Logging

The simplest and most common debugging technique.

printk and Log Levels

/* Generic printk with level */
printk(KERN_ERR "Error: %d\n", err);
printk(KERN_INFO "Initialized\n");
printk(KERN_DEBUG "Debug info\n");

/* Shorthand macros (preferred) */
pr_err("Error: %d\n", err);
pr_info("Initialized\n");
pr_debug("Debug info\n");  /* Compiled out unless DEBUG defined */

Level	Macro	Use For
KERN_EMERG	pr_emerg	System unusable
KERN_ERR	pr_err	Error conditions
KERN_WARNING	pr_warn	Warning
KERN_NOTICE	pr_notice	Normal but significant
KERN_INFO	pr_info	Informational
KERN_DEBUG	pr_debug	Debug messages

Device-Specific Logging

Always prefer dev_* for drivers - they include device name:

dev_err(&pdev->dev, "Failed to init: %d\n", ret);
dev_info(&pdev->dev, "Probe successful\n");
dev_dbg(&pdev->dev, "Register value: 0x%x\n", val);

/* With error return */
return dev_err_probe(&pdev->dev, ret, "Clock failed\n");

Output includes device path:

my_driver 0000:00:1f.6: Failed to init: -22

Dynamic Debug

Enable/disable debug messages at runtime without recompiling:

# Enable all debug in a module
echo 'module my_driver +p' > /sys/kernel/debug/dynamic_debug/control

# Enable specific file
echo 'file my_driver.c +p' > /sys/kernel/debug/dynamic_debug/control

# Enable specific function
echo 'func my_probe +p' > /sys/kernel/debug/dynamic_debug/control

# Add function name and line number
echo 'module my_driver +fpl' > /sys/kernel/debug/dynamic_debug/control

# View current settings
cat /sys/kernel/debug/dynamic_debug/control | grep my_driver

# Disable
echo 'module my_driver -p' > /sys/kernel/debug/dynamic_debug/control

Dynamic debug only works with dev_dbg() and pr_debug(). Build with CONFIG_DYNAMIC_DEBUG=y.

Format Specifiers

Kernel has special format specifiers:

/* Pointers */
pr_info("Device: %pOF\n", np);       /* Device tree path */
pr_info("Address: %pR\n", &res);     /* Resource (BAR) */
pr_info("MAC: %pM\n", mac_addr);     /* MAC address */
pr_info("IPv4: %pI4\n", &ip_addr);   /* IP address */

/* Error pointers */
pr_info("Error: %pe\n", ERR_PTR(-EINVAL));  /* "-EINVAL" */

Viewing Logs

# Continuous watching
dmesg -w

# With human-readable timestamps
dmesg -T

# Filter by level
dmesg --level=err,warn

# journalctl (systemd)
journalctl -k -f

Summary

Function	When to Use
pr_err	Error conditions
dev_err	Driver errors (preferred)
dev_dbg	Debug info (use with dynamic debug)
dev_err_probe	Probe failures with deferred handling

Further Reading

• printk Documentation
• Dynamic Debug