How to mess with loglevel at boot time.

/* printk's without a loglevel use this.. */
#define MESSAGE_LOGLEVEL_DEFAULT CONFIG_MESSAGE_LOGLEVEL_DEFAULT

/* We show everything that is MORE important than this.. */
#define CONSOLE_LOGLEVEL_SILENT  0 /* Mum's the word */
#define CONSOLE_LOGLEVEL_MIN	 1 /* Minimum loglevel we let people use */
#define CONSOLE_LOGLEVEL_QUIET	 4 /* Shhh ..., when booted with "quiet" */
#define CONSOLE_LOGLEVEL_DEBUG	10 /* issue debug messages */
#define CONSOLE_LOGLEVEL_MOTORMOUTH 15	/* You can't shut this one up */

/*
 * Default used to be hard-coded at 7, we're now allowing it to be set from
 * kernel config.
 */
#define CONSOLE_LOGLEVEL_DEFAULT CONFIG_CONSOLE_LOGLEVEL_DEFAULT

extern int console_printk[];

#define console_loglevel (console_printk[0])
#define default_message_loglevel (console_printk[1])
#define minimum_console_loglevel (console_printk[2])
#define default_console_loglevel (console_printk[3])

static inline void console_silent(void)
{
	console_loglevel = CONSOLE_LOGLEVEL_SILENT;
}

static inline void console_verbose(void)
{
	if (console_loglevel)
		console_loglevel = CONSOLE_LOGLEVEL_MOTORMOUTH;
}

static int __init debug_kernel(char *str)
{
        console_loglevel = CONSOLE_LOGLEVEL_DEBUG;
        return 0;
}

static int __init quiet_kernel(char *str)
{
        console_loglevel = CONSOLE_LOGLEVEL_QUIET;
        return 0;
}

early_param("debug", debug_kernel);
early_param("quiet", quiet_kernel);

static int __init loglevel(char *str)
{
        int newlevel;

        /*
         * Only update loglevel value when a correct setting was passed,
         * to prevent blind crashes (when loglevel being set to 0) that
         * are quite hard to debug
         */
        if (get_option(&str, &newlevel)) {
                console_loglevel = newlevel;
                return 0;
        }

        return -EINVAL;
}

early_param("loglevel", loglevel);