Structure of kernel printing and debugging, from the ground up.

KERN_SOH_ASCII is prepended to string passed to printk().

#define KERN_SOH	"\001"		/* ASCII Start Of Header */
#define KERN_SOH_ASCII	'\001'

#define KERN_EMERG	KERN_SOH "0"	/* system is unusable */
#define KERN_ALERT	KERN_SOH "1"	/* action must be taken immediately */
#define KERN_CRIT	KERN_SOH "2"	/* critical conditions */
#define KERN_ERR	KERN_SOH "3"	/* error conditions */
#define KERN_WARNING	KERN_SOH "4"	/* warning conditions */
#define KERN_NOTICE	KERN_SOH "5"	/* normal but significant condition */
#define KERN_INFO	KERN_SOH "6"	/* informational */
#define KERN_DEBUG	KERN_SOH "7"	/* debug-level messages */

#define KERN_DEFAULT	KERN_SOH "d"	/* the default kernel loglevel */

/*
 * Annotation for a "continued" line of log printout (only done after a
 * line that had no enclosing \n). Only to be used by core/arch code
 * during early bootup (a continued line is not SMP-safe otherwise).
 */
#define KERN_CONT	KERN_SOH "c"

Remember to skip leading KERN_SOH_ASCII:

static inline int printk_get_level(const char *buffer)
{
	if (buffer[0] == KERN_SOH_ASCII && buffer[1]) {
		switch (buffer[1]) {
		case '0' ... '7':
		case 'd':	/* KERN_DEFAULT */
		case 'c':	/* KERN_CONT */
			return buffer[1];
		}
	}
	return 0;
}

Unconditional printing at all log levels.

#define pr_emerg(fmt, ...) \
	printk(KERN_EMERG pr_fmt(fmt), ##__VA_ARGS__)
#define pr_alert(fmt, ...) \
	printk(KERN_ALERT pr_fmt(fmt), ##__VA_ARGS__)
#define pr_crit(fmt, ...) \
	printk(KERN_CRIT pr_fmt(fmt), ##__VA_ARGS__)
#define pr_err(fmt, ...) \
	printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
#define pr_warning(fmt, ...) \
	printk(KERN_WARNING pr_fmt(fmt), ##__VA_ARGS__)
#define pr_warn pr_warning
#define pr_notice(fmt, ...) \
	printk(KERN_NOTICE pr_fmt(fmt), ##__VA_ARGS__)
#define pr_info(fmt, ...) \
	printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)
/*
 * Like KERN_CONT, pr_cont() should only be used when continuing
 * a line with no newline ('\n') enclosed. Otherwise it defaults
 * back to KERN_DEFAULT.
 */
#define pr_cont(fmt, ...) \
        printk(KERN_CONT fmt, ##__VA_ARGS__)

/* If you are writing a driver, please use dev_dbg instead */

#ifndef dev_fmt
#define dev_fmt(fmt) fmt
#endif

#ifdef CONFIG_PRINTK

__printf(3, 0)
int dev_vprintk_emit(int level, const struct device *dev,
		     const char *fmt, va_list args);
__printf(3, 4)
int dev_printk_emit(int level, const struct device *dev, const char *fmt, ...);

__printf(3, 4)
void dev_printk(const char *level, const struct device *dev,
		const char *fmt, ...);
__printf(2, 3)
void _dev_emerg(const struct device *dev, const char *fmt, ...);
__printf(2, 3)
void _dev_alert(const struct device *dev, const char *fmt, ...);
__printf(2, 3)
void _dev_crit(const struct device *dev, const char *fmt, ...);
__printf(2, 3)
void _dev_err(const struct device *dev, const char *fmt, ...);
__printf(2, 3)
void _dev_warn(const struct device *dev, const char *fmt, ...);
__printf(2, 3)
void _dev_notice(const struct device *dev, const char *fmt, ...);
__printf(2, 3)
void _dev_info(const struct device *dev, const char *fmt, ...);

#else

static inline __printf(3, 0)
int dev_vprintk_emit(int level, const struct device *dev,
		     const char *fmt, va_list args)
{ return 0; }
static inline __printf(3, 4)
int dev_printk_emit(int level, const struct device *dev, const char *fmt, ...)
{ return 0; }

static inline void __dev_printk(const char *level, const struct device *dev,
				struct va_format *vaf)
{}
static inline __printf(3, 4)
void dev_printk(const char *level, const struct device *dev,
		 const char *fmt, ...)
{}

static inline __printf(2, 3)
void _dev_emerg(const struct device *dev, const char *fmt, ...)
{}
static inline __printf(2, 3)
void _dev_crit(const struct device *dev, const char *fmt, ...)
{}
static inline __printf(2, 3)
void _dev_alert(const struct device *dev, const char *fmt, ...)
{}
static inline __printf(2, 3)
void _dev_err(const struct device *dev, const char *fmt, ...)
{}
static inline __printf(2, 3)
void _dev_warn(const struct device *dev, const char *fmt, ...)
{}
static inline __printf(2, 3)
void _dev_notice(const struct device *dev, const char *fmt, ...)
{}
static inline __printf(2, 3)
void _dev_info(const struct device *dev, const char *fmt, ...)
{}

#endif

#define dev_emerg(dev, fmt, ...)					\
	_dev_emerg(dev, dev_fmt(fmt), ##__VA_ARGS__)
#define dev_crit(dev, fmt, ...)						\
	_dev_crit(dev, dev_fmt(fmt), ##__VA_ARGS__)
#define dev_alert(dev, fmt, ...)					\
	_dev_alert(dev, dev_fmt(fmt), ##__VA_ARGS__)
#define dev_err(dev, fmt, ...)						\
	_dev_err(dev, dev_fmt(fmt), ##__VA_ARGS__)
#define dev_warn(dev, fmt, ...)						\
	_dev_warn(dev, dev_fmt(fmt), ##__VA_ARGS__)
#define dev_notice(dev, fmt, ...)					\
	_dev_notice(dev, dev_fmt(fmt), ##__VA_ARGS__)
#define dev_info(dev, fmt, ...)						\
	_dev_info(dev, dev_fmt(fmt), ##__VA_ARGS__)