Compiles debug level messages into the kernel, which would not
otherwise be available at runtime. These messages can then be
enabled/disabled on a per module basis. This mechanism implicitly
enables all pr_debug() and dev_dbg() calls. The impact of this
compile option is a larger kernel text size of about 2%.

Usage:

Dynamic debugging is controlled by the debugfs file,
dynamic_printk/modules. This file contains a list of the modules that
can be enabled. The format of the file is the module name, followed
by a set of flags that can be enabled. The first flag is always the
'enabled' flag. For example:

<module_name> <enabled=0/1>
.
.
.

<module_name> : Name of the module in which the debug call resides
<enabled=0/1> : whether the messages are enabled or not

From a live system:

snd_hda_intel enabled=0
fixup enabled=0
driver enabled=0

Enable a module:

$echo "set enabled=1 <module_name>" > dynamic_printk/modules

Disable a module:

$echo "set enabled=0 <module_name>" > dynamic_printk/modules

Enable all modules:

$echo "set enabled=1 all" > dynamic_printk/modules

Disable all modules:

$echo "set enabled=0 all" > dynamic_printk/modules

Finally, passing "dynamic_printk" at the command line enables
debugging for all modules. This mode can be turned off via the above
disable command.

source code:

lib/dynamic_printk.c