board: xiaomi: mocha: add Xiaomi Mi Pad A0101 support

The Mi Pad is a tablet computer based on Nvidia Tegra K1 SoC which
originally ran the Android operating system. The Mi Pad has a 7.9" IPS
display with 1536 x 2048 (324 ppi) resolution. 2 GB of RAM and 16/64 GB of
internal memory that can be supplemented with a microSDXC card giving up to
128 GB of additional storage.

Signed-off-by: Svyatoslav Ryhel <[email protected]>

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diff --git a/doc/board/index.rst b/doc/board/index.rst
index b055046e649..84c135e02c1 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -69,4 +69,5 @@ Board-specific doc
    variscite/index
    wexler/index
    xen/index
+   xiaomi/index
    xilinx/index
diff --git a/doc/board/xiaomi/index.rst b/doc/board/xiaomi/index.rst
new file mode 100644
index 00000000000..109ab4a251f
--- /dev/null
+++ b/doc/board/xiaomi/index.rst
@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Xiaomi
+======
+
+.. toctree::
+   :maxdepth: 2
+
+   mocha
diff --git a/doc/board/xiaomi/mocha.rst b/doc/board/xiaomi/mocha.rst
new file mode 100644
index 00000000000..be3e333127b
--- /dev/null
+++ b/doc/board/xiaomi/mocha.rst
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+.. SPDX-License-Identifier: GPL-2.0+
+
+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
+------------
+
+.. code-block:: bash
+
+    $ 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.
+
+.. code-block:: bash
+
+    $ 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.
+
+.. 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 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:
+
+.. code-block:: bash
+
+    $ 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:
+
+.. code-block::
+
+  CONFIG_TEXT_BASE=0x80A00000
+  CONFIG_SKIP_LOWLEVEL_INIT=y
+  # CONFIG_OF_BOARD_SETUP is not set
+  CONFIG_TEGRA_SUPPORT_NON_SECURE=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.