		if (!con->write)
			continue;
		if (!cpu_online(smp_processor_id()) &&
		    !(con->flags & CON_ANYTIME))
			continue;
		if (con->flags & CON_EXTENDED)
			con->write(con, ext_text, ext_len);
		else
			con->write(con, text, len);
	}
}

查看控制台如何注册的,通过s3c244x.c来进行分析：

#define S3C24XX_SERIAL_NAME	"ttySAC"
static struct console s3c24xx_serial_console = {
	.name		= S3C24XX_SERIAL_NAME,
	.device		= uart_console_device,
	.flags		= CON_PRINTBUFFER,
	.index		= -1,
	.write		= s3c24xx_serial_console_write,
	.setup		= s3c24xx_serial_console_setup,
	.data		= &s3c24xx_uart_drv,
};

#define console_initcall(fn)					\
	static initcall_t __initcall_##fn			\
	__used __section(.con_initcall.init) = fn
	
static int __init s3c24xx_serial_console_init(void)
{
	register_console(&s3c24xx_serial_console);
	return 0;
}
console_initcall(s3c24xx_serial_console_init);

void register_console(struct console *newcon)
{
	int i;
	unsigned long flags;
	struct console *bcon = NULL;
	struct console_cmdline *c;

	/* 通过查找已注册的控制台，是否已经注册 */
	if (console_drivers)
		for_each_console(bcon)
			if (WARN(bcon == newcon,
					"console '%s%d' already registered\n",
					bcon->name, bcon->index))
				return;

	/*
	 * before we register a new CON_BOOT console, make sure we don't
	 * already have a valid console
	 * CON_BOOT一般在earlycon中设置，查找是否已经注册了boot控制台
	 */
	if (console_drivers && newcon->flags & CON_BOOT) {
		/* find the last or real console */
		for_each_console(bcon) {
			if (!(bcon->flags & CON_BOOT)) {
				pr_info("Too late to register bootconsole %s%d\n",
					newcon->name, newcon->index);
				return;
			}
		}
	}

	if (console_drivers && console_drivers->flags & CON_BOOT)
		bcon = console_drivers;

	if (preferred_console < 0 || bcon || !console_drivers)
		preferred_console = selected_console;

	/*
	 *	See if we want to use this console driver. If we
	 *	didn't select a console we take the first one
	 *	that registers here.
	 */
	if (preferred_console < 0) { /* 未设置preferred_console 将preferred_console设置为第一个 */
		if (newcon->index < 0)
			newcon->index = 0;
 		if (newcon->setup == NULL || newcon->setup(newcon, NULL) == 0) {
			newcon->flags |= CON_ENABLED;
			if (newcon->device) {
				newcon->flags |= CON_CONSDEV;
				preferred_console = 0;
			}
		}
	}

	/*
	 *	See if this console matches one we selected on
	 *	the command line. 和命令行中的控制台匹配
	 */
	for (i = 0, c = console_cmdline;
	     i < MAX_CMDLINECONSOLES && c->name[0];
	     i++, c++) {
		if (!newcon->match ||
		    newcon->match(newcon, c->name, c->index, c->options) != 0) {
			/* default matching */
			BUILD_BUG_ON(sizeof(c->name) != sizeof(newcon->name));
			if (strcmp(c->name, newcon->name) != 0)
				continue;
			if (newcon->index >= 0 &&
			    newcon->index != c->index)
				continue;
			if (newcon->index < 0)
				newcon->index = c->index;

			if (_braille_register_console(newcon, c))
				return;

			if (newcon->setup &&
			    newcon->setup(newcon, c->options) != 0)
				break;
		}

		newcon->flags |= CON_ENABLED;
		if (i == selected_console) {
			newcon->flags |= CON_CONSDEV;
			preferred_console = selected_console;
		}
		break;
	}

	if (!(newcon->flags & CON_ENABLED))
		return;

	/*
	 * If we have a bootconsole, and are switching to a real console,
	 * don't print everything out again, since when the boot console, and
	 * the real console are the same physical device, it's annoying to
	 * see the beginning boot messages twice
	 */
	//如果有bootconsole，则newcon不需要输出register之前的log，因为如果bootconsole和newcon是同一个设备
    //则之前的log就输出2次
	if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV))
		newcon->flags &= ~CON_PRINTBUFFER;

	/*
	 *	Put this console in the list - keep the
	 *	preferred driver at the head of the list.
	 */
	console_lock();
	/* 将newcon加入链表中 */
	if ((newcon->flags & CON_CONSDEV) || console_drivers == NULL) {
		newcon->next = console_drivers;
		console_drivers = newcon;
		if (newcon->next)
			newcon->next->flags &= ~CON_CONSDEV;
	} else {
		newcon->next = console_drivers->next;
		console_drivers->next = newcon;
	}

	if (newcon->flags & CON_EXTENDED)
		if (!nr_ext_console_drivers++)
			pr_info("printk: continuation disabled due to ext consoles, expect more fragments in /dev/kmsg\n");

	if (newcon->flags & CON_PRINTBUFFER) {
		/*
		 * console_unlock(); will print out the buffered messages
		 * for us.
		 */
		raw_spin_lock_irqsave(&logbuf_lock, flags);
		console_seq = syslog_seq;
		console_idx = syslog_idx;
		raw_spin_unlock_irqrestore(&logbuf_lock, flags);
		/*
		 * We're about to replay the log buffer.  Only do this to the
		 * just-registered console to avoid excessive message spam to
		 * the already-registered consoles.
		 */
		exclusive_console = newcon;
	}
	console_unlock();
	console_sysfs_notify();

	/*
	 * By unregistering the bootconsoles after we enable the real console
	 * we get the "console xxx enabled" message on all the consoles -
	 * boot consoles, real consoles, etc - this is to ensure that end
	 * users know there might be something in the kernel's log buffer that
	 * went to the bootconsole (that they do not see on the real console)
	 */
	pr_info("%sconsole [%s%d] enabled\n",
		(newcon->flags & CON_BOOT) ? "boot" : "" ,
		newcon->name, newcon->index);

    /* 注册后取消注册bootcon */
	if (bcon &&
	    ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV) &&
	    !keep_bootcon) {
		/* We need to iterate through all boot consoles, to make
		 * sure we print everything out, before we unregister them.
		 */
		for_each_console(bcon)
			if (bcon->flags & CON_BOOT)
				unregister_console(bcon);
	}
}

那如何调用函数s3c24xx_serial_console_init呢？

在.init.setup段注册"console="命令

#define __setup_param(str, unique_id, fn, early)			\
	static const char __setup_str_##unique_id[] __initconst		\
		__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 }

#define __setup(str, fn)						\
	__setup_param(str, fn, fn, 0)
	
__setup("console=", console_setup);

setup_arch->parse_early_options->parse_args 解析bootargs（“console=”）

static int __init do_early_param(char *param, char *val,
				 const char *unused, void *arg)
{
	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;
}

void __init parse_early_options(char *cmdline)
{
	parse_args("early options", cmdline, NULL, 0, 0, 0, NULL,
		   do_early_param);
}

/* Arch code calls this early on, or if not, just before other parsing. */
void __init parse_early_param(void)
{
	static int done __initdata;
	static char tmp_cmdline[COMMAND_LINE_SIZE] __initdata;

	if (done)
		return;

	/* All fall through to do_early_param. */
	strlcpy(tmp_cmdline, boot_command_line, COMMAND_LINE_SIZE);
	parse_early_options(tmp_cmdline);
	done = 1;
}

当解析出console命令，调用__setup(“console=”, console_setup);注册的函数：

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

	if (str[0] == 0)
		return 1;

	if (_braille_console_setup(&str, &brl_options))
		return 1;

	/*
	 * 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;
	options = strchr(str, ',');
	if (options)
		*(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 (isdigit(*s) || *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;
}

将控制台加入console_cmdline命令行中

static int __add_preferred_console(char *name, int idx, char *options,
				   char *brl_options)
{
	struct console_cmdline *c;
	int i;

	/*
	 *	See if this tty is not yet registered, and
	 *	if we have a slot free.
	 */
	for (i = 0, c = console_cmdline;
	     i < MAX_CMDLINECONSOLES && c->name[0];
	     i++, c++) {
		if (strcmp(c->name, name) == 0 && c->index == idx) {
			if (!brl_options)
				selected_console = i;
			return 0;
		}
	}
	if (i == MAX_CMDLINECONSOLES)
		return -E2BIG;
	if (!brl_options)
		selected_console = i;
	strlcpy(c->name, name, sizeof(c->name));
	c->options = options;
	braille_set_options(c, brl_options);

	c->index = idx;
	return 0;
}