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