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+                             ====================
+                             DEBUGGING FR-V LINUX
+                             ====================
+
+
+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.
+
+
+==================
+KERNEL PREPARATION
+==================
+
+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
+      kernel.
+
+  (*) "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.
+
+
+=========================
+KERNEL DEBUGGING WITH GDB
+=========================
+
+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]
+
+Or:
+
+        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"...
+        (gdb)
+
+Attach to the board like this:
+
+        (gdb) target remote /dev/ttyS0
+        Remote debugging using /dev/ttyS0
+        start_kernel () at init/main.c:395
+        (gdb)
+
+This should show the appropriate lines from the source too. The kernel can
+then be debugged almost as if it's any other program.
+
+
+===============================
+INTERRUPTING THE RUNNING KERNEL
+===============================
+
+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
+      debugger.
+
+  (*) 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
+invocation.
+