Phant0mas’ Place

Coding, building, debugging

Debug a Running Linux Kernel on the Zedboard With GDB

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Disclaimers

I am sharing what works for me. It may not work for you or it may fail over time. You may suffer data loss or worse. I disclaim all warranties and representations.

Prerequisities

  • Xilinx Microprocessor Debugger (XMD)
  • GDB capable of understanding the target machine

Debugging the Kernel

When developing a device driver, having access to the linux internals using GDB may be the key to meeting that deadline. On the zedboard, debugging the kernel with GDB is actually very easy with the help of the Xilinx Microprocessor Debugger, or XMD for short.

XMD is included with the Vivado Design Suite from Xilinx. In case you are working with the older ISE Design Suite, the included XMD should work as well.

Start debugging

  • Connect the JTAG to the zedboard (or a micro usb to the PROG USB port) and boot it.
  • Open a shell and run xmd.
shell 
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user@workstation ~ $: xmd
Xilinx Microprocessor Debugger (XMD) Engine
Xilinx EDK 14.7 Build EDK_P.20131013
Copyright (c) 1995-2012 Xilinx, Inc.  All rights reserved.

XMD%
XMD%
  • Run connect arm hw. This will start a GDB server at localhost:1234.
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XMD% connect arm hw

JTAG chain configuration
--------------------------------------------------
Device   ID Code        IR Length    Part Name
 1       4ba00477           4        Cortex-A9
 2       23727093           6        XC7Z020

--------------------------------------------------
Enabling extended memory access checks for Zynq.
Writes to reserved memory are not permitted and reads return 0.
To disable this feature, run "debugconfig -memory_access_check disable".

--------------------------------------------------

CortexA9 Processor Configuration
-------------------------------------
Version.............................0x00000003
User ID.............................0x00000000
No of PC Breakpoints................6
No of Addr/Data Watchpoints.........4

Connected to "arm" target. id = 64
Starting GDB server for "arm" target (id = 64) at TCP port no 1234
XMD%

  • Open a new shell and go to the directory where the kernel sources are located, the ones you used to build the kernel, and run arm-xilinx-eabi-gdb vmlinux.

    In my case I am using arm-xilinx-eabi-gdb but any gdb cross-build for arm will do. We are passing vmlinux to the debugger, so it can read the kernel symbols.

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user@workstation ~/git_repos/linux-analog $: arm-xilinx-eabi-gdb vmlinux
GNU gdb (Sourcery CodeBench Lite 2013.11-46) 7.6.50.20130726-cvs
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
...
Reading symbols from /home/manolis/git_repos/linux-analog/vmlinux...done.
(gdb)
  • Then, under the GDB command prompt, enter: target remote localhost:1234
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(gdb) target remote localhost:1234
Remote debugging using localhost:1234
cpu_v7_do_idle () at arch/arm/mm/proc-v7.S:74
74        ret lr
(gdb)
  • And now GDB is ready for debugging those kernel hangs. Enter continue in the gdb prompt so linux can continue it’s normal operation. You can come back anytime and stop it in order to debug it.

Extras

  • Make sure you have enabled the kernel debug symbols in the config file of the kernel. Enabling CONFIG_DEBUG_INFO will do the trick.

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