End of search list.
Here you can see that the default search paths for include directories are now adjusted to point to your cross versions instead of the native include directories. This seemingly obscure detail is critical to being able to develop applications and compile open-source packages for your embedded system. It is one of the most confusing topics to even experienced application developers who are new to embedded systems.
12.2. Host System Requirements
Your development workstation must include several important components and systems. Of course, you need a properly configured cross toolchain. You can download and compile one yourself or obtain one of the many commercial toolchains available. Building one yourself is beyond the scope of this book, although there are several good references available. See Section 12.4.1, 'Suggestions for Additional Reading,' at the end of this chapter for recommendations.
The next major item you need is a Linux distribution targeted for your embedded system architecture. This includes hundreds to potentially thousands of files that will populate your embedded system's file systems. Again, the choices are to build your own or to obtain one of the commercial ones. One of the more popular embedded system distributions available on the Internet is the aforementioned ELDK. The ELDK is available for some PowerPC and other embedded targets. Building an embedded Linux distribution from scratch would require a book of this size in itself and, therefore, is beyond the scope of our discussions here.
In summary, your development host requires four separate and distinct capabilities:
• Cross toolchain and libraries
• Target system packages, including programs, utilities, and libraries
• Host tools such as editors, debuggers, and utilities
• Servers for hosting your target board, covered in the next section
If you install a ready-built embedded Linux development environment on your workstation, either a commercial variety or one freely available in the open source community, the toolchain and components have already been preconfigured to work together. For example, the toolchain has been configured with default directory search paths that match the location of the target header files and system libraries on your development workstation. The situation becomes much more complex if your requirements include having support for multiple architectures and processors on your development workstation. This is the reason that embedded Linux distributions exist.
12.2.1. Hardware Debug Probe
In addition to the components listed previously, you should consider some type of hardware-assisted debugging. This consists of a hardware probe connected to your host (often via Ethernet) and connected to your target via a debug connector on the board. Many solutions are on the market today. We cover this topic in detail in Chapter 14, 'Kernel Debugging Techniques.'
12.3. Hosting Target Boards
Referring back to Figure 12-1, you will notice an Ethernet connection from the target-embedded board to the host-development system. This is not strictly necessary, and, indeed, some smaller embedded devices do not have an Ethernet interface. However, this is the exception rather than the rule. Having an Ethernet connection available on your target board is worth its cost in silicon!
While developing the kernel, you will compile and download kernels to your embedded board many times. Many embedded development systems and bootloaders have support for TFTP and assume that the developer will use it. TFTP is a lightweight protocol for moving files between a TFTP server and TFTP client, similar to FTP.
Using TFTP from your bootloader to load the kernel will save you countless hours waiting for serial downloads even at higher serial baud rates. And loading your ramdisk can take much longer because ramdisk images can grow to many tens of megabytes and more, depending on your requirements. The investment in your time to configure and use TFTP will surely pay off and is highly recommended. There are very few designs that can't afford the real estate to include an Ethernet port during development, even if it is depopulated for production.
12.3.1. TFTP Server
Configuring TFTP on your Linux development host is not difficult. Of course, the details might vary, depending on which Linux distribution you choose for your development workstation. The guidelines presented here are based on Red Hat and Fedora Core Linux distributions.
TFTP is a TCP/IP service that must be enabled on your workstation. To enable TFTP service, you must instruct your server to respond to incoming TFTP packets and spawn your TFTP server. On many Linux distributions, this is done by editing a configuration file used by the xinetd Internet superserver. For example, on the Red Hat and Fedora desktop Linux distributions, this file is /etc/xinetd.d/tftp. Listing 12-4 contains a TFTP configuration from a Fedora Core 2 development workstation to enable the TFTP service. It has been slightly rearranged to fit the page.
Listing 12-4. TFTP Configuration
# default: off
# description: The tftp server serves files using the trivial
# file transfer protocol. The tftp protocol is often used to
# boot diskless workstations, download configuration files to
# network-aware printers, and to start the installation process
# for some operating systems.
service tftp
{
socket_type = dgram
protocol = udp
wait = yes
user = root
server = /usr/sbin/in.tftpd
server_args = -c -s /tftpboot
disable = no
per_source = 11
cps = 100 2
