Documentation/erbium: Document the Minion emulator port.

Describe the Erbium architecture and the Minion board: supported scope,
memory map and interrupts, toolchain constraints, Make and CMake
configurations, how to build the pinned public emulator, and how to run
the NSH and ostest images in it.

Signed-off-by: Afonso Oliveira <afonso.oliveira707@gmail.com>

Documentation/platforms/risc-v/erbium/boards/minion/index.rst

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+=============
+Erbium Minion
+=============
+
+.. tags:: chip:erbium, arch:risc-v, vendor:aifoundry
+
+Memory and interrupts
+=====================
+
+The ELF loader reserves the first 512 bytes of the 16 MiB MRAM window.
+NuttX therefore starts at ``0x40000200``. Initialized data is loaded at its
+execution address. Startup clears BSS and places the heap after the idle
+stack. The linker checks that the image and idle stack fit in MRAM.
+
+The PLIC at ``0xa0000000`` provides sources 1 through 6; UART0 uses source
+3. Receive and transmit use interrupts, without periodic polling.
+The machine timer uses ``mtime`` at ``0x80f40200`` and ``mtimecmp`` at
+``0x80f40208``. Its default frequency is 2 MHz in the emulator.
+
+Toolchain
+=========
+
+Use a GNU RISC-V bare-metal toolchain with RV64 single-precision ABI
+support, such as ``riscv64-unknown-elf-gcc``. The normal RISC-V toolchain
+logic derives ``rv64imfc_zicsr_zifencei`` and ``lp64f`` from Kconfig.
+
+Erbium handles floating-point divide and square root through microcode.
+This standalone image does not supply that microcode, so the port uses
+``-mno-fdiv`` to generate software helper calls for those operations.
+Other single-precision instructions and FPU context switching remain
+enabled, including the FPU portion of ``ostest``. Double-precision
+arithmetic uses software. The atomic instruction extension is not enabled.
+NuttX provides atomic operations through interrupt masking on hart 0.
+Startup also avoids the microcode-dependent ``FENCE.I`` instruction; this
+port expects the emulator's ELF loader to install the executable image.
+NuttX's dynamic ELF module loader is disabled in both configurations because
+its instruction-cache synchronization also uses ``FENCE.I``. This does not
+affect the ``nuttx`` ELF output file loaded by the emulator.
+
+Configurations
+==============
+
+``nsh`` provides the NuttShell console and procfs. ``ostest`` starts the
+NuttX OS test application automatically. Both configurations run on hart 0.
+
+With sibling ``nuttx`` and ``apps`` checkouts, build using Make::
+
+  cd nuttx
+  ./tools/configure.sh -l minion:nsh
+  make -j8
+
+To select the OS tests, start from a clean configuration::
+
+  make distclean
+  ./tools/configure.sh -l minion:ostest
+  make -j8
+
+From the directory containing the sibling ``nuttx`` and ``apps`` checkouts,
+CMake is also supported::
+
+  # Run make distclean first if nuttx was configured with Make.
+  cmake -S nuttx -B build -GNinja -DBOARD_CONFIG=minion:nsh
+  cmake --build build
+
+Build the emulator
+==================
+
+The public ET-platform revision
+``836a4ab600e93c3059bb58c898edbc37744cd8d0`` has been tested with this port.
+No local emulator patches are required. Build only the header-only Erbium
+HAL package and the native emulator; the accelerator firmware and ET
+cross-toolchain are not needed. The host requires CMake 3.21 or later,
+Ninja, a C++17 compiler, and the glog and LZ4 development packages::
+
+  git clone https://github.com/aifoundry-org/et-platform.git
+  git -C et-platform checkout 836a4ab600e93c3059bb58c898edbc37744cd8d0
+  cmake -S et-platform/erbium-hal -B hal-build \
+    -DCMAKE_INSTALL_PREFIX="$PWD/emu-prefix"
+  cmake --install hal-build
+  cmake -S et-platform/sw-sysemu -B emu-build -GNinja \
+    -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH="$PWD/emu-prefix" \
+    -DCMAKE_CXX_FLAGS=-Wno-error=unused-result
+  cmake --build emu-build --target erbium_emu
+
+The last compiler option keeps an existing ignored ``write()`` return value
+in the emulator UART implementation from becoming a build error with GCC
+13. It does not modify the emulator source. The executable is
+``emu-build/erbium_emu``.
+
+Run in the emulator
+===================
+
+Use an emulator with Erbium PLIC PMA access, clocked UART FIFO/interrupts,
+and timer advancement while the core is in WFI. Older emulator builds
+without those features are not supported.
+
+Start an interactive console with::
+
+  erbium_emu -elf_load nuttx -reset_pc 0x40000200 -single_thread \
+    -max_cycles 100000000000
+
+The same command runs the ``ostest`` image without console input. Allow
+enough simulated cycles for the tests' sleeps and timeouts; reaching the
+emulator cycle limit is not a successful test result. A complete run prints
+``Final memory usage:`` followed by ``user_main: Exiting`` and
+``ostest_main: Exiting with status 0``. Inspect the entire log for failures
+and assertions as well.
+
+The board includes an automated runner that accepts prebuilt ELF files::
+
+  python3 boards/risc-v/erbium/minion/tools/test_emu.py \
+    --emu /path/to/erbium_emu --elf nuttx --mode ostest --output logs/ostest
+
+For an ``nsh`` image, select ``--mode nsh``. This checks console commands,
+procfs, uptime advancement during sleep and receive interrupts after idle.
+Add ``--extra-harts``
+to start four harts and exercise secondary-hart parking. The runner fills
+uninitialized emulator memory with a nonzero pattern and records the UART
+log, emulator log, binary checksums and result JSON. It stops the emulator
+after the required completion markers; a timeout or cycle-limit exit before
+completion fails the test. A Release build of the emulator is recommended
+for the complete OS tests, which contain many simulated sleeps.
+
+UART settings
+=============
+
+UART0 supports 8-bit characters, no parity and one stop bit (8N1), using
+``CONFIG_UART0_BAUD`` and the standard serial buffer size options. Other
+line formats are rejected at build time. ``CONFIG_ERBIUM_UART_CLOCK``
+describes the peripheral clock
+after the system divider. The emulator defaults correspond to a 200 MHz
+system clock divided by 30. Runtime termios reconfiguration and hardware
+flow control are not implemented. ``CONFIG_SUPPRESS_UART_CONFIG`` preserves
+a bootloader's UART configuration; it must remain disabled for direct ELF
+loading into a reset emulator. ``CONFIG_SUPPRESS_SERIAL_INTS`` is honored
+for low-level console debugging.
+
+The emulator reads host input into its finite RX FIFO immediately. Feeding
+a long command through a pipe can overrun that FIFO. Automated console
+tests should pace input, for example by waiting for each echoed character.

Documentation/platforms/risc-v/erbium/index.rst

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+======
+Erbium
+======
+
+Erbium is an AIFoundry RISC-V platform with a 64-bit Minion core. This
+initial port targets the ``erbium_emu`` system emulator from
+`ET-platform <https://github.com/aifoundry-org/et-platform>`_.
+
+The port runs in machine mode on hart 0, with a flat address space and SMP
+disabled. Other harts remain parked and never initialize NuttX memory.
+Supported peripherals are the machine timer, PLIC and UART0 console.
+Silicon, protected/kernel builds, SMP, storage, networking
+and other peripherals have not been validated by this port.
+
+.. toctree::
+   :maxdepth: 1
+
+   boards/minion/index
+
+The emulator currently only stores writes to the system reset register; it
+does not reboot the simulated system. The board therefore does not expose
+``BOARDIOC_RESET`` or claim reset support.