board: asus: transformer: add ASUS Transformer T20 family support

The Asus Eee Pad Transformer family are 2-in-1 detachable/slider
tablets developed by Asus that run the Android operating system.
The Eee Pad Transformers feature a 10.1-inch (260 mm) display,
an Nvidia Tegra 2 dual-core chip, 1 GB of RAM, and 16/32 GB of storage.
Transformers board derives from Nvidia Ventana development board.

This patch brings support for all 3 known T20 Transformers:
- Asus Eee Pad Transformer TF101
- Asus Eee Pad Transformer TF101G
- Asus Eee Pad Slider SL101

Tested-by: Robert Eckelmann <longnoserob@gmail.com> # ASUS TF101
Tested-by: Antoni Aloy Torrens <aaloytorrens@gmail.com> # ASUS TF101
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>

2 files changed, 130 insertions, 0 deletions

diff --git a/doc/board/asus/index.rst b/doc/board/asus/index.rst
index 87e535f..2b10328 100644
--- a/doc/board/asus/index.rst
+++ b/doc/board/asus/index.rst
@@ -7,4 +7,5 @@ ASUS
    :maxdepth: 2
 
    grouper_common
+   transformer_t20
    transformer_t30
diff --git a/doc/board/asus/transformer_t20.rst b/doc/board/asus/transformer_t20.rst
new file mode 100644
index 0000000..b5614b1
--- /dev/null
+++ b/doc/board/asus/transformer_t20.rst
@@ -0,0 +1,129 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+U-Boot for the ASUS Eee Pad Transformer device family
+=====================================================
+
+``DISCLAMER!`` Moving your ASUS Eee Pad Transformer/Slider 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 ``tf101.config``, ``tf101g.config`` and
+``sl101.config``.
+
+.. code-block:: bash
+
+    $ export CROSS_COMPILE=arm-linux-gnueabi-
+    $ make transformer_t20_defconfig tf101.config # For TF101
+    $ 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.
+
+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 tf101
+
+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 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 ``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 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.