To use the dynamic debug functionality in the Linux kernel, follow these steps:

1. Make sure your kernel is compiled with dynamic debugging (CONFIG_DYNAMIC_DEBUG).

2. Mount the debug filesystem if it hasn't already been mounted:

$ mount -t debugfs nodev /sys/kernel/debug  

3. Configure the debug through the dynamic_debug/control folder. It accepts a whitespace-separated sequence of words:

• • func <function name>
  • file <filename>
  • module <module name>
  • format <pattern>
  • line <line or line range>
  • + <flag>: This adds the specified flag
  • - <flag>: This removes the specified flag
  • = <flag>: This sets the specified flag

The flags are defined as follows:

• • f: This flag includes the function name in the message
  • l: This flag includes the line number in the message
  • m: This flag includes the module name in the message
  • p: This flag enables the debug message
  • t: This flag includes the thread ID in non-interrupt context messages

By default, all debug messages are disabled. The control file contains all the available debug points and, by default, they have no flags enabled (marked as =_).

To enable all debug statements in a file, type the following:

$ echo -n 'file <filename> +p' > 
/sys/kernel/debug/dynamic_debug/control      

Or to just enable a specific debug statement, type the following:

$ echo -n 'file <filename> line nnnn +p' > 
/sys/kernel/debug/dynamic_debug/control
    

4. To list all enabled debug statements, use the following command:

$ awk '$3 != "=_"' /sys/kernel/debug/dynamic_debug/control 

To make the debug changes persistent, we can pass dyndbg="<query>" or module.dyndbg="<query>" for modules to the kernel in the command-line arguments.

Note that the query string needs to be passed surrounded by quotes so that it is correctly parsed. You can concatenate more than one query in the command-line argument by using a semicolon to separate them, for example:

dyndbg="file mxc_v4l2_capture.c +pfl; file ipu_bg_overlay_sdc.c +pfl" 

If, for example, we are using the bootargs U-Boot argument to pass kernel command-line arguments, we would do the following:

> env set bootargs 'dyndbg=\"file one.c +pfl; file two.c +pfl; file three.c +pfl; file four.c +pfl\"'