Building the kernel

Downloading the source

Download Technologic's Linux source and TS-7800 configuration files and unzip them in a suitable location.

Kernel configuration

A complete discussion of kernel configuration is beyond the scope of this tutorial. In this case, the ts7800_defconfig target gave me a default usable configuration for the 7800, with one small hiccup: the CONFIG_DMA_ENGINE setting ended up off when it should have been on.

Tweaking the kernel

It is usually best to edit the kernel using make menuconfig, which offers a semi-graphical interface to kernel configuration. This interface is navigated using arrow keys to move the cursor, the Tab key to select options from the bottom of the screen, and the space or Enter keys to select options. For instance, to exit without changing anything, press Tab until the <Exit> at the bottom of the screen is highlighted, then press Enter. Running make menuconfig again reopens the editor.

Changing the default console

The TS-7800 normally boots silently, because the default kernel configuration specifies a null console device to keep the display quiet. To change this, use the arrow keys to navigate down to "Boot options," and press Enter. The third line shows the default kernel options, which select the ramdisk, the startup script, and the console. Use the arrow keys to navigate down to this line, press Enter, and change console none to console ttyS0,115200. Then, press Tab to move the cursor to the <Ok> at the bottom of the panel, and press Enter. Now press Tab to select <Exit> and press Enter, bringing you back to the main menu.

For the goal of booting as fast as possible, the console device isn't useful, and indeed, even at a high baud rate, sending kernel messages can take a noticeable fraction of the time the system takes to boot. For debugging and playing around, though, you want the console.

Enabling the DMA engine

Navigate down to "Device drivers" and press Enter. This list is longer than the usual display, so you will have to scroll down to the very end to reach the option for "DMA Engines." Navigate to that with the arrow keys, and press Enter. There are two options at the top of this page that have square brackets indicating a boolean option. The second, "Support for DMA engines," was not enabled by default in the download I started with. Navigate to it with the arrow keys, and press space to toggle its state. Now use Tab and Enter to select <Exit> from each screen to navigate out to the top level of the program, and then <Exit> one more time to leave the program. When asked whether you wish to save your new kernel configuration, tab to <Yes> and press Enter.

Compiling the kernel

Type make. Yes, it really is that simple. This builds a kernel, as well as a collection of modules. Once again, multi-core users might want multiple jobs; try make -j 5. For the purposes of this project, I'm going to ignore kernel modules, and favor compiling-in any needed features. The bootstrap ramdisk technique used to get needed drivers into the kernel early seems excessive, and building a root file system is already complicated enough. This, of course, brings up the question of how to get a kernel booting, the subject of the next section.

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Table of contents

• Before you start
• About the target and architecture
• Cross-compilation
• Building the kernel
• Boot loaders
• Filesystems
• Populating the root file system
• Creating a disk image
• Getting the images back on the card
• The boot process reviewed
• Resources
• About the author

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