Linux Kernel Module Cheat

17.4.3.1. pr_debug != printk(KERN_DEBUG

When CONFIG_DYNAMIC_DEBUG is set, printk(KERN_DEBUG is not the exact same as pr_debug( since printk(KERN_DEBUG messages are visible with:

./run --kernel-cli 'initcall_debug logleve=8'

which outputs lines of type:

<7>[    1.756680] calling  clk_disable_unused+0x0/0x130 @ 1
<7>[    1.757003] initcall clk_disable_unused+0x0/0x130 returned 0 after 111 usecs

which are printk(KERN_DEBUG inside init/main.c in v4.16.

Mentioned at: https://stackoverflow.com/questions/37272109/how-to-get-details-of-all-modules-drivers-got-initialized-probed-during-kernel-b

This likely comes from the ifdef split at init/main.c:

/* If you are writing a driver, please use dev_dbg instead */
#if defined(CONFIG_DYNAMIC_DEBUG)
#include <linux/dynamic_debug.h>

/* dynamic_pr_debug() uses pr_fmt() internally so we don't need it here */
#define pr_debug(fmt, ...) \
    dynamic_pr_debug(fmt, ##__VA_ARGS__)
#elif defined(DEBUG)
#define pr_debug(fmt, ...) \
    printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
#else
#define pr_debug(fmt, ...) \
    no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
#endif