Simple boot menu for booting Linux kernels using Kexec Hardboot and DTB (device tree) on ASUS TF300T

Requirements

Tools on Host

• Cross-compiler toolchain with C++ support
• blobtools, abootimg, fastboot

Things you cross-compile

• Kernel with Kexec Hardboot DTB support: https://github.com/tzagorni/tf300t-dtboot-kernel
• kexec-tools (requires zlib): https://mirrors.edge.kernel.org/pub/linux/utils/kernel/kexec/
• A device tree binary file for the device (tegra30-tf300t.dtb). In a kernel source tree that contains the right device tree source (mine), run:

  make tegra30-tf300t.dtb
  

  The resulting file will be in arch/arm/dts/tegra30-tf300t.dtb

make sure to set $CC/CXX variables etc. for cross compiling and to use static linking

Building the boot menu binary dtbootmenu

pushd src
make
popd

Setup Android Dual-Booting (optional)

ideally, all of this should happen automatically in a "flashable zip" or something. But currently nothing like this is implemented.

On the device, create a directory named boot in the internal "SD card" directory. Place the following files in it

• zImage-android: Kernel image for Android (extract from your ROM's installer zip using abootimg)
• initrd-android: Ramdisk image for Android (extract from your ROM's installer zip using abootimg)
• android.dtbootmenu with the following contents:

  kernel: zImage-android
  initrd: initrd-android
  title: Android
  legacy
  

Flashing

You need:

• Kernel zImage binary (tf300t-dtboot-kernel)
• tegra30-tf300t.dtb file
• kexec binary
• dtbootmenu binary

./create-boot-blob.sh <zImage> <dtb file> <kexec binary> src/dtbootmenu
sudo fastboot flash boot boot.blob