[kdb] [PATCH 0/37] kgdb, kdb and atomic kernel modesetting series

[Jason Wessel]

Back in May 2009 an initial kdb prototype was posted to lkml as an RFC
to see if there was sufficient interest to merge kdb and kgdb together.

There was enough interest to continue on with the project, and it is
now in a state where further work can occur to review, and hopefully
merge the code into a future kernel.

If this is the first time you have heard about kgdb or kdb, you might
consider taking a look at the presentation slides from LPC 2009.

http://kgdb.wiki.kernel.org/index.php/Main_Page#Linux_Plumbers_Conference_presentation_2009

The code included in this series does not include any of the USB code
referenced in the presentation.

Several changes have occurred since the original prototype was posted.
All of the architectures using the kgdb core today have had the
functions implemented to make use of the kdb shell.  This list
includes x86, arm, ppc, mips, sh, sparc, and blackfin.  The proposed
atomic kernel modesetting project now has an initial implementation
for the intel i915 driver, and it should be possible to add atomic
mode setting hooks for any other video driver which makes use of
builtin kernel mode setting.  All the dead code and functions that
were not previously implemented in the RFC have been removed or
implemented.

The directory of the kernel debugger changed as well.  All the kernel
debugger pieces, the debug core, gdbstub and kdb now live under
kernel/debug.

While the patch set may be slightly large, it is broken down into
logical, incremental functionality.  Only the first 12 patches are
needed to see kdb in action on a serial port which has a kgdboc driver
for example.

It is important to understand that this work was directly derived from
the original kdb, and the intent is to deprecate the out of tree kdb
and move all its functionality to this code base.  This version of kdb
is completely wired into the debug core and the kgdboc polled I/O
model using the same API used by the gdbstub (which people call kgdb).

The kdb front end in this patch series works a little differently than
if you were to take the original kdb patch set from:

ftp://oss.sgi.com/projects/kdb/download/v4.4/

In the kernel .config you should enable the following options:

CONFIG_KGDB=y
CONFIG_KGDB_SERIAL_CONSOLE=y
CONFIG_KGDB_LOW_LEVEL_TRAP=y
CONFIG_KGDB_KDB=y
CONFIG_KDB_KEYBOARD=y

To use kdb with a serial port you would use the kgdb/kgdboc way of
doing things.  You would used a kernel command line with:

   console=ttyS0,115200 kgdboc=ttyS0 kgdbwait

That will get you access to kdb just after the console has been
registered.  If you want to use the keyboard, you could use the
following (NOTE it is kbd and not kdb, kbd is short for keyboard):

   console=tty0 kgdboc=kbd kgdbwait

You can also use the keyboard and or serial console:

   console=ttyS0,115200 console=tty0 kgdboc=kbd,ttyS0


In terms of breaking into the debugger after the system is up, you
must use the sysrq-g sequence.  That means you could run:
   echo g > /proc/sysrq-trigger

Or you can use the SysRq key on your key board.  On a typical laptop
you might have to do the following:

press and hold ALT    -- You will be holding this the whole time
press and hold FN
press and release the key with the SysRq label
release FN
press and release g
release ALT

Once you are in kdb you can run help to see a limited list of
commands.

You can also still connect gdb or re-enter kdb without leaving the
exception state, if you are using a serial port.  To get out of kgdb
mode you can type "$3#33", or to get into kgdb mode from kdb, you can
type "kgdb".