Build ZYNQ Device Tree

2020-03-24

Let's try build a boot image for ZYNQ, in a hard way. Part II will cover device tree.

Device Tree is key component for running Linux. It's often be called DTS (Device Tree Structure) or DTB (Device Tree Blob), different depend on whether it is source or compiled binary. We will try to build DTB from source.

In fact, both u-boot and Linux need a DTS/DTB, we will talk it later. For most ARM board, the DTS is fixed, thus we can just grab one from vender and does not need to "build" it. But it's not true for Zynq, since the PL side is not fixed. Many IPs are provided as a peripheral and Linux should load drivers for them.

0. Prerequisites

  • Install Xilinx Vitis tool.

  • Build the hardware (PL/FPGA side) design, and export .xsa file.

1. Install Xilinx Device Tree Generator

In Vivado, we build the PL side design and export the hardware handoff file (.xsa). The FPGA design flow ends here. Then we need to generate device tree source file from .xsa file. To do this, Xilinx provide a tool called Device Tree Generator (DTG) on Github. Using your git tool to clone it and checkout latest release or download the zip file from release page.

git clone git@github.com:Xilinx/device-tree-xlnx.git
git checkout xilinx-v2019.2 -b my-xilinx-v2019.2

If you download the zip file, unzip to some folder and keep it. The next step is launch Vitis and setup repository, since the generator need to work with Vitis.

  1. Launch Vitis, on menu bar, find Xilinx -> Repositories. The Xilinx preferences window opened.
  2. In Local Repositories or Global Repositories, click New... to add a new repository. In the popup window, select device-tree-xlnx folder.
  3. Maybe Vitis need to be restarted to reload repository.

2. Create Vitis Project

  1. Launch Vitis, create a new Platform Project. This opens new project wizard.

  2. In the wizard, select the .xsa file exported from Vivado.

  3. In the Operation system selection, choose device_tree (Only after you setup the repository, it will appear here).

  1. Build the project.

  2. Find your device tree under /ps7_cortexa9_0/deivce_tree_domain/bsp foler.

Beware that there are two top level .dts file here. We will use system-top.dts. zynq-7000.dtsi is included.

Another way to generate dts without create a Vitis project is using Vitis Xilinx -> Generate Device Tree button.

3. Build Device Tree Compiler (DTC)

Device tree compiler (DTC) is a tool to compile .dts file to .dtb. Ubuntu has already provide the binary package of dtc, can be installed by:

sudo apt install detive-tree-compiler

Or you can build new version form dtc source at kernel.org. It's very simple, just need to install build-essential and some packages (I don't know, follow error message to see which package is needed), and type make in dtc source folder.

Building dtc is not cross-compile, so you does not need setup ARCH and CROSS_COMPILE environment variable.

If build from source, make sure add dtc to PATH.

4. Build Device Tree Blob

The generated .dts file using the c like #include lines to include extra .dtsi files. This need precompiled or preprocess before use:

gcc -E -nostdinc -undef -D__DTS__ -x assembler-with-cpp -o system-top.dts.tmp system-top.dts

This will generate system-top.dts.tmp with correct syntax for dtc. Or you can replace all #include with dts style /include/ so you can skip preprocess step.

Then we can build the .dtb file.

dtc -I dts -O dtb -o system-top.dtb system-top.dts.tmp

Reference

https://xilinx-wiki.atlassian.net/

FPGAZYNQLinuxVitis
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Using Vivado Project With Git

ZYNQ Hard Startup Guide

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