[PATCH 3/3] Documentation: IMX: add doc for the Variscite DT8MCustomBoard

[Michael Kopfensteiner <michael.kopfensteiner@vahle.com>]

This commit adds a page for the newly added Variscite board.

The information about downloading the NXP firmware blobs was obtained
from NXP's public U-Boot sources for the i.MX processors [1].

[1] https://github.com/nxp-imx/uboot-imx

Signed-off-by: Michael Kopfensteiner <michael.kopfensteiner@vahle.com>
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 .../imx/variscite-dt8mcustomboard-imx8mp.rst  | 138 ++++++++++++++++++
 1 file changed, 138 insertions(+)
 create mode 100644 Documentation/boards/imx/variscite-dt8mcustomboard-imx8mp.rst

diff --git a/Documentation/boards/imx/variscite-dt8mcustomboard-imx8mp.rst b/Documentation/boards/imx/variscite-dt8mcustomboard-imx8mp.rst
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+Variscite DT8MCustomBoard with DART-MX8M-PLUS SOM
+=================================================
+
+This board is an eval-kit for the Variscite DART-MX8M-PLUS SOM. The latter is a
+SOM based on the i.MX8M Plus processor. As seen in official Variscite documents there exist
+several hardware revisions for this board. Currently only revision 3.0 could was tested
+with Barebox.
+
+The Variscite DART-MX8M-PLUS SOM is available in different configurations. For a rough overview,
+these are some of the possible options:
+
+* Processor: NXP i.MX8M Plus, either @1.6GHz or @1.8GHz
+* 1 to 8 GB LPDDR4 RAM
+* 8 to 128 GB eMMC
+* (optional) GigE PHY on module
+* (optional) Wifi + Bluetooth Chip on module
+
+Besides that the SOM offers a lot of interfaces. Among some of the interfaces that are
+made available by the eval-board are:
+
+* USB3
+* GigE Ethernet
+* PCIe
+* SD-card slot
+* HDMI
+
+More Information about the eval-board can be found at
+https://www.variscite.com/product/single-board-computers/var-dt8mcustomboard/
+
+More Information about the targeted SOM can be found at
+https://www.variscite.com/product/system-on-module-som/cortex-a53-krait/dart-mx8m-plus-nxp-i-mx-8m-plus
+
+Providing necessary binary files
+--------------------------------
+
+Barebox requires some blobs to successfully bringup the system. These blobs
+serve different use cases. Barebox's build system will look for these files
+in the configured firmware directory (``firmware`` by default). The build
+systems expects these files to have certain names.
+
+Hence the very first thing before building Barebox is to obtain these files and
+placing them in the firmware folder.
+
+The DDR4 training files are part of a set of files that is provided by NXP.
+They are provided under the terms of a proprietary EULA one has to agree to,
+before getting access to the blobs. They are provided as self-extracting
+archive. To get a hand on them, perform the following::
+
+   $ wget https://www.nxp.com/lgfiles/NMG/MAD/YOCTO/firmware-imx-8.10.bin
+   $ chmod u+x firmware-imx-8.10.bin
+   $ ./firmware-imx-8.10.bin
+
+Assuming that the downloaded executable was run from inside the toplevel directory of the Barebox repo,
+the necessary DDR4 training files can simply be hardlinked (or copied)::
+
+   $ ln firmware-imx-8.10/firmware/ddr/synopsys/lpddr4_pmu_train_1d_dmem_202006.bin firmware/lpddr4_pmu_train_1d_dmem.bin
+   $ ln firmware-imx-8.10/firmware/ddr/synopsys/lpddr4_pmu_train_1d_imem_202006.bin firmware/lpddr4_pmu_train_1d_imem.bin
+   $ ln firmware-imx-8.10/firmware/ddr/synopsys/lpddr4_pmu_train_2d_dmem_202006.bin firmware/lpddr4_pmu_train_2d_dmem.bin
+   $ ln firmware-imx-8.10/firmware/ddr/synopsys/lpddr4_pmu_train_2d_imem_202006.bin firmware/lpddr4_pmu_train_2d_imem.bin
+
+Another required binary is the Secure Monitor Firmware (BL31). This is build by some ARM Trusted Firmware project (ATF).
+One fork is provided by NXP and can be downloaded from https://github.com/nxp-imx/imx-atf. Variscite does maintain it's
+own fork of NXP's ATF project. This can be found at https://github.com/varigit/imx-atf/.
+
+Once the ATF has been built successfully, the resulting BL31 binary needs to be placed in the ``firmware`` directory
+under the filename ``imx8mp-bl31.bin``.
+
+After those files are in place, one can proceed with the usual Barebox build routine.
+
+Compiling Barebox
+-----------------
+
+A quick way to configure and compile Barebox is by using the provided defconfig
+for the DT8MCustomBoard::
+
+   export ARCH=arm
+   export CROSS_COMPILE=/path/to/your/toolchain/bin/aarch64-v8a-linux-gnu-
+   make variscite_imx8mp_dart_cb_defconfig
+   make
+
+When the build succeeds, the Barebox image `barebox-variscite-imx8mp-dart-cb.img`
+can be found in the `images`  subdirectory.
+
+Boot Configuration
+------------------
+
+The DT8MCustomBoard allows the user to choose whether to proceed with the *internal*
+or *external boot mode*. *Internal boot mode* refers to booting from the eMMC memory,
+while *external boot mode* refers to booting from an SD-card.
+
+The mode is selected with switch SW7, located below the buttons on the board.
+
+Set the switch to **ON** for the BootROM to perform an *internal boot*. Otherwise
+set the switch to **OFF** to follow the *external boot* procedure.
+
+If in doubt, refer to the silk screen on the board, to select the correct switch
+position.
+
+If the BootROM cannot find a valid bootloader image in any of the checked sources,
+it'll try several fallbacks until it finally ends in USB download mode or finds
+a valid bootloader image to load.
+
+Starting Barebox
+----------------
+
+An easy solution to start Barebox bare metal is to use the *external boot* mode and
+copy Barebox onto a SD-card.
+
+To copy the Barebox binary onto a SD-card, use the `dd` tool on linux::
+
+   dd if=images/barebox-variscite-imx8mp-dart-cb.img of=/dev/mmcblk0 bs=512 seek=1 skip=1
+
+For more details and an explanation on how to start Barebox from an existing
+U-Boot look here:
+
+Currently Supported Features
+----------------------------
+
+The Barebox binary configured by the `variscite_imx8mp_dart_cb_defconfig` does currently
+not support all possible features of the DT8MCustomBoard. Yet the binary does contain
+everything necessary to boot an operating system on the i.MX8MP.
+
+Some of the currently supported features:
+* general i.MX8MP bringup, including DRAM initialisation
+* working eMMC and SD-card support
+* serial console on UART 1 - available through the micro-USB connector on the board
+* working gigabit ethernet on the first port (labeled ETH, named `eth0` in Barebox and linux)
+* working LED and GPIO support
+
+Some functionality that is currently missing or untested:
+* secondary ethernet interface (labeled ETH2) will currently not work
+* secure boot (not tested)
+* loading of the HDMI firmware
+* OP-TEE integration (not tested - early loading currently not supported by the startup code)
+* running on other hardware revisions of the DT8MCustomBoard than v3.0 (not tested)
+
+A lot of the missing features are already solved within other i.MX8M boards, hence it might be
+fairly simple to adapt those solutions to this board as well.
\ No newline at end of file
-- 
2.30.2