linux kgdb arm,KGDB for ARM-Linux Usage Notes

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 .

Theory of Operation

Adding KGDB Support to your Board Port

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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

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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

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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

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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.