Kernel logging

Kernel log messages are the primary source of information for diagnosing boot failures, driver problems, and runtime errors. This page covers how to get those messages out of the device: via serial console, early boot console, runtime log commands, and log-level control.

Serial console

The Qualcomm^® Linux kernel uses the Qualcomm Generic Interface (GENI) serial driver to support the UART console on Qualcomm SoCs.

Required kernel configuration

Verify that the following Kconfig options are enabled:

zcat /proc/config.gz | grep -E "CONFIG_SERIAL_QCOM_GENI"
# Expected:
# CONFIG_SERIAL_QCOM_GENI=y
# CONFIG_SERIAL_QCOM_GENI_CONSOLE=y

If either is missing, enable them in the kernel defconfig or a .cfg fragment and rebuild.

Add the serial console to the kernel command line

Add the console= parameter to KERNEL_CMDLINE_EXTRA in meta-qcom/conf/machine/include/qcom-<SoC>.conf:

KERNEL_CMDLINE_EXTRA ?= "root=/dev/disk/by-partlabel/system rw rootwait \
                          console=ttyMSM0,115200n8 pcie_pme=nomsi earlycon"

Parameter	Meaning
`console=ttyMSM0,115200n8`	Route kernel log output to the first Qualcomm GENI UART at 115200 baud, no parity, 8 data bits
`earlycon`	Enable the early console before driver init (see below)

Connect hardware

Connect a USB-to-UART cable between the device and the host:

Device UART header → USB-UART adapter → Host USB port

On the host, open the serial port at 115200 baud using any terminal emulator (minicom, screen, PuTTY, or Tera Term):

# Linux example
minicom -D /dev/ttyUSB0 -b 115200

For upstream documentation, see Linux Serial Console.

Early boot messages (earlycon)

The earlycon kernel parameter enables a minimal early console before the full UART driver is initialised. It provides kernel messages from the very start of the boot sequence, including the unpack of the initramfs and early device tree processing, that would otherwise be invisible. Add to the kernel command line:

earlycon

On Qualcomm SoCs the early console uses the same ttyMSM0 UART. Messages appear immediately from the first line of start_kernel().

earlycon and console=ttyMSM0,... are independent. Use both together to get uninterrupted output from early boot through the login prompt.

printk log levels

printk is the kernel’s logging function. Every message carries a log level that controls whether it appears on the console. Table: printk log levels

Level	Macro	Value	Meaning
Emergency	`pr_emerg` / `KERN_EMERG`	0	System is unusable
Alert	`pr_alert` / `KERN_ALERT`	1	Action must be taken immediately
Critical	`pr_crit` / `KERN_CRIT`	2	Critical conditions
Error	`pr_err` / `KERN_ERR`	3	Error conditions
Warning	`pr_warn` / `KERN_WARNING`	4	Warning conditions
Notice	`pr_notice` / `KERN_NOTICE`	5	Normal but significant condition
Info	`pr_info` / `KERN_INFO`	6	Informational
Debug	`pr_debug` / `KERN_DEBUG`	7	Debug-level messages

Messages with a level numerically lower than the current console log level are printed to the console. The default console log level on Qualcomm Linux is 7 (all messages printed). With the default console log level of 7, messages at levels 0–6 (Emergency through Info) reach the console. Set the level to 8 to include debug output.

Runtime log level control

Read and set the console log level at runtime via /proc/sys/kernel/printk. The file contains four space-separated values: <current> <default> <minimum> <boot-time-default>.

# Read current log level
cat /proc/sys/kernel/printk
# Example output: 7  4  1  7

# Set console log level to 4 (warnings and above only)
echo "4" > /proc/sys/kernel/printk

# Suppress all but emergencies (useful for release/performance tests)
echo "1" > /proc/sys/kernel/printk

Boot-time log level

Set the log level on the kernel command line to control verbosity before /proc is mounted:

loglevel=4      # warnings and above
quiet           # equivalent to loglevel=4 with reduced output
loglevel=0      # silence all output (only KERN_EMERG passes)

Read the kernel log buffer

dmesg

The most common way to read the kernel ring buffer:

dmesg                         # full buffer
dmesg | head -50              # first 50 lines (early boot)
dmesg | tail -50              # most recent 50 lines
dmesg --level=err,warn        # errors and warnings only
dmesg -T                      # human-readable timestamps
dmesg -w                      # follow (like tail -f)

/proc/kmsg

virtual proc file that provides a live stream of kernel messages. Each read returns new messages only:

cat /proc/kmsg   # streams kernel log (Ctrl+C to stop)

Persistent logs across reboots

On systems running systemd-journald, kernel messages are stored persistently and can be queried across boots:

journalctl -k           # kernel messages from the current boot
journalctl -k -b -1     # kernel messages from the previous boot

On systems without journald, kernel messages may be written to syslog files:

cat /var/log/kern.log    # Debian/Ubuntu style
cat /var/log/messages    # RHEL/embedded style

Disable console logging

To silence the console for production or performance testing:

# Kernel command-line options (choose one):
quiet                    # suppress informational messages
console=null             # discard all console output
loglevel=0               # only KERN_EMERG messages pass

Alternatively, disable the early console drivers in Kconfig for a fully silent build:

# CONFIG_SERIAL_EARLYCON is not set
# CONFIG_SERIAL_MSM_CONSOLE is not set

Debug builds

To include debug symbols and extra debug Kconfig in the Yocto image, pass DEBUG_BUILD=1 to the bitbake invocation. This enables debug.config fragments in the kernel build:

# Inside the kas shell
DEBUG_BUILD=1 bitbake qcom-multimedia-image

Or add it permanently to build/conf/local.conf:

DEBUG_BUILD = "1"

Introduction

Architecture

Key Documents

Technologies

System

Release Notes

Serial console

Required kernel configuration

Add the serial console to the kernel command line

Connect hardware

Early boot messages (earlycon)

printk log levels

Runtime log level control

Boot-time log level

Read the kernel log buffer

dmesg

/proc/kmsg

Persistent logs across reboots

Disable console logging

Debug builds

​Serial console

​Required kernel configuration

​Add the serial console to the kernel command line

​Connect hardware

​Early boot messages (earlycon)

​printk log levels

​Runtime log level control

​Boot-time log level

​Read the kernel log buffer

​dmesg

​/proc/kmsg

​Persistent logs across reboots

​Disable console logging

​Debug builds

Serial console

Required kernel configuration

Add the serial console to the kernel command line

Connect hardware

Early boot messages (earlycon)

printk log levels

Runtime log level control

Boot-time log level

Read the kernel log buffer

dmesg

/proc/kmsg

Persistent logs across reboots

Disable console logging

Debug builds