U-Boot for the Xiaomi Mi Pad tablet

DISCLAMER! Moving your Xiaomi Mi Pad to use U-Boot assumes replacement of the vendor bootloader. Vendor Android firmwares will no longer be able to run on the device. This replacement IS reversible.

Quick Start

  • Build U-Boot

  • Boot U-Boot

  • Process and flash U-Boot

  • Boot Linux

  • Self Upgrading

  • Chainload configuration

Build U-Boot

$ export CROSS_COMPILE=arm-none-eabi-
$ make mocha_defconfig
$ make

After the build succeeds, you will obtain the final u-boot-dtb-tegra.bin image, ready for booting or further processing.

Boot U-Boot

Existing tegrarcm loader can be used to pre-load U-Boot you have build into RAM and basically perform a tethered cold-boot.

$ tegrarcm --bct mocha.bct --bootloader u-boot-dtb-tegra.bin --loadaddr 0x80108000

U-Boot will try to load Linux kernel and if fails, it will turn the tablet off. While pre-loading U-Boot, hold the volume down button which will trigger the bootmenu.

Process and flash U-Boot

DISCLAMER! All questions related to the re-crypt work should be asked in re-crypt repo issues. NOT HERE!

re-crypt is a tool that processes the u-boot-dtb-tegra.bin binary into form usable by device. This process is required only on the first installation or to recover the device in case of a failed update.

$ git clone https://gitlab.com/grate-driver/re-crypt.git
$ cd re-crypt # place your u-boot-dtb-tegra.bin here
$ ./re-crypt.py --dev mocha

The script will produce bct.img and ebt.img ready to flash.

Permanent installation can be performed by pre-loading just built U-Boot into RAM via tegrarcm. While pre-loading U-Boot, hold the volume down button which will trigger the bootmenu. There, select fastboot using the volume and power buttons.

After, on host PC, do:

$ fastboot flash 0.1 bct.img
$ fastboot flash 0.2 ebt.img
$ fastboot reboot

Device will reboot.

Boot Linux

To boot Linux, U-Boot will look for an extlinux.conf on MicroSD and then on eMMC. Additionally, if the volume down button is pressed while booting, the device will enter bootmenu. Bootmenu contains entries to mount MicroSD and eMMC as mass storage, fastboot, reboot, reboot RCM, poweroff, enter U-Boot console and update bootloader (check the next chapter).

Flashing bct.img and ebt.img eliminates vendor restrictions on eMMC and allows the user to use/partition it in any way the user desires.

Self Upgrading

Place your u-boot-dtb-tegra.bin on the first partition of the MicroSD card and insert it into the tablet. Enter bootmenu, choose update the bootloader option with the Power button and U-Boot should update itself. Once the process is completed, U-Boot will ask to press any button to reboot.

Chainload configuration

To build U-Boot without SPL suitable for chainloading adjust mocha_defconfig:

CONFIG_TEXT_BASE=0x80A00000
CONFIG_SKIP_LOWLEVEL_INIT=y
# CONFIG_OF_BOARD_SETUP is not set
CONFIG_TEGRA_PRAM=y

After the build succeeds, you will obtain the final u-boot-dtb.bin file, ready for booting using vendor bootloader’s fastboot or which can be further processed into a flashable image.