Combining the Linux Device Tree and Kernel Image for ARM

8 Oct 2013 • 2 minute read

Back in 2010, I wrote two articles about a SystemC model used to load the Linux kernel and its various artifacts into memory for ARM virtual platforms. The first was A SystemC TLM 2.0 ARM Linux Boot Loader and the second was More on the SystemC ARM Linux Boot Loader. At the time I covered the kernel, Linux kernel command line, and RAM-based file system.

Not too long after those articles most ARM systems started using the Linux Device Tree. The Device Tree takes the place of the kernel command line, but does much more by providing information about the hardware that is present in the system. This creates a separation between the kernel source code and the hardware such that the list of hardware can be modified without requiring the entire kernel to be modified. This is a very nice feature for virtual platform developers as it is often possible to just remove hardware descriptions from the device tree as the models are being developed, or if they have problems.

Like the kernel and file system, the device tree can be loaded into memory using the same SystemC loader model. We have been doing this with the Cadence Virtual System Platform for a couple of years now. If you are interested in examples about using the device tree, I covered it in an article about running a Linaro filesystem on the Zynq Virtual Platform last year. Although it's easy to forget, the kernel source tree also has documentation. For the device tree, look in the Documentation/devicetree directory of any kernel source tree.

The most recent development I wanted to cover today is support for combining the kernel image and the device tree into a single file. It turns out that some bootloaders may have trouble getting the device tree into memory, or maybe it's just easier to eliminate dealing with the extra file on the target system.

I have used the appended device tree feature on the ARM Versatile Express platform with kernel versions in the 3.9 to 3.11 range. Using it is straightforward. First, make sure the kernel configuration has the feature enabled. It is in the "Boot options --->" menu and called "Use appended device tree blob to zImage" as shown in the screenshot below.

Preparing the zImage is easy -- just cat the compiled device tree .dtb file to the end of the zImage:

$ cat arch/arm/boot/dts/vexpress-v2p-ca15-tc1.dtb >> arch/arm/boot/zImage

That's it -- the need to separately load the device tree into memory is gone. Just the zImage can be loaded into memory and the kernel will automatically find the appended device tree and use it.

The trick is remembering that the new zImage has the device tree appended. It may be better to rename the file so you know the device tree has been appended. Also, don't forget this has to be done after every kernel build or device tree compile.

Jason Andrews

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