Documentation: Add HiFive Unleashed documentation

Change-Id: Ic97955e36feeaa18b5a0dbd502c722c06dcc1b31
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/25792
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

Documentation/mainboard/sifive/hifive-unleashed.md

@@ -0,0 +1,104 @@
+# SiFive HiFive Unleashed
+
+This page describes how to run coreboot on the [HiFive Unleashed] development
+board from [SiFive], the first RISC-V board on the market with enough resources
+to run a multiuser operating system.
+
+For general setup instructions, please refer to the [Getting Started Guide].
+
+
+## TODO
+
+The following things are still missing from this coreboot port:
+
+- Trampoline in the MBR block to support boot mode 1
+- SiFive UART driver
+- CBMEM support
+- FU540 clock configuration
+- FU540 RAM init
+- Placing the ramstage in DRAM
+- Starting the U54 cores
+- FU540 PIN configuration and GPIO access macros
+- FU540 OTP driver and serial number read-out
+- Support for booting Linux on RISC-V
+
+
+## Configuration
+
+Run `make menuconfig` and select _SiFive_/_HiFive Unleashed_ in the _Mainboard_
+menu.
+
+
+### Boot modes
+
+A total of 16 boot modes can be configured using the switches labeled `MSEL0`
+through `MSEL3`. The most important ones are as follows:
+
+- **MSEL=1**: Jump directly into the SPI flash, bypassing ROM1
+- **MSEL=11**: Load FSBL from SD-card
+- **MSEL=15**: Default boot mode; Load FSBL/coreboot from a GPT partition on
+  SPI flash
+
+
+## Flashing coreboot
+
+The HiFive Unleashed has an 32 MiB SPI flash (**ISSI IS25WP256D**), that can be
+programmed from within Linux running on the board, via USB/JTAG, or directly
+with an SPI programmer.
+
+### Internal programming
+
+The SPI flash can be accessed as `/dev/mtd0` from Linux.
+
+### USB/JTAG
+
+To program the flash via USB/JTAG, connect the USB port to a computer. If the
+board is powered on, two new serial ports, for example `/dev/ttyUSB0` and
+`/dev/ttyUSB1` will appear. The first is JTAG, and the second is connected to
+the SoC's UART.
+
+- Download and build the [RISC-V fork of OpenOCD].
+- Download the [OpenOCD script] for Freedom Unleashed.
+- Start OpenOCD with `openocd -f openocd.cfg`
+- Connect to OpenOCD's command interface (via telnet) and enter the line
+  marked with `> `:
+```
+> flash write_image erase unlock build/coreboot.rom 0x20000000
+auto erase enabled
+auto unlock enabled
+wrote 33554432 bytes from file build/coreboot.rom in 1524.943848s (21.488 KiB/s)
+```
+  Note that programming the whole flash with OpenOCD isn't fast. In this
+  example it took just over 25 minutes. This process can be sped up
+  considerably by building/flashing a smaller image; OpenOCD does not check if
+  the image and the flash have the same size.
+
+
+### External programming
+
+External programming with an SPI adapter and [flashrom] may work, but has not
+been tested. Please study the [schematics] before going this route.
+
+
+## Error codes
+
+The zeroth-stage bootloader (ZSBL) in ROM1 can print error codes on the serial
+console in certain situations.
+
+```
+// Error codes are formatted as follows:
+// [63:60]    [59:56]  [55:0]
+// bootstage  trap     errorcode
+// If trap == 1, then errorcode is actually the mcause register with the
+// interrupt bit shifted to bit 55.
+```
+(--- from the [SiFive forum](https://forums.sifive.com/t/loading-fsbl-from-sd/1156/4))
+
+
+[HiFive Unleashed]: https://www.crowdsupply.com/sifive/hifive-unleashed
+[SiFive]: https://www.sifive.com/
+[Getting Started Guide]: https://www.sifive.com/documentation/boards/hifive-unleashed/hifive-unleashed-getting-started-guide/
+[RISC-V fork of OpenOCD]: https://github.com/riscv/riscv-openocd
+[OpenOCD script]: https://github.com/sifive/freedom-u-sdk/blob/057a47f657fa33e2c60df7f183884a68e90381cc/bsp/env/freedom-u500-unleashed/openocd.cfg
+[flashrom]: https://flashrom.org/Flashrom
+[schematics]: https://www.sifive.com/documentation/boards/hifive-unleashed/hifive-unleashed-schematics/