doc: board: Add Calxeda Highbank/Midway documentation

The Calxeda servers are using U-Boot as the primary bootloader, which
was shipped as part of a firmware upgrade package.
Even though the machines are considered legacy at this point, the port
still works, so deserves some documentation.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/board/highbank/highbank.rst

@@ -0,0 +1,78 @@
+Calxeda Highbank/Midway board support
+=====================================
+
+The Calxeda ECX-1000 ("Highbank") and ECX-2000 ("Midway") were ARM based
+servers, providing high-density cluster systems. A single motherboard could
+host between 12 and 48 nodes, each with their own quad-core ARMv7
+processor, private DRAM and peripherals, connected through a high-bandwith
+and low-latency "fabric" network. Multiple motherboards could be connected
+together, to extend this fabric.
+
+For the purpose of U-Boot we just care about a single node, this can be
+used as a single system, just using the fabric to connect to some Ethernet
+network. Each node boots on its own, either from a local hard disk, or
+via the network.
+
+The earlier ECX-1000 nodes ("Highbank") contain four ARM Cortex-A9 cores,
+a Cortex-M3 system controller, three 10GBit/s MACs and five SATA
+controllers. The DRAM is limited to 4GB.
+
+The later ECX-2000 nodes ("Midway") use four Cortex-A15 cores, alongside
+two Cortex-A7 management cores, and support up to 32GB of DRAM, while
+keeping the other peripherals.
+
+For the purpose of U-Boot those two SoCs are very similar, so we offer
+one build target. The subtle differences are handled at runtime.
+Calxeda as a company is long defunct, and the remaining systems are
+considered legacy at this point.
+
+Bgilding U-Boot
+---------------
+There is only one defconfig to cover both systems::
+
+    $ make highbank_defconfig
+    $ make
+
+This will create ``u-boot.bin``, which could become part of the firmware update
+package, or could be chainloaded by the existing U-Boot, see below for more
+details.
+
+Boot process
+------------
+Upon powering up a node (which would be controlled by some BMC style
+management controller on the motherboard), the system controller ("ECME")
+would start and do some system initialisation (fabric registration,
+DRAM init, clock setup). It would load the device tree binary, some secure
+monitor code (``a9boot``/``a15boot``) and a U-Boot binary from SPI flash
+into DRAM, then power up the actual application cores (ARM Cortex-A9/A15).
+They would start executing ``a9boot``/``a15boot``, registering the PSCI SMC
+handlers, then dropping into U-Boot, but in non-secure state (HYP mode on
+the A15s).
+
+U-Boot would act as a mere loader, trying to find some ``boot.scr`` file on
+the local hard disks, or reverting to PXE boot.
+
+Updating U-Boot
+---------------
+The U-Boot binary is loaded from SPI flash, which is controlled exclusively
+by the ECME. This can be reached via IPMI using the LANplus transport protocol.
+Updating the SPI flash content requires vendor specific additions to the
+IPMI protocol, support for which was never upstreamed to ipmitool or
+FreeIPMI. Some older repositories for `ipmitool`_, the `pyipmi`_ library and
+a Python `management script`_ to update the SPI flash can be found on Github.
+
+A simpler and safer way to get an up-to-date U-Boot running, is chainloading
+it via the legacy U-Boot::
+
+    $ mkimage -A arm -O u-boot -T standalone -C none -a 0x8000 -e 0x8000 \
+      -n U-Boot -d u-boot.bin u-boot-highbank.img
+
+Then load this image file, either from hard disk, or via TFTP, from the
+existing U-Boot, and execute it with bootm::
+
+    => tftpboot 0x8000 u-boot-highbank.img
+    => bootm
+
+.. _`ipmitool`: https://github.com/Cynerva/ipmitool
+.. _`pyipmi`: https://pypi.org/project/pyipmi/
+.. _`management script`: https://github.com/Cynerva/cxmanage

doc/board/highbank/index.rst

@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Highbank
+========
+
+.. toctree::
+   :maxdepth: 2
+
+   highbank

doc/board/index.rst

@@ -16,6 +16,7 @@ Board-specific doc
    coreboot/index
    emulation/index
    google/index
+   highbank/index
    intel/index
    kontron/index
    microchip/index