Debugging the Linux Kernel With JTAG

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device, I suggest you ask whether it supports Linu

units (MMU), Linux binary formats, and loadable

debugger) protocol as well, you can be sure that it

Cast of characters

I will use my setup of a FemtoLinux project throug

Linux kernel on ARM, so it's beneficial to describe

reduce the Linux system's call latency and overhe

5.5 monolithic applications to Linux without redesi

Linux kernel hacking would be nearly impossible

For development, we use a LPC3250 board by E

based on NXP CPU with ARM926 core. The hard

particular interest in the scope of this article, but w

thought out and specifically designed for develope

combined with USB-to-serial bridge clearly shows

Naturally, the board has a 20-pin JTAG interface f

JTAG adapter, we use the Flyswatter by Tin Can Tsurprised me very positively--before I found it, I ha

functional JTAG debugger for less than $100.

I guess that one of the reasons Tin Can Tool were

they didn't invest in the debugging software at all,

packages OpenOCD and GDB, which leads us to

tools for JTAG debugging.

Software tools for JTAG

In the scope of this article, OpenOCD--the on-chip

software middleware running between the debugg

does more than that, but as far as JTAG debuggin

commands to USB commands, which it sends to t

OpenOCD implements a remote gdbserver protoc

of gdbserver, which just happens to debug the Lin

running under Linux.

OpenOCD supports ARM7, ARM9, XScale, and C

using MIPS or PowerPC, you'll have to look for a

debugger, such as Abatron BDI3000. OpenOCD wadapters, in particular FT2232-based ones.

FT2232 is a very popular chip manufactured by F

used in many low-end JTAG adapters, such as Tin

support, OpenOCD has an extensive list of comm

interface that include target state command, memo

commands, JTAG low-level commands, and much

documentation that explains in detail every comm

encourage you to read it.

GDB is in the house

The last part of the setup puzzle is GDB, or the G

debugger for UNIX systems, including Linux, and i

facilities for program tracing, altering, and debuggi

ugging the Linux kernel with JTAG http://www.eetimes.com/design/embedded/4207333/Debugging-the-Lin...

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this article, so I'll cover only what is relevant for JT

Most commonly GDB is used as a single applicati

this mode, GDB client, in other words the front en

PC) and GDB server, the back end, on another (u

is used to control the application, and GDB client i

using GDB's remote TCP-based protocol.

This configuration is what you would normally use

In the JTAG debugging case, the OpenOCD daem

gdbserver, while the GDB client will remain the sa

One important drawback of GDB is a lack of graph

Display Debugger (DDD), a GUI wrapper for GDB.

it has a very slow response time (by modern stand

Fortunately, there is a better alternative--Insight. U

of GDB with integrated GUI. (See http://sourcewar

upside is that the GUI is much nicer and easier tobuilt in, you'll probably have to compile it for your

you to use an external GDB version from your cro

Configuring OpenOCD

Assuming you've installed all the software I've me

you need to take before starting debugging is to c

may already include configuration files for your JT

work." Actually, this was almost the case with Flys

CPUTAPID parameter), but your mileage may vary

Refer to OpenOCD documentation for a full explan

basic commands using in my configuration as an

my configuration file looks like that in Listing 1.

Click on image

The first part is the interface configuration, which

The "interface" command identifies the driver that

ft2232_vid_pid commands specify vendor Id and pro

The rest are the target board configuration comma

JTAG TAP in the chain. You will need at least one

the "target create" commands.

Here is what an actual JTAG debugging session lboard is connected to Flyswatter via 20-pin JTAG

to a Linux PC. Linux is running OpenOCD daemo


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Click on image

Of course, if you prefer a command line interface,

to all standard GDB commands, you can send JT

monitor command. For instance, you can check all

shown in Listing 2, or you can print information a

command shown in Listing 3. (For a full list of su

documentation.)

Click on image

Click on image

The instructions that I've described so far assumelinked kernel image. This is true most of the time--

initialization debugging, and this code is of course

Sometimes, however, you may wish to debug a dri

problem here is that after you load the module usi

GDB/Insight is not aware of this module's symbols

the symbols names are and to where they've been

You can find the base address of each module by

can see that on my system, the nfs module has b

cat /proc/modules


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nfs 271912 1 - Live 0xf8716000

...

The offset is printed in the last column. When you

module's symbol table using the following comma

add-symbol-file nsf.ko 0xf8716000

When you are finished, you can continue debuggi

A good start

This article is by no means a definitive guide to J

enough information to get started. As usually the c

everything I've described here.

For instance, if you're unwilling to spend the time

not an issue, Wind River Workbench On-Chip De

all processors currently used in embedded system

environment. Needless to say, it supports both Vx

Another slightly less expensive option is Abatron

processors and can be used with any client that s

way as I described in this article. The major differe

interface and runs a GDB server emulation softwa

device itself, which is very convenient.

If you prefer an Eclipse-based integrated environm

purchase an add-on software product LinuxScope-

debugger for the Eclipse IDE that has been optimi

Sasha Sirotkincurrently works on the FemtoLinu

for embedded ARM, MIPS, and PowerPC system

site atwww.femtolinux.com. Sasha can be reach

Resources:1. FemtoLinux, www.femtolinux.com

2. EmbeddedArtists, www.embeddedartists.com

3. Tin Can Tools, www.tincantools.com

4. OpenOCD, http://openocd.berlios.de/web/

5. Insight, http://sourceware.org/insight/

6. LinuxScope-JTD, www.ultsol.com/mfgs_comp_li

7. Wind River OCD, www.windriver.com/products/

8. Abatron BDI3000, www.abatron.ch/products/bdi


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Debugging the Linux Kernel With JTAG

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