Christopher Hallinan - Embedded Linux Primer - A Practical, Real-World Approach

setenv bootargs console=ttyS0,115200 root=/dev/nfs rw \

ip=dhcp nfsroot=192.168.1.9:/home/chris/sandbox/pdna-target

Then we load a kernel via our TFTP server. Listing 12-8 shows what this might look like on a PowerPC embedded target.

Listing 12-8. Loading Kernel via TFTP Server

=> tftpboot 200000 uImage-pdna <<< Entered at U-Boot prompt

Using FEC ETHERNET device

TFTP from server 192.168.1.9; our IP address is 192.168.1.68

Filename 'uImage-pdna'.

Load address: 0x200000

Loading: ##################################################

##################################################

#########################################

done

Bytes transferred = 911984 (dea70 hex)

=>

When we boot the kernel, we see specific evidence of our NFS root mount configuration. Listing 12-9 reproduces selected output from the kernel boot messages to demonstrate this. This output has been formatted (many lines omitted and whitespace added) for readability.

Listing 12-9. Booting with NFS Root Mount

Uncompressing Kernel Image ... OK

Linux version 2.6.14 (chris@pluto) (gcc version 3.3.3 (DENX ELDK 3.1.1 3.3.3-10)) #1 Mon Jan 2 11:58:48 EST 2006

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Kernel command line: console=ttyS0,115200 root=/dev/nfs rw nfsroot=192.168.1.9:/home /chris/sandbox/pdna-target ip=dhcp

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Sending DHCP requests ... OK

IP-Config: Got DHCP answer from 192.168.1.9, my address is 192.168.1.68

IP-Config: Complete:

device=eth0, addr=192.168.1.68, mask=255.255.255.0,

gw=255.255.255.255, host=192.168.1.68, domain=,

nis-domain=(none), bootserver=192.168.1.9,

rootserver=192.168.1.9,

rootpath=/home/chris/sandbox/pdna-target

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.

Looking up port of RPC 100003/2 on 192.168.1.9

Looking up port of RPC 100005/1 on 192.168.1.9

VFS: Mounted root (nfs filesystem).

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BusyBox v0.60.5 (2005.06.07-07:03+0000) Built-in shell (msh)

Enter 'help' for a list of built-in commands.

#

From Listing 12-9, first we see the kernel banner followed by the kernel command line. We specified four items in this kernel command line:

• Console device (/dev/console)

• Root device (/dev/nfs)

• NFS root path (/home/chris/sandbox/pdna-target)

• IP kernel-level autoconfiguration method (dhcp)

Shortly thereafter, we see the kernel attempting kernel-level autoconfiguration via DHCP. When the server responds and the DHCP exchange completes, the kernel displays the detected configuration in the following lines. You can see from this listing that the DHCP server has assigned the target the IP address 192.168.1.68. Compare the detected settings with those specified in Listing 12-6. That was similar to the DHCP server configuration that resulted in this configuration.

When the kernel has completed the IP autoconfiguration, it is capable of mounting the root file system using the supplied parameters. You can see this from the three lines ending with the VFS (virtual file subsystem) message announcing that it has mounted the root NFS file system. After the NFS root file system has been mounted, initialization completes as described in Chapter 5, "Kernel Initialization."

It is also possible to pass target IP settings to the kernel in a static fashion instead of having the kernel obtain IP settings from a DHCP or BOOTP server. IP settings can be passed via the kernel command line directly. In this case, the kernel command line might look similar to this:

console=console=ttyS0,115200 \ ip=192.168.1.68:192.168.1.9::255.255.255.0:pdna:eth0:off \

root=/dev/nfs rw nfsroot=192.168.1.9:/home/chris/pdna-target

• Many features of a development environment greatly facilitate efficiency for embedded cross-development. Most of these fall under the category of tools and utilities. We cover this aspect in detail in the next chapter, where we cover development tools.

• A properly configured development host is a critical asset for the embedded developer.

• Toolchains employed for cross-development must be properly configured to match your host system's target Linux environment.

• Your development host must have target components installed that your toolchain and binary utilities can reference. These components include target header files, libraries, target binaries, and their associated configuration files. In short, you need to assemble or obtain an embedded Linux distribution.

• Configuring target servers such as TFTP, DHCP, and NFS will greatly increase your productivity as an embedded Linux developer. This chapter introduced configuration examples for each.

GCC online documentation

http://gcc.gnu.org/onlinedocs/

Building and testing gcc/glibc cross toolchains

http://kegel.com/crosstool/

The TFTP Protocol, Version 2

RFC 1350

www.ietf.org/rfc/rfc1350.txt?number=1350

Bootstrap Protocol (BOOTP)

RFC 951

www.ietf.org/rfc/rfc0951.txt?number=951

Dynamic Host Configuration Protocol

RFC 2131

www.ietf.org/rfc/rfc2131.txt?number=2131

A typical embedded Linux distribution includes many useful tools. Some are complex and require a great deal of proficiency to master. Others are simple and have been all but ignored by developers of embedded systems. Some tools might require customization for a particular environment. Many will run "right out of the box" and provide the developer with useful information without much effort. This chapter presents a cross-section of the most important (and frequently neglected) tools available to the embedded Linux engineer.

It is impossible to provide complete details on the tools and utilities presented in this chapter. That would take an entire book by itself! Rather than provide a complete reference, our goal is to provide an introduction on the basic usage of each one. You are encouraged to pursue additional study on these and other important development tools. The man page (or other documentation) for each tool is a great place to start.

The GNU Debugger (GDB) is introduced first, followed by a brief look at the Data Display Debugger, a graphical front end for GDB. Next we introduce a series of utilities designed to give the developer a look at the behavior of programs and the system as a whole. These include strace, ltrace, top, and ps, often overlooked by inexperienced Linux developers. We then present some crash dump and memory-analysis tools. The chapter concludes by introducing some of the more useful binary utilities.

If you spend much time developing Linux applications, you will undoubtedly spend many hours getting to know the GNU Debugger. GDB is arguably the most important tool in the developer's toolbox. It has a long history, and its capabilities have blossomed to include low-level hardware-specific debugging support for a wide variety of architectures and microprocessors. It should be noted that the user manual for GDB is nearly as large as this book. Our intention here is to introduce GDB to get you started. You are encouraged to study the user manual referenced later under Section 13.7.1, "Suggestions for Additional Reading."