intel platforms: add list of Chromebook dev. environments

getting_started/intel_debug/introduction.rst

@@ -137,4 +137,65 @@ drivers and should work when changing the ``dsp_driver`` parameter.
 The selection of firmware, topology, and UCM files is based on platform
 capabilities, codec names, and DMI options. While the SOF team and the
 community try to cover all possible cases, errors will happen when the
-wrong file is selected at any of the three layers.
+wrong file is selected at any of the three layers.
+
+4. Chromebooks and SOF
+----------------------
+
+As stated above, starting from 2019/2020 Intel Chromeboooks have been
+configured with the *community* key which means they can run
+audio firmware signed by anyone. The entire filesystem is locked by
+default instead but there are a couple options to disable security for
+development purposes. In all cases the very first step is to switch the
+Chromebook to (non-secure) `Developer Mode
+<https://chromium.googlesource.com/chromiumos/docs/+/HEAD/developer_mode.md>`_.
+Developer Mode is in fact the only strictly required step if you only
+want to install and run your own SOF firmware and are not interested in
+changing anything else in ChromeOS.
+
+If you need the flexibility to make more changes, Chromebooks can run
+Linux in several, non-mutually exclusive ways. All the options listed
+below will let you run any SOF firmware at will. One of the biggest
+differences between them is how to install and run your own Linux
+kernel.
+
+- **ChromeOS** has direct hardware access but ChromeOS development
+  cannot happen on ChromeOS itself, it requires a separate workstation
+  similar to how most embedded development typically does. Setting up
+  the ``cros_sdk`` is documented in the `ChromeOS developer guide
+  <https://chromium.googlesource.com/chromiumos/docs/+/HEAD/developer_guide.md>`_.
+  The ``cros_sdk`` is a complete environment that lets you modify
+  anything in ChromeOS and even build an entire system image. The
+  ``cros_sdk`` requires significant disk space and there is some
+  learning effort if you're not already familiar with Gentoo's build
+  system "Portage"; especially for the Linux kernel.
+
+- `Crostini
+  <https://chromium.googlesource.com/chromiumos/docs/+/HEAD/containers_and_vms.md>`_
+  is a secure Linux Virtual Machine that does not have direct access to
+  the hardware and cannot be used for SOF (it does not require Developer
+  Mode). Listed here for completeness. It may be possible to use
+  Crostini as your pseudo-separate ``cros_sdk`` workstation but a
+  different, more powerful system that you never have to reboot is an
+  obviously much better ``cros_sdk`` option.
+
+- **Crouton** is a non-secure chroot that does allow direct hardware
+  access and can be used for SOF. It lets you install a choice of
+  popular Linux distributions which you can use to develop on the device
+  itself: make regular backups! The Zephyr project has `very detailed
+  specific instructions
+  <https://docs.zephyrproject.org/2.7.0/boards/xtensa/intel_adsp_cavs25/doc/index.html>`_
+  on how to use Crouton for SOF. Most of these instructions are not
+  Zephyr-specific. With Crouton you can configure and compile a Linux
+  kernel as usual, however the kernel *installation* process is similar
+  to the ``cros_sdk`` process with a couple of small twists.
+
+- Finally, it is possible to **dual-boot** or completely replace
+  ChromeOS by a regular Linux distribution on *some* Chromebooks and
+  forget it is a Chromebook entirely. This comes at a price however: it
+  is the least secure option and the more likely to make your device
+  permanently unusable ("brick"). That level of risk is highly dependent
+  on your particular Chromebook model. If that does not scare you then
+  https://chrx.org/ is a good starting point; pay special attention to
+  the "note on security". This is the only option that lets you manage
+  kernel installations as a typical Linux distribution does.