Diff for "FAQ/LinuxKernelDebug101" - Linux Kernel Newbies

Differences between revisions 1 and 2

Whats the easiest way to debug ?

The easiest way is the one you know. For most of us thats printf(), or its kernel version:

#include <device.h>
...
     printk(KERN_INFO DRVNAME "%s %s\n", info1, info2);
     printk(KERN_ERR DRVNAME "something bad: %s\n", errstr);
     printk("logged at (what?) default\n");

In lines 1,2, note the lack of comma after KERN_INFO and DRVNAME. These are string values that are catenated, per C language spec, to the front of the format string.

KERN_INFO and friends are defined here: http://lxr.linux.no/lxr/http/source/include/linux/kernel.h#L44

The logging system reads the numbers, and steers the message accorgingly.

Anything with less typing ?

For drivers with a struct device, device.h provides an easier way:

#include <device.h>
...
     dev_info("%s %s\n", info1, info2);
     dev_err("something bad: %s\n", errstr);

Messages issued by these statements include driver name, etc.

dev_dbg()

This symbol has additional feature - the statement typically disappears at compile-time, allowing developer to keep debug code w/o runtime penalties.

#define DEBUG 1
#include <device.h>
...
     dev_dbg("%s %s\n", info1, info2);

I'm still not seeing the debug messages !

If you're looking at the console for messages, may be you need to raise its verbosity:

     echo 8 > /proc/sys/kernel/printk

this will cause dev_dbg() messages, which are logged at level "<7>", to be reported to the console.

-  ⇤ ← Revision 1 as of 2006-08-04 21:39:06 → 
  Size: 554
  Editor: 71-33-135-229
  Comment: needs prettyfication
+   ← Revision 2 as of 2006-08-06 17:10:14 → ⇥
  Size: 1556
  Editor: 71-33-135-229
  Comment:
-Deletions are marked like this.
+Additions are marked like this.
 Line 1:
-Describe FAQ/LinuxKernelDebug101 here.
+== Whats the easiest way to debug ? ==
 Line 3:
- easiest way is, as always, printk("format", ...)
+The easiest way is the one you know.  For most of us thats printf(),
or its kernel version: 
{{{
#include <device.h>
...
     printk(KERN_INFO DRVNAME "%s %s\n", info1, info2);
     printk(KERN_ERR DRVNAME "something bad: %s\n", errstr);
     printk("logged at (what?) default\n");
}}}
-Line 5:
+Line 13:
- almost as easy is dev_info() and friends: 
   dev_info, dev_warn, dev_err, dev_dbg
   they do logging at the KERN_* levels (INFO, WARN, ERROR, DEBUG) respectively
   convenient shorthand, more readable.
+In lines 1,2, note the lack of comma after KERN_INFO and DRVNAME.  
These are string values that are catenated, per C language spec, 
to the front of the format string.
-Line 10:
+Line 17:
- dev_dbg has additional features.
   compiles out, unless #define DEBUG 1 is done before #include <device.h>
     so to activate, recompile, reload
   to see them on console, lower console logging level
+KERN_INFO and friends are defined here:
http://lxr.linux.no/lxr/http/source/include/linux/kernel.h#L44

The logging system reads the numbers, and steers the message accorgingly.


== Anything with less typing ? ==

For drivers with a struct device, device.h provides an easier way:

{{{
#include <device.h>
...
     dev_info("%s %s\n", info1, info2);
     dev_err("something bad: %s\n", errstr);

}}}

Messages issued by these statements include driver name, etc.

== dev_dbg() ==

This symbol has additional feature - the statement typically disappears at compile-time,
allowing developer to keep debug code w/o runtime penalties.

{{{
#define DEBUG 1
#include <device.h>
...
     dev_dbg("%s %s\n", info1, info2);
}}}

== I'm still not seeing the debug messages ! ==

If you're looking at the console for messages, may be you need to raise its verbosity:
{{{
-Line 15:
+Line 54:
+}}} 
this will cause dev_dbg() messages, which are logged at level "<7>", to be reported to the console.