reference console_loglevel ERROR in kernel module - linux

Question

I have a Linux module, which has a debug function and I just want to call that function in debug mode.
Now I have code like this:
if (console_loglevel >= CONSOLE_LOGLEVEL_DEBUG)
dump_my_message(dev, my_msg);
But when build this code in linux-next, it will throw below error:
CHK include/generated/uapi/linux/version.h
Kernel: arch/x86/boot/bzImage is ready (#2)
Building modules, stage 2.
MODPOST 2738 modules
ERROR: "console_printk" [drivers/mymodule.ko] undefined!
scripts/Makefile.modpost:91: recipe for target '__modpost' failed
make[1]: *** [__modpost] Error 1
Makefile:1117: recipe for target 'modules' failed
make: *** [modules] Error 2
Can you help to find out how to make this work? thanks!!!

Answer 1

The compilation error is caused by the fact that console_printk symbol is not exported so it cannot be used by modules.
What you really should use, however, is Dynamic debug functionality and its pr_debug()/dev_dbg() functions.
Basically what you need is to ensure that CONFIG_DYNAMIC_DEBUG is enabled in your kernel, use dev_dbg() in all places where you want to write some debugging code and then dynamically enable your debug messages, for example:
To enable all the messages in your module, add dyndbg=+p at the end of your insmod/modprobe call.
To selectively enable only some of the messages, use query language described in the documentation. For example, to only enable messages from functions foo() and bar() in your module, use:
insmod mymodule.ko dyndbg="func foo +p; func bar +p"