NOTE: This is an early access release of kgdb and thus there may be unknown
issues that still need to be fixed. Please send all bug reports
to dsaxena@mvista.com.
Theory of Operation
Adding KGDB Support to your Board Port
------------------------------------------------------------
Adding KGDB support to your ARM port is fairly simple. You simply
need to create the three functions:
- kgdb_serial_init(void)
This function is called to initialize the serial port. In
the future, this will include a parameter with the cmdline
options sent to kgdb.
- kgdb_serial_putchar(unsigned char)
Output a single character over the serial port
- unsigned char kgdb_serial_getchar(void)
Receive a single character over the serial port
Once those three functions have been created, the rest of KGDB is
board independent and should work without any problems.
Console Messages over KGDB
------------------------------------------------------------
Note that if you have only one serial port on your board, you cannot
use that serial port for both serial console and KGDB. In this case,
you can add a 'console=ttyKGDB' option to your kernel command line
and KGDB will output console messages to the GDB client.
Using KGDB
------------------------------------------------------------
To use kgdb you need to first turn on both the CONFIG_KGDB and
CONFIG_KGDB_SERIAL config options. In addition to this, you can
turn on CONFIG_KGDB_CONSOLE to enable console output to be
viewed by the GDB client program.
Once KGDB is enabled, the kernel will automatically breakpoint()
at arch/arm/kernel/trap.s:trap_init(). Note that this is the
earliest point at which KGDB can be intialized due to the fact
that it relies on the undefined instruction handler. Once KGDB
is entered, you can connect to it from the client via
'target remote /dev/ttySx', where ttySx is the serial port on
the host machine that is connected to your board.
Note that you must let the kernel breakpoint() before trying to
connect the debugger, otherwise the two will get out of sync and
you will not be able to start the debug session.
If you don't want the kernel to stop immediately at boot up, you can
add the 'nohalt' option to the command line. If you have CONFIG_MAGICSYSRQ
turned on, you can send a SysReq-G to the system console at anytime to
force a breakpoint. In minicom, this is accomplished via the following
key combination: Ctrl-A f g.
Once you've established a connection between the GDB client and the KGDB
stub, you can debug as you normally would. Consult the GDB documentation
if you need help with debugging commands.
Finding Bugs
------------------------------------------------------------
If you think you've found a bug in the communication layer between
the client and KGDB, add the following commands to the beggining of
your gdb session:
set debug remote 1
set remotelogfile logfile
This will output all packet data to the screen save it to a logfile.
You can send the packet logfile along with a description of what you
were doing at the time to me.
from http://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=1335%2F1