Up to now, all our cheap Android devices were based on older Linux kernel (3.0.x, 3.4.x) that still used board files (arch/arm/board, but we’ve recently seen companies like Amlogic and Rockchip release source code with Linux kernel 3.10.x. One of the key differences between these version are the move from board files to flattened device tree and multi-platform support. If it is fully implemented, a single kernel image should be able to boot multiple hardware platforms, and all low level configuration handled by the device tree file. Since I’ve connected the serial port of Tronsmart Vega S89 for debugging, and it’s a slow news day, I thought I might try to boot the Linux kernel I compiled myself, but one of the challenge was to get the device tree file. I’ll show how to extract it from the firmware. It should also be possible to get it directly from the flash, but “cat /proc/mtd” does not show a complete list of partition as in previous versions.
I’ve performed the steps below in Ubuntu 14.04. The first thing is to install some tools: the device tree compiler that we’ll use to decompile the dtb (binary) file into a dtd (text) file, and split_bootimg.pl a standard PERL script to extract files from boot.img:
sudo apt-get install device-tree-compiler
sudo cp split_bootimg.pl /usr/local/bin
sudo chmod +x /usr/local/bin/split_bootimg.pl
I’ll use M8 / TM8 firmware (Amlogic S802) as an example. The exact procedure will vary between firmware files, but if you can boot.img, the procedure should be platform independent and work for any ARM SoC. After having downloaded and extracted the firmware file (TM8 ap6330_03102014A_0410_ROOT.rar), let’s create a working directory, and unzip the “OTA” file.
We now get a bunch of files, including boot.img. Great! Time to run split_bootimg.pl script to extract its content:
Page size: 2048 (0x00000800)
Kernel size: 7209567 (0x006e025f)
Ramdisk size: 2024995 (0x001ee623)
Second size: 17699 (0x00004523)
Writing boot.img-kernel ... complete.
Writing boot.img-ramdisk.gz ... complete.
Writing boot.img-second.gz ... complete.
So we’ve got the kernel, a ramdisk, and a “second file” that happens to be the dtb file. We can now decompile it with dtc (device tree compiler) as follows:
dtc -I dtb boot.img-second.gz -O dts -o meson8_tm8.dtd
That’s it. Here’s M8 device tree file.
I’ve done the same for Tronsmart Vega S89 (Elite). S89 firmware is usually distributed as an IMG file to be used with AML Flash Burning tool, but I haven’t found a way to extract such file yet. however, I’ve found an “OTA” firmware, to be updated via SD Card, on freaktab, and could extract the device tree file for Tronsmart Vega S89 Elite & Vega S89. Both M8 and S89 Elite DTD files are very similar, but the maximum CPU frequency seems to be higher in M8, and there are other apparently minor differences. Vega S89 DTD file appears to be much different however.
Jean-Luc started CNX Software in 2010 as a part-time endeavor, before quitting his job as a software engineering manager, and starting to write daily news, and reviews full time later in 2011.
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