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The kernel contains a GDB stub that talks GDB remote protocol across a serial
port. This permits GDB to single step through the kernel, set breakpoints and
trap exceptions that happen in kernel space and interrupt execution. It also
permits the NMI interrupt button or serial port events to jump the kernel into
the debugger.
On the CPUs that have on-chip UARTs (FR400, FR403, FR405, FR555), the
GDB stub hijacks a serial port for its own purposes, and makes it
generate level 15 interrupts (NMI). The kernel proper cannot see the serial
port in question under these conditions.
On the MB93091-VDK CPU boards, the GDB stub uses UART1, which would otherwise
be /dev/ttyS1. On the MB93093-PDK, the GDB stub uses UART0. Therefore, on the
PDK there is no externally accessible serial port and the serial port to
which the touch screen is attached becomes /dev/ttyS0.
Note that the GDB stub runs entirely within CPU debug mode, and so should not
incur any exceptions or interrupts whilst it is active. In particular, note
that the clock will lose time since it is implemented in software.
Firstly, a debuggable kernel must be built. To do this, unpack the kernel tree
and copy the configuration that you wish to use to .config. Then reconfigure
the following things on the "Kernel Hacking" tab:
(*) "Include debugging information"
Set this to "Y". This causes all C and Assembly files to be compiled
to include debugging information.
(*) "In-kernel GDB stub"
Set this to "Y". This causes the GDB stub to be compiled into the
(*) "Immediate activation"
Set this to "Y" if you want the GDB stub to activate as soon as possible
and wait for GDB to connect. This allows you to start tracing right from
the beginning of start_kernel() in init/main.c.
(*) "Console through GDB stub"
Set this to "Y" if you wish to be able to use "console=gdb0" on the
command line. That tells the kernel to pass system console messages to
GDB (which then prints them on its standard output). This is useful when
debugging the serial drivers that'd otherwise be used to pass console
messages to the outside world.
Then build as usual, download to the board and execute. Note that if
"Immediate activation" was selected, then the kernel will wait for GDB to
attach. If not, then the kernel will boot immediately and GDB will have to
interrupt it or wait for an exception to occur before doing anything with
the kernel.
Set the serial port on the computer that's going to run GDB to the appropriate
baud rate. Assuming the board's debug port is connected to ttyS0/COM1 on the
computer doing the debugging:
stty -F /dev/ttyS0 115200
Then start GDB in the base of the kernel tree:
frv-uclinux-gdb linux [uClinux]
frv-uclinux-gdb vmlinux [MMU linux]
When the prompt appears:
GNU gdb frv-031024
Copyright 2003 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "--host=i686-pc-linux-gnu --target=frv-uclinux"...
Attach to the board like this:
(gdb) target remote /dev/ttyS0
Remote debugging using /dev/ttyS0
start_kernel () at init/main.c:395
This should show the appropriate lines from the source too. The kernel can
then be debugged almost as if it's any other program.
The kernel can be interrupted whilst it is running, causing a jump back to the
GDB stub and the debugger:
(*) Pressing Ctrl-C in GDB. This will cause GDB to try and interrupt the
kernel by sending an RS232 BREAK over the serial line to the GDB
stub. This will (mostly) immediately interrupt the kernel and return it
to the debugger.
(*) Pressing the NMI button on the board will also cause a jump into the
(*) Setting a software breakpoint. This sets a break instruction at the
desired location which the GDB stub then traps the exception for.
(*) Setting a hardware breakpoint. The GDB stub is capable of using the IBAR
and DBAR registers to assist debugging.
Furthermore, the GDB stub will intercept a number of exceptions automatically
if they are caused by kernel execution. It will also intercept BUG() macro