.. SPDX-License-Identifier: GPL-2.0+
.. sectionauthor:: Sam Protsenko <semen.protsenko@linaro.org>

WinLink E850-96 board
=====================

Overview
--------

WinLink's E850-96 board [1]_ is based on Samsung Exynos850 SoC and follows
96Boards Consumer Edition specification [2]_. That makes it possible to use
96Boards mezzanine boards [3]_ along with it. It's an open-hardware board and
the hardware design files [4]_ were published, along with the supported
software [5]_ and related documentation.

U-Boot can be used on E850-96 instead of the original Samsung LittleKernel based
bootloader [6]_. Because FWBL1 [7]_ doesn't verify bootloader's signature, there
is no need to sign a U-Boot binary. That means U-Boot binary can be flashed into
``bootloader`` partition (instead of LittleKernel bootloader) and it will just
work.

Because BL2 bootloader already sets up DRAM and runs the final bootloader
(U-Boot) from DRAM, there is no need in U-Boot SPL. It's enough to have only
U-Boot proper (``u-boot.bin``).

Boot Flow
---------

The boot path for Exynos850 is shown on the figure below.

.. image:: img/exynos850-boot-architecture.svg
  :alt: Exynos850 SoC boot flow

Legend:

* ``BL0``: Boot ROM code
* ``BL1``: Software part of Boot ROM
* ``EPBL``: Exynos Primary Boot Loader
* ``BL2``: Initializes CMU and DRAM and runs the final bootloader
* ``Bootloader``: Final bootloader (e.g. U-Boot); also called BL33 in terms of
  ARM boot flow
* ``EL3_MON``: EL3 monitor (trusted firmware, handles SMC calls); also called
  BL31 in terms of ARM boot flow
* ``LDFW``: Loadable Firmware

Build Procedure
---------------

.. warning::
  At the moment both eMMC and USB features are not enabled in U-Boot. Flashing
  U-Boot binary **WILL** effectively brick your board. The ``dltool`` [8]_ can
  be used then to perform USB boot and flash LittleKernel bootloader binary [7]_
  to unbrick and revive the board. Flashing U-Boot binary might be helpful for
  developers or anybody who want to check current state of U-Boot enablement on
  E850-96 (which is mostly serial console and related blocks).

Build U-Boot binary from source code (using AArch64 baremetal GCC toolchain):

.. prompt:: bash $

  export PATH=<toolchain path>/bin:$PATH
  export CROSS_COMPILE=<toolchain prefix>
  make e850-96_defconfig
  make

Boot E850-96 board into fastboot mode as described in board software doc [9]_,
and flash U-Boot binary into ``bootloader`` eMMC partition:

.. prompt:: bash $

  fastboot flash bootloader u-boot.bin
  fastboot reboot

U-Boot will boot up to the shell.

References
----------

.. [1] https://www.96boards.org/product/e850-96b/
.. [2] https://www.96boards.org/products/ce/
.. [3] https://www.96boards.org/products/mezzanine/
.. [4] https://www.96boards.org/documentation/consumer/e850-96b/hardware-docs/
.. [5] https://gitlab.com/Linaro/96boards/e850-96/
.. [6] https://gitlab.com/Linaro/96boards/e850-96/lk
.. [7] https://gitlab.com/Linaro/96boards/e850-96/images
.. [8] https://gitlab.com/Linaro/96boards/e850-96/tools/dltool
.. [9] https://gitlab.com/Linaro/96boards/e850-96/doc