doc: describe Milk-V Mars board

Add instructions to build U-Boot for the Milk-V Mars board

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>

doc/board/starfive/index.rst

@@ -6,4 +6,5 @@ StarFive
 .. toctree::
    :maxdepth: 1
 
+   milk-v_mars.rst
    visionfive2

doc/board/starfive/milk-v_mars.rst

@@ -0,0 +1,111 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Milk-V Mars
+===========
+
+U-Boot for the Milk-V Mars uses the same U-Boot binaries as the VisionFive 2
+board. In U-Boot SPL the actual board is detected and the device-tree patched
+accordingly.
+
+Building
+~~~~~~~~
+
+1. Add the RISC-V toolchain to your PATH.
+2. Setup ARCH & cross compilation environment variable:
+
+.. code-block:: none
+
+   export CROSS_COMPILE=<riscv64 toolchain prefix>
+
+The M-mode software OpenSBI provides the supervisor binary interface (SBI) and
+is responsible for the switch to S-Mode. It is a prerequisite to build U-Boot.
+Support for the JH7110 was introduced in OpenSBI 1.2. It is recommended to use
+a current release.
+
+.. code-block:: console
+
+	git clone https://github.com/riscv/opensbi.git
+	cd opensbi
+	make PLATFORM=generic FW_TEXT_START=0x40000000 FW_OPTIONS=0
+
+Now build the U-Boot SPL and U-Boot proper.
+
+.. code-block:: console
+
+	cd <U-Boot-dir>
+	make starfive_visionfive2_defconfig
+	make OPENSBI=$(opensbi_dir)/build/platform/generic/firmware/fw_dynamic.bin
+
+This will generate the U-Boot SPL image (spl/u-boot-spl.bin.normal.out) as well
+as the FIT image (u-boot.itb) with OpenSBI and U-Boot.
+
+Device-tree selection
+~~~~~~~~~~~~~~~~~~~~~
+
+Depending on the board version U-Boot set variable $fdtfile to either
+starfive/jh7110-starfive-visionfive-2-v1.2a.dtb or
+starfive/jh7110-starfive-visionfive-2-v1.3b.dtb.
+
+To overrule this selection the variable can be set manually and saved in the
+environment
+
+::
+
+    setenv fdtfile my_device-tree.dtb
+    env save
+
+or the configuration variable CONFIG_DEFAULT_FDT_FILE can be used to set to
+provide a default value.
+
+Boot source selection
+~~~~~~~~~~~~~~~~~~~~~
+
+The board provides the DIP switches MSEL[1:0] to select the boot device out of
+SPI flash, eMMC, SD-card, UART. To select booting from SD-card set the DIP
+switches MSEL[1:0] to 10.
+
+Preparing the SD-Card
+~~~~~~~~~~~~~~~~~~~~~
+
+The device firmware loads U-Boot SPL (u-boot-spl.bin.normal.out) from the
+partition with type GUID 2E54B353-1271-4842-806F-E436D6AF6985. You are free
+to choose any partition number.
+
+With the default configuration U-Boot SPL loads the U-Boot FIT image
+(u-boot.itb) from partition 2 (CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION=0x2).
+When formatting it is recommended to use GUID
+BC13C2FF-59E6-4262-A352-B275FD6F7172 for this partition.
+
+The FIT image (u-boot.itb) is a combination of OpenSBI's fw_dynamic.bin,
+u-boot-nodtb.bin and the device tree blob.
+
+Format the SD card (make sure the disk has GPT, otherwise use gdisk to switch)
+
+.. code-block:: bash
+
+	sudo sgdisk --clear \
+	  --set-alignment=2 \
+	  --new=1:4096:8191 --change-name=1:spl --typecode=1:2E54B353-1271-4842-806F-E436D6AF6985\
+	  --new=2:8192:16383 --change-name=2:uboot --typecode=2:BC13C2FF-59E6-4262-A352-B275FD6F7172  \
+	  --new=3:16384:1654784 --change-name=3:system --typecode=3:EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 \
+	  /dev/sdb
+
+Copy U-Boot to the SD card
+
+.. code-block:: bash
+
+	sudo dd if=u-boot-spl.bin.normal.out of=/dev/sdb1
+	sudo dd if=u-boot.itb of=/dev/sdb2
+
+	sudo mount /dev/sdb3 /mnt/
+	sudo cp u-boot-spl.bin.normal.out /mnt/
+	sudo cp u-boot.itb /mnt/
+	sudo cp Image.gz /mnt/
+	sudo cp initramfs.cpio.gz /mnt/
+	sudo cp jh7110-starfive-visionfive-2.dtb /mnt/
+	sudo umount /mnt
+
+Booting
+~~~~~~~
+
+Once you plugin the sdcard and power up, you should see the U-Boot prompt.