[RFC PATCH] doc: arch: Add document for RISC-V architecture

[Yu Chien Peter Lin]

This patch adds a brief introduction to the RISC-V architecture and
the typical boot process used on a variety of RISC-V platforms.

Signed-off-by: Yu Chien Peter Lin <peterlin at andestech.com>
---
Hi RISC-V community,

Please leave a comment if there is anything I've missed that should
be mentioned in the document. Thanks.
---
 doc/arch/index.rst |  1 +
 doc/arch/riscv.rst | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 doc/arch/riscv.rst

diff --git a/doc/arch/index.rst b/doc/arch/index.rst
index b3e85f9bf3..b8da4b8c8e 100644
--- a/doc/arch/index.rst
+++ b/doc/arch/index.rst
@@ -11,6 +11,7 @@ Architecture-specific doc
    m68k
    mips
    nios2
+   riscv
    sandbox/index
    sh
    x86
diff --git a/doc/arch/riscv.rst b/doc/arch/riscv.rst
new file mode 100644
index 0000000000..243e7e7e2e
--- /dev/null
+++ b/doc/arch/riscv.rst
@@ -0,0 +1,43 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright (C) 2023, Yu Chien Peter Lin <peterlin at andestech.com>
+
+RISC-V
+======
+
+Overview
+--------
+
+This document outlines the U-Boot boot process for the RISC-V architecture.
+RISC-V is an open-source instruction set architecture (ISA) based on the
+principles of reduced instruction set computing (RISC). It has been designed
+to be flexible and customizable, allowing it to be adapted to different use
+cases, from embedded systems to high performance servers.
+
+Typical Boot Process
+--------------------
+
+RISC-V production boot images typically include a U-Boot SPL for platform-specific
+initialization. The U-Boot SPL then loads a FIT image (u-boot.itb), which contains
+an SBI (Supervisor Binary Interface) firmware such as `OpenSBI <https://github.com/riscv-software-src/opensbi>`_, as well as a regular
+U-Boot (or U-Boot proper) running in S-mode. Finally, the S-mode Operating System
+is loaded.
+
+In between the boot stages, the hartid is passed through the a0 register, and the
+start address of the devicetree is passed through the a1 register.
+
+The following diagram illustrates the boot process::
+
+	<----------( M-mode )--------><-------( S-mode )------>
+	+------------+   +---------+    +--------+   +--------+
+	| U-Boot SPL |-->|   SBI   |--->| U-Boot |-->|   OS   |
+	+------------+   +---------+    +--------+   +--------+
+
+To examine the boot process with the QEMU virt machine, you can follow the steps
+in the following document:
+:doc:`../board/emulation/qemu-riscv.rst`
+
+Toolchain
+---------
+
+You can build the `RISC-V GNU toolchain <https://github.com/riscv-collab/riscv-gnu-toolchain>`_ from scratch, or download a
+pre-built toolchain from the `releases page <https://github.com/riscv-collab/riscv-gnu-toolchain/releases>`_.
-- 
2.34.1