boards/mtk_adsp: Add a doc page

Simple docs for this board family.  Not a lot of complexity currently.

Signed-off-by: Andy Ross <[email protected]>

boards/mediatek/index.rst

@@ -0,0 +1,166 @@
+.. _boardname_linkname:
+
+Mediatek Audio DSPs
+###################
+
+Zephyr can be built and run on the Audio DSPs included in various
+members of the Mediatek MT8xxx series of ARM SOCs used in Chromebooks
+from various manufacturers.
+
+Two of these DSPs are in the market already, implemented via the
+MT8195 ("Kompanio 1380") and MT8186 ("Kompanio 520") SOCs.
+Development has been done on and validation performed on at least
+these devices, though more exist:
+
+  ======  =============  ===================================  =================
+  SOC     Product Name   Example Device                       ChromeOS Codename
+  ======  =============  ===================================  =================
+  MT8195  Kompanio 1380  HP Chromebook x360 13b               dojo
+  MT8186  Kompanio 520   Lenovo 300e Yoga Chromebook Gen 4    steelix
+  ======  =============  ===================================  =================
+
+Hardware
+********
+
+These devices are Xtensa DSP cores, very similar to the Intel ADSP
+series in concept (with the notable difference that these are all
+single-core devices, no parallel SMP is available, but at the same
+time there are fewer worries about the incoherent cache).
+
+Their memory space is split between dedicated, fast SRAM and ~16MB of
+much slower system DRAM.  Zephyr currently loads and links into the
+DRAM area, a convention it inherits from SOF (these devices have
+comparatively large caches which are used for all accesses, unlike
+with intel_adsp).  SRAM is used for interrupt vectors and stacks,
+currently.
+
+There is comparatively little on-device hardware.  The architecture is
+that interaction with the off-chip audio hardware (e.g. I2S codecs,
+DMIC inputs, etc...) is managed by the host kernel.  The DSP receives
+its data via a single array of custom DMA controllers.
+
+Beyond that the Zephyr-visible hardware is limited to a bounty of
+timer devices (of which Zephyr uses two), and a "mailbox"
+bidirectional interrupt source it uses to communicate with the host
+kernel.
+
+Programming and Debugging
+*************************
+
+These devices work entirely in RAM, so there is no "flash" process as
+such.  Their memory state is initialized by the host Linux
+environment.  This process works under the control of a
+``mtk_adsp_load.py`` python script, which has no dependencies outside
+the standard library and can be run (as root, of course) on any
+reasonably compatible Linux environment with a Python 3.8 or later
+interpreter.  A chromebook in development mode with the dev packages
+installed works great.  See the ChromiumOS developer library for more
+detail:
+
+* `Developer mode <https://www.chromium.org/chromium-os/developer-library/guides/device/developer-mode/>`__
+* `Dev-Install: Installing Developer and Test packages onto a Chrome OS device <https://www.chromium.org/chromium-os/developer-library/guides/device/install-software-on-base-images/>`__
+
+Once you have the device set up, the process is as simple as copying
+the ``zephyr.img`` file from the build directory to the device
+(typically via ssh) and running it with the script.  For example for
+my mt8186 device named "steelix":
+
+.. code-block:: console
+
+   user@dev_host:~$ west build -b mt8186_adsp samples/hello_world
+   ...
+   ... # build output
+   ...
+   user@dev_host:~$ scp build/zephyr/zephyr.img root@steelix:
+   user@dev_host:~$ scp soc/mediatek/mtk_adsp/mtk_adsp_load.py root@steelix:
+   user@dev_host:~$ ssh steelix
+
+   root@steelix:~ # ./mtk_adsp_load.py load zephyr.img
+   *** Booting Zephyr OS build v3.6.0-5820-gd2a89b3c089e ***
+   Hello World! mt8186_adsp/mt8186_adsp
+
+Debugging
+=========
+
+Given the limited I/O facilities, debugging support remains limited on
+these platforms.  Users with access to hardware-level debug and trace
+tools (e.g. from Cadence) will be able to use them as-is.  Zephyr
+debugging itself is limited to printk/logging techniques at the
+moment.  In theory a bidirectional console like winstream can be used
+with gdb_stub, which has support on Xtensa and via the SDK debuggers,
+but this is still unintegrated.
+
+Toolchains
+**********
+
+The MT8195 toolchain is already part of the Zephyr SDK, so builds for
+the ``mt8195_adsp`` board should work out of the box simply following
+the generic Zephyr build instructions in the Getting Started guide.
+
+The MT8186 toolchain is not, and given the proliferation of Xtensa
+toolchains in the SDK may not be.  The overlay files for the device
+are maintained by the SOF project, however, and building a toolchain
+yourself using crosstools-ng is not difficult or time-consuming.  This
+script should work for most users:
+
+.. code-block:: shell
+
+   #!/bin/sh
+
+   TC=mtk_mt818x_adsp
+
+   # Grab source (these are small)
+   git clone https://github.com/crosstool-ng/crosstool-ng
+   git clone https://github.com/thesofproject/xtensa-overlay
+
+   # Build ct-ng itself
+   cd crosstool-ng
+   ./bootstrap
+   ./configure --enable-local
+   make -j$(nproc)
+
+   mkdir overlays
+   (cd overlays; ln -s ../../xtensa-overlay/xtensa_mt8186.tar.gz xtensa_${TC}.tar.gz)
+
+   # Construct a .config file
+   cat >.config <<EOF
+   CT_CONFIG_VERSION="3"
+   CT_EXPERIMENTAL=y
+   CT_OVERLAY_LOCATION="overlays"
+   CT_OVERLAY_NAME="${TC}"
+   CT_ARCH_XTENSA=y
+   CT_XTENSA_CUSTOM=y
+   CT_TARGET_VENDOR="${TC}_zephyr"
+   CT_TARGET_CFLAGS="-ftls-model=local-exec"
+   CT_CC_GCC_CONFIG_TLS=n
+   CT_GDB_CROSS_EXTRA_CONFIG_ARRAY="--enable-xtensa-use-target-regnum --disable-xtensa-remote-g-packet"
+   EOF
+
+   # Build
+   ./ct-ng olddefconfig
+   ./ct-ng build.$(nproc)
+
+After this completes, you will find your toolchain in ``~/x-tools``
+and can use it to build by setting it as your Zephyr cross compiler:
+
+.. code-block:: shell
+
+   export CROSS_COMPILE=$HOME/x-tools/xtensa-mtk_mt818x_adsp_zephyr-elf/bin/xtensa-mtk_mt818x_adsp_zephyr-elf-
+   export ZEPHYR_TOOLCHAIN_VARIANT=cross-compile
+
+Closed-source Tools
+===================
+
+Zephyr can also be built by the proprietary Cadence xcc and xt-clang
+toolchains.  Support for those tools is beyond the scope of this
+document, but it works similarly, by specifying your toolchain and
+core identities and paths via the environment, for example:
+
+.. code-block:: shell
+
+   export XTENSA_TOOLS_ROOT=/path/to/XtDevTools
+   export XTENSA_CORE=hifi5_7stg_I64D128
+   export TOOLCHAIN_VER=RI-2021.6-linux
+   export ZEPHYR_TOOLCHAIN_VARIANT=xt-clang
+   export XTENSA_TOOLCHAIN_PATH=$XTENSA_TOOLS_ROOT/install/tools
+   west build -b mt8186_adsp samples/hello_world