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.. SPDX-License-Identifier: GPL-2.0+
U-Boot for the ASUS/Google Nexus 7 (2012)
=========================================
``DISCLAMER!`` Moving your ASUS/Google Nexus 7 (2012) to use U-Boot assumes
replacement of the vendor ASUS bootloader. Vendor android firmwares will no
longer be able to run on the device. This replacement IS reversible.
Quick Start
-----------
- Build U-Boot
- Process U-Boot
- Flashing U-Boot into the eMMC
- Boot
- Self Upgrading
Build U-Boot
------------
Device support is implemented by applying config fragment to a generic board
defconfig. Valid fragments are ``tilapia.config``, ``grouper_E1565.config``
and ``grouper_PM269.config``.
.. code-block:: bash
$ export CROSS_COMPILE=arm-linux-gnueabi-
$ make grouper_common_defconfig grouper_E1565.config # For maxim based grouper
$ make
After the build succeeds, you will obtain the final ``u-boot-dtb-tegra.bin``
image, ready for further processing.
Process 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. You need to know your
tablet's individual SBK to continue.
Permanent installation can be performed either by using the nv3p protocol or by
pre-loading just built U-Boot into RAM.
Processing for the NV3P protocol
********************************
.. code-block:: bash
$ 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 grouper --sbk <your sbk>
where SBK has next form ``0xXXXXXXXX`` ``0xXXXXXXXX`` ``0xXXXXXXXX`` ``0xXXXXXXXX``
The script will produce a ``repart-block.bin`` ready to flash.
Processing for pre-loaded U-Boot
********************************
The procedure is the same, but the ``--split`` argument is used with the
``re-crypt.py``. The script will produce ``bct.img`` and ``ebt.img`` ready
to flash.
Flashing U-Boot into the eMMC
-----------------------------
``DISCLAMER!`` All questions related to NvFlash should be asked in the proper
place. NOT HERE! Flashing U-Boot will erase all eMMC, so make a backup before!
Permanent installation can be performed either by using the nv3p protocol or by
pre-loading just built U-Boot into RAM.
Flashing with the NV3P protocol
*******************************
Nv3p is a custom Nvidia protocol used to recover bricked devices. Devices can
enter it either by using ``wheelie`` with the correct ``blob.bin`` file or by
pre-loading vendor bootloader with the Fusée Gelée.
With nv3p, ``repart-block.bin`` is used. It contains BCT and a bootloader in
encrypted state in form, which can just be written RAW at the start of eMMC.
.. code-block:: bash
$ wheelie --blob blob.bin
$ nvflash --resume --rawdevicewrite 0 1024 repart-block.bin
When flashing is done, reboot the device.
Flashing with a pre-loaded U-Boot
*********************************
U-Boot pre-loaded into RAM acts the same as when it was booted "cold". Currently
U-Boot supports bootmenu entry fastboot, which allows to write a processed copy
of U-Boot permanently into eMMC.
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:
.. code-block:: bash
$ fastboot flash 0.1 bct.img
$ fastboot flash 0.2 ebt.img
$ fastboot reboot
Device will reboot.
Boot
----
To boot Linux, U-Boot will look for an ``extlinux.conf`` on eMMC. Additionally,
if the Volume Down button is pressed while booting, the device will enter
bootmenu. Bootmenu contains entries to mount eMMC as mass storage, fastboot,
reboot, reboot RCM, poweroff, enter U-Boot console and update bootloader (check
the next chapter).
Flashing ``repart-block.bin`` 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 eMMC (using
ability of u-boot to mount it). Enter bootmenu, choose update bootloader option
with Power button and U-Boot should update itself. Once the process is
completed, U-Boot will ask to press any button to reboot.
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