linux kernel driver debug

1. print

printk(): never

pr_debug(): always good

dev_dbg(): prefered when you have a struct device object

netdev_dbg(): prefered when you have a struct netdevice object

[something]_dbg(): prefered when you have a that something object

see the implementation in source code:

/* If you are writing a driver, please use dev_dbg instead */
#if defined(CONFIG_DYNAMIC_DEBUG)
/* 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

It shows the CONFIG_DYNAMIC_DEBUG/DEBUG option can be used to enable or disable the print.

dynamic debug:

Benefits:
Almost no overhead when log code is not enabled.
Turn on/off debug log at runtime.
No need to recompile the kernel.

Control interface: uboot boot argus, debugfs

# mount -t debugfs none /sys/kernel/debug/
# cd /sys/kernel/debug/dynamic_debug/
# echo “file xxx.c +p” > control
# echo “file svcsock.c line 1603 +p” > control
# echo “file drivers/usb/core/* +p” > control
# echo “file xxx.c -p” > control

retrive the log information

dmesg

dmesg -C clear

dmesg -c show and then clear.

tail /var/log/syslog-f dynamic to show the log information in case the syslogd is running.

The debug log default size is 64K, you can configure it by menuconfig, note that the size is calculted by 2^n.

dmesg -n 5
Set console logging filter to KERN_WARNING or more severe.
dmesg -l warn
Only print the logs of KERN_WARNING in the kernel ring buffer.

Another option is dynamic debug by configuring the debugfs

enable it in kernel : CONFIG_DYNAMIC_DEBUG=y menuconfig

using pr_debug(), dev_dbg() ... in driver source code

echo “<matches> <ops><flags>” > <debugfs>/dynamic_debug/control

e.g.

echo “file xxx.c+p” > <debugfs>/dynamic_debug/control

 

2. using file system proc/ memory large file or sequence file

3. using ioctrl to get/set specific data

Below shall enable the kernel debug. CONFIG_DEBUG_INFO =y

4. gdb list command to idenfiy the line of oops

e.g.

(gdb ) list *(cppi41_dma_probe+0x2c4)

 

5. addr2line command to identify the line of oops

e.g.  addr2line -fe vmlinux ce000678

6. objdump -dS .o

-d: display assembler conetents of executable sections

-S intermix source code with disassembly